LTC2344HUH-18#PBF - ANALOG DEVICES

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TYPICAL APPLICATION

FEATURES DESCRIPTION

Quad, 18-Bit, 400ksps/ch

Differential SoftSpan ADC with

Wide Input Common Mode Range

The LTC

2344-18 is an 18-bit, low noise 4-channel simul-

taneous sampling successive approximation register (SAR)

ADC with differential, wide common mode range inputs.

Operating from a 5V supply and using the internal reference

and buffer, each channel of this SoftSpan

ADC can be

independently configured on a conversion-by-conversion

basis to accept ±4.096V, 0V to 4.096V, ±2.048V, or 0V to

2.048V signals. Individual channels may also be disabled

to increase throughput on the remaining channels.

The wide input common mode range and 102dB CMRR of

the LTC2344-18 analog inputs allow the ADC to directly

digitize a variety of signals, simplifying signal chain de-

sign. This input signal flexibility, combined with ±4LSB

INL, no missing codes at 18 bits, and 95dB SNR, makes

the LTC2344-18 an ideal choice for many applications

requiring wide dynamic range.

The LTC2344-18 supports pin-selectable SPI CMOS (1.8V

to 5V) and LVDS serial interfaces. Between one and four

lanes of data output may be employed in CMOS mode,

allowing the user to optimize bus width and throughput.

L, LT, LT C , LT M, Linear Technology and the Linear logo are registered trademarks and

SoftSpan is a trademark of Linear Technology Corporation. All other trademarks are the property

of their respective owners. Protected by U.S. Patents, including 7705765, 7961132, 8319673,

9197235.

APPLICATIONS

n 400ksps per Channel Throughput

n Four Simultaneous Sampling Channels

n ±4LSB INL (Maximum, ±4.096V Range)

n Guaranteed 18-Bit, No Missing Codes

n Differential, Wide Common Mode Range Inputs

n Per-Channel SoftSpan Input Ranges:

±4.096V, 0V to 4.096V, ±2.048V, 0V to 2.048V

±5V, 0V to 5V, ±2.5V, 0V to 2.5V

n 95dB Single-Conversion SNR (Typical)

n −114dB THD (Typical) at fIN = 2kHz

n 102dB CMRR (Typical) at fIN = 200Hz

n Rail-to-Rail Input Overdrive Tolerance

n Guaranteed Operation to 125°C

n Integrated Reference and Buffer (4.096V)

n SPI CMOS (1.8V to 5V) and LVDS Serial I/O

n Internal Conversion Clock, No Cycle Latency

n 81mW Power Dissipation (Typical)

n 32-Lead (5mm × 5mm) QFN Package

n Programmable Logic Controllers

n Industrial Process Control

n Medical Imaging

n High Speed Data Acquisition

Integral Nonlinearity vs

Output Code and Channel

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

ALL CHANNELS

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

234418 TA01b

0.1µF2.2µF0.1µF

1.8V TO 5V5V

SAMPLE

CLOCK

234418 TA01a

S/H

MUX

VDD VDDLBYP OVDD

FOUR SIMULTANEOUS

SAMPLING CHANNELS

DIFFERENTIAL INPUTS

IN+/IN– WITH

WIDE INPUT COMMON MODE RANGE

FULLY

DIFFERENTIAL

BIPOLAR

ARBITRARY

UNIPOLAR

SDO0

SDO3

SCKO

SCKI

SDI

BUSY

CNV

• • •

LVDS/CMOS

IN0+

IN0–

IN3+

IN3–

18-BIT

SAR ADC

CMOS OR LVDS

I/O INTERFACE

0.1µF

REFIN

47µF

GNDREFBUF

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS

Supply Voltage (VDD) ..................................................6V

Supply Voltage (OVDD) ................................................6V

Internal Regulated Supply Bypass (VDDLBYP) ... (Note 3)

Analog Input Voltage

IN0+ to IN3+,

IN0– to IN3– (Note 4) .................. –0.3V to (VDD + 0.3V)

REFIN .................................................... –0.3V to 2.8V

REFBUF, CNV (Note 4) ............. –0.3V to (VDD + 0.3V)

Digital Input Voltage (Note 4) ..... –0.3V to (OVDD + 0.3V)

Digital Output Voltage (Note 4) .. –0.3V to (OVDD + 0.3V)

Power Dissipation .............................................. 500mW

Operating Temperature Range

LTC2344C ................................................ 0°C to 70°C

LTC2344I .............................................–40°C to 85°C

LTC2344H .......................................... –40°C to 125°C

Storage Temperature Range .................. –65°C to 150°C

(Notes 1, 2)

GND

31 30 29 28 27 26 25

9 10 11 12

TOP VIEW

UH PACKAGE

32-LEAD (5mm × 5mm) PLASTIC QFN

13 14 15 16

IN3

–

IN3

IN2

–

IN2

IN1

–

IN1

IN0

–

IN0

SDO+/SDO3

SCKO–

/SDO2

SCKO+

/SCKO

OVDD

GND

SCKI–/SCKI

SCKI+/SDO1

SDI–/SDO0

GND

VDD

GND

VDDLBYP

BUSY

SDO–/SDI

GND

REFIN

GND

REFBUF

LVDS/CMOS

CNV

SDI+

TJMAX = 150°C, θJA = 44°C/W

EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC2344CUH-18#PBF LTC2344CUH-18#TRPBF 234418 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C

LTC2344IUH-18#PBF LTC2344IUH-18#TRPBF 234418 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C

LTC2344HUH-18#PBF LTC2344HUH-18#TRPBF 234418 32-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through

designated sales channels with #TRMPBF suffix.

http://www.linear.com/product/LTC2344-18#orderinfo

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

ELECTRICAL CHARACTERISTICS

CONVERTER CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN+ Absolute Input Range

(IN0+ to IN3+)

(Note 6) l0 VDD V

VIN– Absolute Input Range

(IN0– to IN3–)

(Note 6) l0 VDD V

VIN+ – VIN– Input Differential Voltage

Range

SoftSpan 7: ±VREFBUF Range (Note 6)

SoftSpan 6: ±VREFBUF/1.024 Range (Note 6)

SoftSpan 5: 0V to VREFBUF Range (Note 6)

SoftSpan 4: 0V to VREFBUF/1.024 Range (Note 6)

SoftSpan 3: ±0.5 • VREFBUF Range (Note 6)

SoftSpan 2: ±0.5 • VREFBUF/1.024 Range (Note 6)

SoftSpan 1: 0V to 0.5 • VREFBUF Range (Note 6)

– VREFBUF

– VREFBUF/1.024

–0.5 • VREFBUF

–0.5 • VREFBUF/1.024

VREFBUF

VREFBUF/1.024

VREFBUF

VREFBUF/1.024

0.5 • VREFBUF

0.5 • VREFBUF/1.024

0.5 • VREFBUF

VCM Input Common Mode Voltage

Range

(Note 6) l0 VDD V

VIN+ – VIN– Input Differential Overdrive

Tolerance

(Note 7) l−VDD VDD V

IIN Analog Input Leakage Current l–1 1 µA

CIN Analog Input Capacitance Sample Mode

Hold Mode

CMRR Input Common Mode

Rejection Ratio

VIN+ = VIN− = 3.6VP-P 200Hz Sine l88 102 dB

VIHCNV CNV High Level Input Voltage l1.3 V

VILCNV CNV Low Level Input Voltage l0.5 V

IINCNV CNV Input Current VIN = 0V to VDD l–10 10 μA

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Resolution l18 Bits

No Missing Codes l18 Bits

Transition Noise SoftSpans 7 and 6: ±4.096V and ±4V Ranges

SoftSpans 5 and 4: 0V to 4.096V and 0V to 4V Ranges

SoftSpans 3 and 2: ±2.048V and ±2V Ranges

SoftSpan 1: 0V to 2.048V Range

1.6

3.2

6.3

LSBRMS

INL Integral Linearity Error (Note 9) l–4 ±1.5 4 LSB

DNL Differential Linearity Error (Note 10) l−0.9 ±0.5 0.9 LSB

ZSE Zero-Scale Error (Note 11) l−570 ±50 570 μV

Zero-Scale Error Drift ±2 μV/°C

FSE Full-Scale Error VREFBUF = 4.096V (REFBUF Overdriven) (Note 11) l−0.13 ±0.025 0.13 %FS

Full-Scale Error Drift ±2.5 ppm/°C

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 5)

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 8)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

DYNAMIC ACCURACY

INTERNAL REFERENCE CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

SINAD Signal-to-(Noise +

Distortion) Ratio

SoftSpans 7 and 6: ±4.096V and ±4V Ranges, fIN = 2kHz

SoftSpans 5 and 4: 0V to 4.096V and 0V to 4V Ranges, fIN = 2kHz

SoftSpans 3 and 2: ±2.048V and ±2V Ranges, fIN = 2kHz

SoftSpan 1: 0V to 2.048V Range, fIN = 2kHz

91.0

85.0

85.1

79.3

95.0

89.1

83.3

SNR Signal-to-Noise Ratio SoftSpans 7 and 6: ±4.096V and ±4V Ranges, fIN = 2kHz

SoftSpans 5 and 4: 0V to 4.096V and 0V to 4V Ranges, fIN = 2kHz

SoftSpans 3 and 2: ±2.048V and ±2V Ranges, fIN = 2kHz

SoftSpan 1: 0V to 2.048V Range, fIN = 2kHz

91.1

85.2

85.1

79.3

95.0

89.1

83.3

THD Total Harmonic Distortion SoftSpans 7 and 6: ±4.096V and ±4V Ranges, fIN = 2kHz

SoftSpans 5 and 4: 0V to 4.096V and 0V to 4V Ranges, fIN = 2kHz

SoftSpans 3 and 2: ±2.048V and ±2V Ranges, fIN = 2kHz

SoftSpan 1: 0V to 2.048V Range, fIN = 2kHz

–114

–111

–110

–109

–99

–95

SFDR Spurious Free Dynamic

Range

SoftSpans 7 and 6: ±4.096V and ±4V Ranges, fIN = 2kHz

SoftSpans 5 and 4: 0V to 4.096V and 0V to 4V Ranges, fIN = 2kHz

SoftSpans 3 and 2: ±2.048 and ±2V Ranges, fIN = 2kHz

SoftSpan 1: 0V to 2.048V Range, fIN = 2kHz

115

112

111

110

Channel-to-Channel

Crosstalk

One Channel Converting 3.6VP-P 200Hz Sine in ±2.048V Range,

Crosstalk to All Other Channels

−109 dB

–3dB Input Bandwidth 22 MHz

Aperture Delay 1 ns

Aperture Delay Matching 150 ps

Aperture Jitter 3 psRMS

Transient Response Full-Scale Step, 0.005% Settling 210 ns

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VREFIN Internal Reference Output Voltage 2.043 2.048 2.053 V

Internal Reference Temperature Coefficient (Note 13) l5 20 ppm/°C

Internal Reference Line Regulation VDD = 4.75V to 5.25V 0.1 mV/V

Internal Reference Output Impedance 20 kΩ

VREFIN REFIN Voltage Range REFIN Overdriven (Note 6) 1.25 2.2 V

The l denotes the specifications which apply over the full operating temperature range,

otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Notes 8, 12)

The l denotes the specifications which apply over the full

operating temperature range, otherwise specifications are at TA = 25°C. (Note 8)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

REFERENCE BUFFER CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VREFBUF Reference Buffer Output Voltage REFIN Overdriven, VREFIN = 2.048V l4.091 4.096 4.101 V

REFBUF Voltage Range REFBUF Overdriven (Notes 6, 14) l2.5 5 V

REFBUF Input Impedance VREFIN = 0V, Buffer Disabled 13 kΩ

IREFBUF REFBUF Load Current VREFBUF = 5V, 4 Channels Enabled (Notes 14, 15)

VREFBUF = 5V, Acquisition Mode (Note 14)

l1.5

0.39

1.9 mA

The l denotes the specifications which apply over the full

operating temperature range, otherwise specifications are at TA = 25°C. (Note 8)

DIGITAL INPUTS AND DIGITAL OUTPUTS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

CMOS Digital Inputs and Outputs

VIH High Level Input Voltage l0.8•OVDD V

VIL Low Level Input Voltage l0.2•OVDD V

IIN Digital Input Current VIN = 0V to OVDD l–10 10 μA

CIN Digital Input Capacitance 5 pF

VOH High Level Output Voltage IOUT = –500μA lOVDD–0.2 V

VOL Low Level Output Voltage IOUT = 500μA l0.2 V

IOZ Hi-Z Output Leakage Current VOUT = 0V to OVDD l–10 10 μA

ISOURCE Output Source Current VOUT = 0V –50 mA

ISINK Output Sink Current VOUT = OVDD 50 mA

LVDS Digital Inputs and Outputs

VID Differential Input Voltage l200 350 600 mV

RID On-Chip Input Termination

Resistance

CS = 0V, VICM = 1.2V

CS = OVDD

l80 106

130 Ω

MΩ

VICM Common-Mode Input Voltage l0.3 1.2 2.2 V

IICM Common-Mode Input Current VIN+ = VIN– = 0V to OVDD l–10 10 μA

VOD Differential Output Voltage RL = 100Ω Differential Termination l275 350 425 mV

VOCM Common-Mode Output Voltage RL = 100Ω Differential Termination l1.1 1.2 1.3 V

IOZ Hi-Z Output Leakage Current VOUT = 0V to OVDD l–10 10 μA

The l denotes the specifications which apply over the

full operating temperature range, otherwise specifications are at TA = 25°C. (Note 8)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

POWER REQUIREMENTS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

CMOS I/O Mode

VDD Supply Voltage l4.75 5.00 5.25 V

OVDD Supply Voltage l1.71 5.25 V

IVDD Supply Current 400ksps Sample Rate, 4 Channels Enabled

400ksps Sample Rate, 4 Channels Enabled, VREFBUF = 5V (Note 14)

Acquisition Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

15.1

13.5

1.2

17.6

16.0

2.0

225

500

μA

µA

IOVDD Supply Current 400ksps Sample Rate, 4 Channels Enabled (CL = 25pF)

Acquisition Mode

Power Down Mode

2.0

3.0

μA

PDPower Dissipation 400ksps Sample Rate, 4 Channels Enabled

Acquisition Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

6.0

0.33

1.2

2.6

LVDS I/O Mode

VDD Supply Voltage l4.75 5.00 5.25 V

OVDD Supply Voltage l2.375 5.25 V

IVDD Supply Current 400ksps Sample Rate, 4 Channels Enabled

400ksps Sample Rate, 4 Channels Enabled, VREFBUF = 5V (Note 14)

Acquisition Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

17.4

15.7

2.8

20.5

18.1

3.8

225

500

μA

µA

IOVDD Supply Current 400ksps Sample Rate, 4 Channels Enabled (RL = 100Ω)

Acquisition Mode (RL = 100Ω)

Power Down Mode

7.4

9.1

8.2

μA

PDPower Dissipation 400ksps Sample Rate, 4 Channels Enabled

Acquisition Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

106

0.33

126

1.2

2.6

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

fSMPL Maximum Sampling Frequency 4 Channels Enabled

3 Channels Enabled

2 Channels Enabled

1 Channel Enabled

400

500

666

1000

ksps

tCYC Time Between Conversions 4 Channels Enabled, fSMPL = 400ksps

3 Channels Enabled, fSMPL = 500ksps

2 Channels Enabled, fSMPL = 666ksps

1 Channel Enabled, fSMPL = 1000ksps

2500

2000

1500

1000

The l denotes the specifications which apply over the full operating temperature

range, otherwise specifications are at TA = 25°C. (Note 8)

ADC TIMING CHARACTERISTICS

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 8)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

ADC TIMING CHARACTERISTICS

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 8)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

tCONV Conversion Time N Channels Enabled, 1 ≤ N ≤ 4 l425 • N –20 475 • N –20 525 • N –20 ns

tACQ Acquisition Time

(tACQ = tCYC – tCONV – tBUSYLH)

4 Channels Enabled, fSMPL = 400ksps

3 Channels Enabled, fSMPL = 500ksps

2 Channels Enabled, fSMPL = 666ksps

1 Channel Enabled, fSMPL = 1000ksps

390

415

440

465

600

575

550

525

tCNVH CNV High Time l40 ns

tCNVL CNV Low Time l390 ns

tBUSYLH CNV↑ to BUSY Delay CL = 25pF l30 ns

tQUIET Digital I/O Quiet Time from CNV↑l20 ns

tPDH PD High Time l40 ns

tPDL PD Low Time l40 ns

tWAKE REFBUF Wake-Up Time CREFBUF = 47μF, CREFIN = 0.1μF 200 ms

CMOS I/O Mode

tSCKI SCKI Period (Notes 16, 17) l10 ns

tSCKIH SCKI High Time l4 ns

tSCKIL SCKI Low Time l4 ns

tSSDISCKI SDI Setup Time from SCKI↑(Note 16) l2 ns

tHSDISCKI SDI Hold Time from SCKI↑(Note 16) l1 ns

tDSDOSCKI SDO Data Valid Delay from SCKI↑CL = 25pF (Note 16) l7.5 ns

tHSDOSCKI SDO Remains Valid Delay from SCKI↑CL = 25pF (Note 16) l1.5 ns

tSKEW SDO to SCKO Skew (Note 16) l–1 0 1 ns

tDSDOBUSYL SDO Data Valid Delay from BUSY↓CL = 25pF (Note 16) l0 ns

tEN Bus Enable Time After CS↓(Note 16) l15 ns

tDIS Bus Relinquish Time After CS↑(Note 16) l15 ns

LVDS I/O Mode

tSCKI SCKI Period (Note 18) l4 ns

tSCKIH SCKI High Time (Note 18) l1.5 ns

tSCKIL SCKI Low Time (Note 18) l1.5 ns

tSSDISCKI SDI Setup Time from SCKI (Notes 10, 18) l1.2 ns

tHSDISCKI SDI Hold Time from SCKI (Notes 10, 18) l–0.2 ns

tDSDOSCKI SDO Data Valid Delay from SCKI (Notes 10, 18) l6 ns

tHSDOSCKI SDO Remains Valid Delay from SCKI (Notes 10, 18) l1 ns

tSKEW SDO to SCKO Skew (Note 10) l–0.4 0 0.4 ns

tDSDOBUSYL SDO Data Valid Delay from BUSY↓(Note 10) l0 ns

tEN Bus Enable Time After CS↓l50 ns

tDIS Bus Relinquish Time After CS↑l15 ns

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

CMOS Timings

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2: All voltage values are with respect to ground.

Note 3: VDDLBYP is the output of an internal voltage regulator, and should

only be connected to a 2.2μF ceramic capacitor to bypass the pin to GND,

as described in the Pin Functions section. Do not connect this pin to any

external circuitry.

Note 4: When these pin voltages are taken below ground or above VDD or

OVDD, they will be clamped by internal diodes. This product can handle

currents of up to 100mA below ground or above VDD or OVDD without

latch-up.

Note 5: VDD = 5V unless otherwise specified.

Note 6: Recommended operating conditions.

Note 7: Exceeding these limits on any channel may corrupt conversion

results on other channels. Refer to Absolute Maximum Ratings section for

pin voltage limits related to device reliability.

Note 8: VDD = 5V, OVDD = 2.5V, fSMPL = 400ksps, internal reference and

buffer, fully differential input signal drive in SoftSpan ranges 7 and 6,

bipolar input signal drive in SoftSpan ranges 3 and 2, unipolar input signal

drive in SoftSpan ranges 5, 4 and 1, unless otherwise specified.

Note 9: Integral nonlinearity is defined as the deviation of a code from a

straight line passing through the actual endpoints of the transfer curve.

The deviation is measured from the center of the quantization band.

Note 10: Guaranteed by design, not subject to test.

Note 11: For bipolar SoftSpan ranges 7, 6, 3, and 2, zero-scale error

is the offset voltage measured from –0.5LSB when the output code

flickers between 00 0000 0000 0000 0000 and 11 1111 1111 1111 1111.

Full-scale error for these SoftSpan ranges is the worst-case deviation of

the first and last code transitions from ideal and includes the effect of

offset error. For unipolar SoftSpan ranges 5, 4, and 1, zero-scale error is

the offset voltage measured from 0.5LSB when the output code flickers

between 00 0000 0000 0000 0000 and 00 0000 0000 0000 0001. Full-

scale error for these SoftSpan ranges is the worst-case deviation of the

last code transition from ideal and includes the effect of offset error.

Note 12: All specifications in dB are referred to a full-scale input in the

relevant SoftSpan input range, except for crosstalk, which is referred to

the crosstalk injection signal amplitude.

Note 13: Temperature coefficient is calculated by dividing the maximum

change in output voltage by the specified temperature range.

Note 14: When REFBUF is overdriven, the internal reference buffer must

be disabled by setting REFIN = 0V.

Note 15: IREFBUF varies proportionally with sample rate and the number of

active channels.

Note 16: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V,

and OVDD = 5.25V.

Note 17: A tSCKI period of 10ns minimum allows a shift clock frequency of

up to 100MHz for rising edge capture.

Note 18: VICM = 1.2V, VID = 350mV for LVDS differential input pairs.

ADC TIMING CHARACTERISTICS

Figure 1. Voltage Levels for Timing Specifications

LVDS Timings (Differential)

0.8 • OVDD

0.2 • OVDD

50% 50%

234418 F01

0.2 • OVDD

0.8 • OVDD

0.2 • OVDD

0.8 • OVDD

tDELAY

tWIDTH

tDELAY

+200mV

–200mV

0V 0V

234418 F01b

–200mV

+200mV

–200mV

+200mV

tDELAY

tWIDTH

tDELAY

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TYPICAL PERFORMANCE CHARACTERISTICS

Integral Nonlinearity

vs Output Code and Range

Integral Nonlinearity

vs Output Code and Range

Integral Nonlinearity

vs Output Code and Range

Integral Nonlinearity

vs Output Code

DC Histogram (Zero-Scale) DC Histogram (Near Full-Scale)

Integral Nonlinearity

vs Output Code and Channel

Differential Nonlinearity

vs Output Code and Channel

TA = 25°C, VDD=5V, OVDD=2.5V, Internal

Reference and Buffer (VREFBUF =4.096V), fSMPL = 400ksps, unless otherwise noted.

32k Point FFT fSMPL=400kHz,

fIN=2kHz

±4.096V RANGE

σ = 1.5

CODE

–10

–8

–6

–4

–2

20000

40000

60000

80000

COUNTS

234418 G07

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

ALL CHANNELS

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

234418 G01

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

SNR = 95.3dB

THD = –112dB

SINAD = 95.2dB

SFDR = 114dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 G09

ALL RANGES

ALL CHANNELS

OUTPUT CODE

65536

131072

196608

262144

–1.00

–0.75

–0.50

–0.25

0.25

0.50

0.75

1.00

DNL ERROR (LSB)

234418 G02

±2.048V AND ±2V

RANGES

BIPOLAR DRIVE (IN

–

= 2.5V)

ONE CHANNEL

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

234418 G03

±4.096V AND ±4V

RANGES

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

ONE CHANNEL

±2.048V AND ±2V

RANGES

OUTPUT CODE

–131072

–65536

65536

131072

–4.0

–3.0

–2.0

–1.0

1.0

2.0

3.0

4.0

INL ERROR (LSB)

234418 G04

0V TO 4.096V AND 0V TO 4V RANGES

UNIPOLAR DRIVE (IN

–

= 0V)

ONE CHANNEL

0V TO 2.048V RANGE

OUTPUT CODE

65536

131072

196608

262144

–4.0

–3.0

–2.0

–1.0

1.0

2.0

3.0

4.0

INL ERROR (LSB)

234418 G05

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

±4.096V RANGE

ARBITRARY DRIVE

/IN

–

COMMON MODE

SWEPT 0V TO 5V

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

234418 G06

σ = 2.2

±4.096V RANGE

σ = 1.6

CODE

131052

131056

131060

131064

131068

131072

20000

40000

60000

80000

COUNTS

234418 G08

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TYPICAL PERFORMANCE CHARACTERISTICS

32k Point FFT fSMPL = 400kHz,

fIN=2kHz

SNR, SINAD vs VREFBUF,

fIN=2kHz

THD, Harmonics vs VREFBUF,

fIN=2kHz

SNR, SINAD

vs Input Frequency

THD, Harmonics

vs Input Frequency

THD, Harmonics vs Input

Common Mode, fIN = 2kHz

32k Point Arbitrary Tw o -Tone FFT

fSMPL=400kHz, IN+=–7dBFS 2kHz

Sine, IN–=–7dBFS 3.1kHz Sine

SNR, SINAD vs Input Level,

fIN=2kHz

CMRR vs Input Frequency and

Channel

TA = 25°C, VDD=5V, OVDD=2.5V, Internal

Reference and Buffer (VREFBUF =4.096V), fSMPL = 400ksps, unless otherwise noted.

SFDR = 119dB

SNR = 95.3dB

±4.096V RANGE

ARBITRARY DRIVE

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 G10

0V TO 4.096V RANGE

UNIPOLAR DRIVE (IN

–

= 0V)

SNR = 89.4dB

THD = –109dB

SINAD = 89.4dB

SFDR = 111dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 G11

SNR

SINAD

±V

REFBUF

RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

REFBUF VOLTAGE (V)

2.5

3.5

4.5

100

SNR, SINAD (dBFS)

234418 G12

THD

2ND

3RD

±V

REFBUF

RANGE

FULLY DIFFERENIAL DRIVE (IN

–

= –IN

)

REFBUF VOLTAGE (V)

2.5

3.5

4.5

–140

–135

–130

–125

–120

–115

–110

–105

–100

THD, HARMONICS (dBFS)

234418 G13

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

SNR

SINAD

FREQUENCY (Hz)

100

10k

100k

102

106

SNR, SINAD (dBFS)

234418 G14

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

THD

2ND

3RD

FREQUENCY (Hz)

100

10k

100k

–130

–120

–110

–100

–90

–80

–70

THD, HARMONICS (dBFS)

234418 G15

2ND

THD

3RD

±4.096V RANGE

P–P

FULLY DIFFERENTIAL DRIVE

INPUT COMMON MODE (V)

–150

–140

–130

–120

–110

–100

THD, HARMONICS (dBFS)

234418 G16

SNR

SINAD

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

INPUT LEVEL (dBFS)

–40

–30

–20

–10

95.0

95.2

95.4

95.6

95.8

96.0

SNR, SINAD (dBFS)

234418 G17

±4.096V RANGE

= IN

–

= 3.6V

P–P

SINE

ALL CHANNELS

FREQUENCY (Hz)

100

10k

100k

100

110

120

130

CMRR (dB)

234418 G18

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TYPICAL PERFORMANCE CHARACTERISTICS

SNR, SINAD vs Temperature,

fIN=2kHz

THD, Harmonics vs Temperature,

fIN = 2kHz

INL, DNL vs Temperature

Positive Full-Scale Error vs

Temperature and Channel

Negative Full-Scale Error vs

Temperature and Channel

Zero-Scale Error vs

Temperature and Channel

Crosstalk vs Input Frequency and

Channel

TA = 25°C, VDD=5V, OVDD=2.5V, Internal

Reference and Buffer (VREFBUF =4.096V), fSMPL = 400ksps, unless otherwise noted.

Supply Current vs Temperature

Power-Down Current

vs Temperature

CH1

±4.096V RANGE

IN0

= –IN0

–

= 3.6V

P–P

SINE

ALL CHANNELS CONVERTING

CH3

FREQUENCY (Hz)

100

10k

100k

–135

–130

–125

–120

–115

–110

–105

–100

–95

–90

–85

–80

CROSSTALK (dB)

234418 G19

SNR

SINAD

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

TEMPERATURE (°C)

–55

–35

–15

105

125

92.0

92.5

93.0

93.5

94.0

94.5

95.0

95.5

96.0

96.5

97.0

SNR, SINAD (dBFS)

234418 G20

THD

2ND

3RD

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

TEMPERATURE (°C)

–55

–35

–15

105

125

–125

–120

–115

–110

–105

–100

–95

THD, HARMONICS (dBFS)

234418 G21

MAX INL

MIN INL

MAX DNL

MIN DNL

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

TEMPERATURE (°C)

–55

–35

–15

105

125

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL, DNL ERROR (LSB)

234418 G22

±4.096V RANGE

REFBUF OVERDRIVEN

REFBUF

= 4.096V

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–0.100

–0.075

–0.050

–0.025

0.000

0.025

0.050

0.075

0.100

FULL-SCALE ERROR (%)

234418 G23

±4.096V RANGE

REFBUF OVERDRIVEN

REFBUF

= 4.096V

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–0.100

–0.075

–0.050

–0.025

0.000

0.025

0.050

0.075

0.100

FULL-SCALE ERROR (%)

234418 G24

±4.096V RANGE

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–10

–8

–6

–4

–2

ZERO-SCALE ERROR (LSB)

234418 G25

OVDD

VDD

TEMPERATURE (°C)

–55

–35

–15

105

125

–4

–2

SUPPLY CURRENT (mA)

234418 G26

OVDD

VDD

TEMPERATURE (°C)

–55

–35

–15

105

125

0.01

0.1

100

1000

POWER-DOWN CURRENT (µA)

234418 G27

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TYPICAL PERFORMANCE CHARACTERISTICS

Internal Reference Output

vs Temperature PSRR vs Frequency

Supply Current vs Sampling Rate

Power Dissipation vs Sampling

Rate, N Channels Enabled

Step Response

(Large-Signal Settling) Step Response (Fine Settling)

Offset Error vs Input Common

Mode and Channel

TA = 25°C, VDD=5V, OVDD=2.5V, Internal

Reference and Buffer (VREFBUF =4.096V), fSMPL = 400ksps, unless otherwise noted.

= IN

–

= 0V

FREQUENCY (Hz)

100

10k

100k

100

110

120

130

140

150

PSRR (dB)

234418 G30

±4.096V RANGE

ALL CHANNELS

INPUT COMMON MODE (V)

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

OFFSET ERROR (LSB)

234418 G28

15 UNITS

TEMPERATURE (°C)

–55

–35

–15

105

125

2.044

2.045

2.046

2.047

2.048

2.049

2.050

2.051

2.052

INTERNAL REFERENCE OUTPUT (V)

234418 G29

OVDD

VDD

SAMPLING FREQUENCY (kHz)

100

200

300

400

SUPPLY CURRENT (mA)

234418 G31

N = 4

N = 1

N = 2

N = 3

SAMPLING FREQUENCY (kHz)

200

400

600

800

1000

POWER DISSIPATION (mW)

234418 G32

±2.048V RANGE

= 400.1767kHz SQUARE WAVE

–

= 2.048V

DRIVEN BY 50Ω SOURCE

SETTLING TIME (ns)

–50

100

150

200

250

300

350

400

450

–131072

–98304

–65536

–32768

32768

65536

98304

131072

OUTPUT CODE (LSB)

234418 G33

±2.048V RANGE

= 400.1767kHz

SQUARE WAVE

–

= 2.048V

DRIVEN BY 50Ω SOURCE

SETTLING TIME (ns)

–50

100

150

200

250

300

350

400

450

–500

–400

–300

–200

–100

100

200

300

400

500

DEVIATION FROM FINAL VALUE (LSB)

234418 G34

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

PIN FUNCTIONS

Pins that are the Same for All Digital I/O Modes

IN0+/IN0– to IN3+/IN3– (Pins 8/7, 6/5, 4/3, and 2/1):

Positive and Negative Analog Inputs, Channels 0 to 3. The

converter simultaneously samples and digitizes (VIN+ –

VIN–) for all channels. Wide input common mode range

(0V ≤ VCM ≤ VDD) and high common mode rejection allow

the inputs to accept a wide variety of signal swings. Full-

scale input range is determined by the channel’s SoftSpan

configuration.

GND (Pins 9, 11, 20, 29, 31, 32, 33): Ground. Solder all

GND pins to a solid ground plane.

REFIN (Pin 10): Bandgap Reference Output/Reference

Buffer Input. An internal bandgap reference nominally

outputs 2.048V on this pin. An internal reference buffer

amplifies VREFIN to create the converter master reference

voltage VREFBUF = 2•VREFIN on the REFBUF pin. When

using the internal reference, bypass REFIN to GND (Pin

11) close to the pin with a 0.1μF ceramic capacitor to filter

the bandgap output noise. If more accuracy is desired,

overdrive REFIN with an external reference in the range

of 1.25V to 2.2V.

REFBUF (Pin 12): Internal Reference Buffer Output. An

internal reference buffer amplifies VREFIN to create the

converter master reference voltage VREFBUF = 2•VREFIN on

this pin, nominally 4.096V when using the internal bandgap

reference. Bypass REFBUF to GND (Pin 11) close to the

pin with a 47μF ceramic capacitor. The internal reference

buffer may be disabled by grounding its input at REFIN.

With the buffer disabled, overdrive REFBUF with an ex-

ternal reference voltage in the range of 2.5V to 5V. When

using the internal reference buffer, limit the loading of any

external circuitry connected to REFBUF to less than 10µA.

Using a high input impedance amplifier to buffer VREFBUF

to any external circuits is recommended.

PD (Pin 13): Power Down Input. When this pin is brought

high, the LTC2344-18 is powered down and subsequent

conversion requests are ignored. If this occurs during a

conversion, the device powers down once the conversion

completes. If this pin is brought high twice without an

intervening conversion, an internal global reset is initi-

ated, equivalent to a power-on-reset event. Logic levels

are determined by OVDD.

LVDS/CMOS (Pin 14): I/O Mode Select. Tie this pin to OVDD

to select LVDS I/O mode, or to ground to select CMOS I/O

mode. Logic levels are determined by OVDD.

CNV (Pin 15): Conversion Start Input. A rising edge on

this pin puts the internal sample-and-holds into the hold

mode and initiates a new conversion. CNV is not gated

by CS, allowing conversions to be initiated independent

of the state of the serial I/O bus.

BUSY (Pin 26): Busy Output. The BUSY signal indicates

that a conversion is in progress. This pin transitions low-

to-high at the start of each conversion and stays high until

the conversion is complete. Logic levels are determined

by OVDD.

VDDLBYP (Pin 28): Internal 2.5V Regulator Bypass Pin. The

voltage on this pin is generated via an internal regulator

operating off of VDD. This pin must be bypassed to GND

close to the pin with a 2.2μF ceramic capacitor. Do not

connect this pin to any external circuitry.

VDD (Pin 30): 5V Power Supply. The range of VDD is 4.75V

to 5.25V. This pin must be bypassed to GND close to the

pin with a 0.1μF ceramic capacitor.

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

PIN FUNCTIONS

CMOS I/O Mode

SDI+ (Pin 16): LVDS Positive Serial Data Input. In CMOS

I/O mode, this pin is Hi-Z.

SDO0 to SDO3 (Pins 17, 18, 23, 24): CMOS Serial Data

Outputs, Channels 0 to 3. The most recent conversion

result along with channel configuration information is

clocked out onto the SDO pins on each rising edge of SCKI.

Output data formatting is described in the Digital Interface

section. Leave unused SDO outputs unconnected. Logic

levels are determined by OVDD.

SCKI (Pin 19): CMOS Serial Clock Input. Drive SCKI with

the serial I/O clock. SCKI rising edges latch serial data in

on SDI and clock serial data out on SDO0 to SDO3. For

standard SPI bus operation, capture output data at the

receiver on rising edges of SCKI. SCKI is allowed to idle

either high or low. Logic levels are determined by OVDD.

OVDD (Pin 21): I/O Interface Power Supply. In CMOS I/O

mode, the range of OVDD is 1.71V to 5.25V. Bypass OVDD

to GND (Pin 20) close to the pin with a 0.1μF ceramic

capacitor.

SCKO (Pin 22): CMOS Serial Clock Output. SCKI rising

edges trigger transitions on SCKO that are skew-matched

to the serial output data streams on SDO0 to SDO3. The

resulting SCKO frequency is half that of SCKI. Rising and

falling edges of SCKO may be used to capture SDO data at

the receiver (FPGA) in double data rate (DDR) fashion. For

standard SPI bus operation, SCKO is not used and should

be left unconnected. SCKO is forced low at the falling edge

of BUSY. Logic levels are determined by OVDD.

SDI (Pin 25): CMOS Serial Data Input. Drive this pin with the

desired 12-bit SoftSpan configuration word (see Table1a),

latched on the rising edges of SCKI. If all channels will be

configured to operate only in SoftSpan 7, tie SDI to OVDD.

Logic levels are determined by OVDD.

CS (Pin 27): Chip Select Input. The serial data I/O bus is

enabled when CS is low and is disabled and Hi-Z when

CS is high. CS also gates the external shift clock, SCKI.

Logic levels are determined by OVDD.

LVDS I/O Mode

SDI+/SDI– (Pins 16, 17): LVDS Positive and Negative Serial

Data Input. Differentially drive SDI+/SDI– with the desired

12-bit SoftSpan configuration word (see Table 1a), latched

on both the rising and falling edges of SCKI+/SCKI–. The

SDI+/SDI– input pair is internally terminated with a 100Ω

differential resistor when CS = 0.

SCKI+/SCKI– (Pins 18, 19): LVDS Positive and Negative

Serial Clock Input. Differentially drive SCKI+/SCKI– with

the serial I/O clock. SCKI+/SCKI– rising and falling edges

latch serial data in on SDI+/SDI– and clock serial data out

on SDO+/SDO–. Idle SCKI+/SCKI– low, including when

transitioning CS. The SCKI+/SCKI– input pair is internally

terminated with a 100Ω differential resistor when CS = 0.

OVDD (Pin 21): I/O Interface Power Supply. In LVDS I/O

mode, the range of OVDD is 2.375V to 5.25V. Bypass OVDD

to GND (Pin 20) close to the pin with a 0.1μF ceramic

capacitor.

SCKO+/SCKO– (Pins 22, 23): LVDS Positive and Negative

Serial Clock Output. SCKO+/SCKO– outputs a copy of the

input serial I/O clock received on SCKI+/SCKI–, skew-

matched with the serial output data stream on SDO+/SDO–.

Use the rising and falling edges of SCKO+/SCKO– to cap-

ture SDO+/SDO– data at the receiver (FPGA). The SCKO+/

SCKO– output pair must be differentially terminated with

a 100Ω resistor at the receiver (FPGA).

SDO+/SDO– (Pins 24, 25): LVDS Positive and Negative

Serial Data Output. The most recent conversion result

along with channel configuration information is clocked

out onto SDO+/SDO– on both rising and falling edges of

SCKI+/SCKI–, beginning with channel 0. The SDO+/SDO–

output pair must be differentially terminated with a 100Ω

resistor at the receiver (FPGA).

CS (Pin 27): Chip Select Input. The serial data I/O bus is

enabled when CS is low, and is disabled and Hi-Z when

CS is high. CS also gates the external shift clock, SCKI+/

SCKI–. The internal 100Ω differential termination resistors

on the SCKI+/SCKI– and SDI+/SDI– input pairs are disabled

when CS is high. Logic levels are determined by OVDD.

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

CONFIGURATION TABLES

Table 1a. SoftSpan Configuration Table. Use This Table with Table 1b to Choose Independent Binary SoftSpan Codes SS[2:0] for Each

Channel Based on Desired Analog Input Range. Combine SoftSpan Codes to Form 12-Bit SoftSpan Configuration Word S[11:0]. Use

Serial Interface to Write SoftSpan Configuration Word to LTC2344-18, as shown in Figure 19

BINARY SoftSpan CODE

SS[2:0] ANALOG INPUT RANGE FULL SCALE RANGE BINARY FORMAT OF

CONVERSION RESULT

111 ±VREFBUF 2 • VREFBUF Tw o ’s Complement

110 ±VREFBUF/1.024 2 • VREFBUF/1.024 Tw o ’s Complement

101 0V to VREFBUF VREFBUF Straight Binary

100 0V to VREFBUF/1.024 VREFBUF/1.024 Straight Binary

011 ±0.5 • VREFBUF VREFBUF Tw o ’s Complement

010 ±0.5 • VREFBUF/1.024 VREFBUF/1.024 Tw o ’s Complement

001 0V to 0.5 • VREFBUF 0.5 • VREFBUF Straight Binary

000 Channel Disabled Channel Disabled All Zeros

Table 1b. Reference Configuration Table. The LTC2344-18 Supports Three Reference Configurations. Analog Input Range Scales with

the Converter Master Reference Voltage, VREFBUF

REFERENCE CONFIGURATION VREFIN VREFBUF BINARY SoftSpan CODE

SS[2:0] ANALOG INPUT RANGE

Internal Reference with

Internal Buffer 2.048V 4.096V

111 ±4.096V

110 ±4V

101 0V to 4.096V

100 0V to 4V

011 ±2.048V

010 ±2V

001 0V to 2.048V

External Reference with

Internal Buffer

(REFIN Pin Externally

Overdriven)

1.25V

(Min Value) 2.5V

111 ±2.5V

110 ±2.441V

101 0V to 2.5V

100 0V to 2.441V

011 ±1.25V

010 ±1.221V

001 0V to 1.25V

2.2V

(Max Value) 4.4V

111 ±4.4V

110 ±4.297V

101 0V to 4.4V

100 0V to 4.297V

011 ±2.2V

010 ±2.148V

001 0V to 2.2V

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

REFERENCE CONFIGURATION VREFIN VREFBUF BINARY SoftSpan CODE

SS[2:0] ANALOG INPUT RANGE

External Reference

Unbuffered

(REFBUF Pin

Externally Overdriven,

REFIN Pin Grounded)

0V 2.5V

(Min Value)

111 ±2.5V

110 ±2.441V

101 0V to 2.5V

100 0V to 2.441V

011 ±1.25V

010 ±1.221V

001 0V to 1.25V

0V 5V

(Max Value)

111 ±5V

110 ±4.883V

101 0V to 5V

100 0V to 4.883V

011 ±2.5V

010 ±2.441V

001 0V to 2.5V

CONFIGURATION TABLES

Table 1b. Reference Configuration Table (Continued). The LTC2344-18 Supports Three Reference Configurations. Analog Input Range

Scales with the Converter Master Reference Voltage, VREFBUF

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

FUNCTIONAL BLOCK DIAGRAM

CMOS I/O Mode

SDO0

SDO3

SCKO

SDI

SCKI

BUSY

18-BIT

SAR ADC

CMOS

SERIAL

I/O

INTERFACE

234418 BD01

18 BITS

REFERENCE

BUFFER

REFBUFREFINGND

VDDLBYP

VDD OVDD

LTC2344-18

CONTROL

LOGIC

2.048V

REFERENCE

2.5V

REGULATOR

LVDS/CMOSPDCNV

IN0+

IN0–S/H

IN1+

IN1–S/H

IN2+

IN2–S/H

IN3+

IN3–S/H

4-CHANNEL MULTIPLEXER

20k 2×

• • •

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

LVDS I/O Mode

FUNCTIONAL BLOCK DIAGRAM

SDO+

SDO–

SCKO+

SCKO–

SDI+

SDI–

SCKI+

SCKI–

BUSY

18-BIT

SAR ADC

LVDS

SERIAL

I/O

INTERFACE

234418 BD02

18 BITS

REFERENCE

BUFFER

REFBUFREFINGND

VDDLBYP

VDD OVDD

LTC2344-18

CONTROL

LOGIC

2.048V

REFERENCE

2.5V

REGULATOR

LVDS/CMOSPDCNV

IN0+

IN0–S/H

IN1+

IN1–S/H

IN2+

IN2–S/H

IN3+

IN3–S/H

4-CHANNEL MULTIPLEXER

20k 2×

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

TIMING DIAGRAM

LVDS I/O Mode

CMOS I/O Mode

1 2 3 4 5 6 7 8 9 10 11 12

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 0 C1 C0

0 C1 C0

SS1SS2 SS0 D17

CNV

CS = PD = 0

CONVERT

DON’T CARE

ACQUIREBUSY

SDO3

SCKO

SDO0

SCKI

SDI

DON’T CARE

SAMPLE N SAMPLE N + 1

SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1

CHANNEL 0

CONVERSION N

CHANNEL 1

CONVERSION N

CHANNEL 3

CONVERSION N

CHANNEL 0

CONVERSION N

234418 TD01

CONVERSION RESULT CHAN ID SoftSpan CONVERSION RESULT

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17DON’T CARE

CONVERSION RESULT CHAN ID SoftSpan CONVERSION RESULT

• • •

13 14 15 16 17 18 19 20 21 22 23 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 2624

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17 D16 D15

CNV

(CMOS)

CS = PD = 0

234418 TD02

CONVERT

DON’T CARE

ACQUIRE

BUSY

(CMOS)

SCKO

(LVDS)

SDO

(LVDS)

SCKI

(LVDS)

SDI

(LVDS)

DON’T CARE

SAMPLE N

SAMPLE

N + 1

SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1

CHANNEL 0

CONVERSION N

CHANNEL 1

CONVERSION N

CHANNEL 3

CONVERSION N

CONVERSION RESULT CHAN ID SoftSpan

90 91 92 93 94 95 96

D0 SS1SS2 SS0 D17

CHANNEL 0

CONVERSION N

CONVERSION

RESULT

CHAN ID SoftSpan

0 C1 C0 0 C1 C0

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

APPLICATIONS INFORMATION

OVERVIEW

The LTC2344-18 is an 18-bit, low noise 4-channel si-

multaneous sampling successive approximation register

(SAR) ADC with differential, wide common mode range

inputs. Using the integrated low-drift reference and buffer

(VREFBUF=4.096V nominal), each channel of this SoftSpan

ADC can be independently configured on a conversion-

by-conversion basis to accept ±4.096V, 0V to 4.096V,

±2.048V, or 0V to 2.048V signals. The input signal range

may be expanded up to ±5V using an external 5V refer-

ence. Individual channels may also be disabled to increase

throughput on the remaining channels.

The wide input common mode range and high CMRR

(102dB typical, VIN+ = VIN– = 3.6VP-P 200Hz Sine) of the

LTC2344-18 analog inputs allow the ADC to directly digi-

tize a variety of signals, simplifying signal chain design.

This input signal flexibility, combined with ±4LSB INL,

no missing codes at 18-bits, and 95dB SNR, makes the

LTC2344-18 an ideal choice for many applications requir-

ing wide dynamic range.

The LTC2344-18 supports pin-selectable SPI CMOS (1.8V

to 5V) and LVDS serial interfaces, enabling it to com-

municate equally well with legacy microcontrollers and

modern FPGAs. In CMOS mode, applications may employ

between one and four lanes of serial output data, allowing

the user to optimize bus width and data throughput. The

LTC2344-18 typically dissipates 81mW when converting

four analog input channels simultaneously at 400ksps

per channel throughput. An optional power-down mode

may be employed to further reduce power consumption

during inactive periods.

CONVERTER OPERATION

The LTC2344-18 operates in two phases. During the ac-

quisition phase, the sampling capacitors in each channel’s

sample-and-hold (S/H) circuit connect to their respective

analog input pins and track the differential analog input

voltage (VIN+ – VIN–). A rising edge on the CNV pin transi-

tions all channels’ S/H circuits from track mode to hold

mode, simultaneously sampling the input signals on all

channels and initiating a conversion. During the conversion

phase, each channel’s sampling capacitors are connected,

one channel at a time, to an 18-bit charge redistribution

capacitor D/A converter (CDAC). The CDAC is sequenced

through a successive approximation algorithm, effectively

comparing the sampled input voltage with binary-weighted

fractions of the channel’s SoftSpan full-scale range (e.g.,

VFSR/2, VFSR/4 … VFSR/262144) using a differential

comparator. At the end of this process, the CDAC output

approximates the channel’s sampled analog input. Once

all channels have been converted in this manner, the ADC

control logic prepares the 18-bit digital output codes from

each channel for serial transfer.

TRANSFER FUNCTION

The LTC2344-18 digitizes each channel’s full-scale voltage

range into 218 levels. In conjunction with the ADC master

reference voltage, VREFBUF, a channel’s SoftSpan configu-

ration determines its input voltage range, full-scale range,

LSB size, and the binary format of its conversion result, as

shown in Tables 1a and 1b. For example, employing the

internal reference and buffer (VREFBUF = 4.096V nominal),

SoftSpan 7 configures a channel to accept a ±4.096V bi-

polar analog input voltage range, which corresponds to a

8.192V full-scale range with a 31.25μV LSB. Other SoftSpan

configurations and reference voltages may be employed to

convert both larger and smaller bipolar and unipolar input

ranges. Conversion results are output in two’s comple-

ment binary format for all bipolar SoftSpan ranges, and

in straight binary format for all unipolar SoftSpan ranges.

The ideal two’s complement transfer function is shown in

Figure 2, while the ideal straight binary transfer function

is shown in Figure 3.

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

INPUT VOLTAGE (V)

OUTPUT CODE (TWO’S COMPLEMENT)

–1

LSB

234418 F02

011...111

011...110

000...001

000...000

100...000

100...001

111...110

LSB

BIPOLAR

ZERO

111...111

FSR/2 – 1LSB–FSR/2

FSR = +FS – –FS

1LSB = FSR/262144

Figure 2. LTC2344-18 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

OUTPUT CODE (STRAIGHT BINARY)

234418 F03

111...111

111...110

100...001

100...000

000...000

000...001

011...110

UNIPOLAR

ZERO

011...111

FSR – 1LSB0V

FSR = +FS

1LSB = FSR/262144

Figure 3. LTC2344-18 Straight Binary Transfer Function

ANALOG INPUTS

Each channel of the LTC2344-18 simultaneously samples

the voltage difference (VIN+ – VIN–) between its analog

input pins over a wide common mode input range while

attenuating unwanted signals common to both input

pins by the common-mode rejection ratio (CMRR) of

the ADC. Wide common mode input range coupled with

APPLICATIONS INFORMATION

high CMRR allows the IN+/IN– analog inputs to swing

with an arbitrary relationship to each other, provided

each pin remains between ground and VDD. This feature

of the LTC2344-18 enables it to accept a wide variety of

signal swings, including traditional classes of analog input

signals such as pseudo-differential unipolar, pseudo-

differential bipolar, and fully differential, simplifying

signal chain design.

In all SoftSpan ranges, each channel’s analog inputs can

be modeled by the equivalent circuit shown in Figure 4.

At the start of acquisition, the 80pF sampling capacitors

(CIN) connect to the analog input pins IN+/IN– through the

sampling switches, each of which has approximately 90Ω

(RIN) of on-resistance. The initial voltage on both sampling

capacitors at the start of acquisition is approximately equal

to the sampled common-mode voltage (VIN++VIN–)/2

from the prior conversion. The external circuitry connected

to IN+ and IN– must source or sink the charge that flows

through RIN as the sampling capacitors settle from their

initial voltages to the new input pin voltages over the course

of the acquisition interval. During conversion and power

down modes, the analog inputs draw only a small leakage

current. The diodes at the inputs provide ESD protection.

Figure 4. Equivalent Circuit for Differential Analog Inputs,

Single Channel Shown

RIN

90Ω

RIN

90Ω

CIN

80pF

CIN

80pF

VDD

BIAS

VOLTAGE

–234418 F04

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

APPLICATIONS INFORMATION

Bipolar SoftSpan Input Ranges

For channels configured in SoftSpan ranges 7, 6, 3, or

2, the LTC2344-18 digitizes the differential analog input

voltage (VIN+ – VIN–) over a bipolar span of ±VREFBUF,

±VREFBUF/1.024, ±0.5 • VREFBUF, or ±0.5 • VREFBUF/1.024,

respectively, as shown in Table 1a. These SoftSpan ranges

are useful for digitizing input signals where IN+ and IN–

swing above and below each other. Traditional examples

include fully differential input signals, where IN+ and

IN– are driven 180 degrees out-of-phase with respect

to each other centered around a common mode voltage

(VIN+ + VIN–)/2, and pseudo-differential bipolar input

signals, where IN+ swings above and below a reference

level, driven on IN–. Regardless of the chosen SoftSpan

range, the wide common mode input range and high CMRR

of the IN+/IN– analog inputs allow them to swing with an

arbitrary relationship to each other, provided each pin

remains between ground and VDD. The output data format

for all bipolar SoftSpan ranges is two’s complement.

Unipolar SoftSpan Input Ranges

For channels configured in SoftSpan ranges 5, 4, or 1, the

LTC2344-18 digitizes the differential analog input voltage

(VIN+ – VIN–) over a unipolar span of 0V to VREFBUF, 0V

to VREFBUF/1.024, or 0V to 0.5 • VREFBUF, respectively, as

shown in Table 1a. These SoftSpan ranges are useful for

digitizing input signals where IN+ remains above IN–. A

traditional example includes pseudo-differential unipolar

input signals, where IN+ swings above a ground reference

level, driven on IN–. Regardless of the chosen SoftSpan

range, the wide common mode range and high CMRR of

the IN+/IN– analog inputs allow them to swing with an

arbitrary relationship to each other, provided each pin

remains between ground and VDD. The output data format

for all unipolar SoftSpan ranges is straight binary.

INPUT DRIVE CIRCUITS

The initial voltage on each channel’s sampling capacitors

at the start of acquisition must settle to the new input

pin voltages during the acquisition interval. The external

circuitry connected to IN+ and IN– must source or sink

the charge that flows through RIN as this settling occurs.

The LTC2344-18 sampling network RC time constant of

7.2ns implies an 18-bit settling time to a full-scale step of

approximately 13•(RIN•CIN)=94ns. The impedance and

self-settling of external circuitry connected to the analog

input pins will increase the overall settling time required.

Low impedance sources can directly drive the inputs of

the LTC2344-18 without gain error, but high impedance

sources should be buffered to ensure sufficient settling

during acquisition and to optimize the linearity and distor-

tion performance of the ADC. Settling time is an important

consideration even for DC input signals, as the voltages on

the sampling capacitors will differ from the analog input

pin voltages at the start of acquisition.

Most applications should use a buffer amplifier to drive the

analog inputs of the LTC2344-18. The amplifier provides

low output impedance, enabling fast settling of the analog

signal during the acquisition phase. It also provides isola-

tion between the signal source and the charge flow at the

analog inputs when entering acquisition.

Input Filtering

The noise and distortion of an input buffer amplifier and

other supporting circuitry must be considered since they

add to the ADC noise and distortion. Noisy input signals

should be filtered prior to the buffer amplifier with a low-

bandwidth filter to minimize noise. The simple one-pole

RC lowpass filter shown in Figure 5 is sufficient for many

applications.

At the output of the buffer, a lowpass RC filter network

formed by the 90Ω sampling switch on-resistance (RIN)

and the 80pF sampling capacitance (CIN) limits the input

bandwidth on each channel to 22MHz, which is fast enough

to allow for sufficient transient settling during acquisition

while simultaneously filtering driver wideband noise. A

buffer amplifier with low noise density should be selected

to minimize SNR degradation over this bandwidth. An

additional filter network may be placed between the buf-

fer output and ADC input to further minimize the noise

contribution of the buffer and reduce disturbances to the

buffer from ADC acquisition transients. A simple one-pole

lowpass RC filter is sufficient for many applications. It is

important that the RC time constant of this filter be small

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

enough to allow the analog inputs to completely settle to

18-bit resolution within the ADC acquisition time (tACQ),

as insufficient settling can limit INL and THD performance.

Also note that the minimum acquisition time varies with

sampling frequency (fSMPL) and the number of enabled

channels.

High quality capacitors and resistors should be used in

the RC filters since these components can add distortion.

NPO/COG and silver mica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can

generate distortion from self-heating and from damage

that may occur during soldering. Metal film surface mount

resistors are much less susceptible to both problems.

Buffering Arbitrary and Fully Differential Analog Input

Signals

The wide common mode input range and high CMRR of

the LTC2344-18 allow each channel’s IN+ and IN– pins

to swing with an arbitrary relationship to each other,

provided each pin remains between ground and VDD. This

unique feature of the LTC2344-18 enables it to accept a

wide variety of signal swings, simplifying signal chain

APPLICATIONS INFORMATION

design. In many applications, connecting a channel’s IN+

and IN– pins directly to the existing signal chain circuitry

will not allow the channel’s sampling network to settle to

18-bit resolution within the ADC acquisition time (tACQ). In

these cases, it is recommended that two unity-gain buffers

be inserted between the signal source and the ADC input

pins, as shown in Figure 6a. Table 2 lists several amplifier

and lowpass filter combinations recommended for use

in this circuit. The LT6237 combines fast settling, high

linearity, and low offset with 1.1nV/√Hz input-referred

noise density, enabling it to achieve the full ADC data sheet

SNR and THD specifications, as shown in the FFT plots in

Figures6b to 6e. In applications where slightly degraded

SNR performance is acceptable, it is possible to drive the

LTC2344-18 using the lower-power LT6234. The LT6234

combines fast settling, good linearity, and low offset with

1.9nV/√Hz input-referred noise density, enabling it to drive

the LTC2344-18 with 1.9dB SNR loss compared with the

LT6237 when a 24.9Ω, 1nF filter is employed. As shown in

Table 2, the LT6237 may be used without a lowpass filter

at a loss of ≤1dB SNR due to increased wideband noise.

Figure 5. Unipolar Signal Chain with Input Filtering

Table 2. Recommended Amplifier and Filter Combinations for the Buffer Circuits in Figures 6a and 9. AC Performance Measured

Using Circuit in Figure 6a, ±4.096V Range for Fully Differential Input Drive, ±2.048V Range for Bipolar Input Drive

AMPLIFIER RFILT

(Ω)

CFILT

(nF) INPUT SIGNAL DRIVE SNR

(dB)

THD

(dB)

SINAD

(dB)

SFDR

(dB)

½ LT6237 24.9 1 FULLY DIFFERENTIAL 95.2 −114 95.2 115

½ LT6234 24.9 1 FULLY DIFFERENTIAL 93.3 −114 93.3 115

½ LT6237 24.9 1 BIPOLAR 89.3 –110 89.3 111

½ LT6234 24.9 1 BIPOLAR 87.6 −110 87.6 111

½ LT6237 0 0 BIPOLAR 88.6 −110 88.6 111

½ LT6234 0 0 BIPOLAR 83.3 −109 83.3 111

–

LTC2344-18

234418 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

UNIPOLAR

INPUT SIGNAL

IN0+

IN0–

BUFFER

AMPLIFIER

LOWPASS

SIGNAL FILTER

10nF

160Ω

BW = 100kHz

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

APPLICATIONS INFORMATION

–2V

LTC2344-18

234418 F06a

ONLY CHANNEL 0 SHOWN FOR CLARITY

–

0.1µF

47µF

IN0+

IN0–

AMPLIFIER

AMPLIFIER REFINREFBUF

OPTIONAL

LOWPASS FILTERS

CFILT

RFILT

IN+

IN–

DIFFERENTIAL INPUTS

IN+/IN– WITH

WIDE INPUT COMMON MODE RANGE

FULLY

DIFFERENTIAL

BIPOLAR

ARBITRARY

UNIPOLAR

Figure 6a. Buffering Arbitrary, Fully Differential, Bipolar, and Unipolar Signals.

See Table 2 For Recommended Amplifier and Filter Combinations

Figure 6b. Two-Tone Test. IN+ = –7dBFS 2kHz Sine, IN– = –7dBFS

3.1kHz Sine, 32k Point FFT, fSMPL = 400ksps. Circuit Shown in

Figure 6a with LT6237 Amplifiers, RFILT = 24.9Ω, CFILT = 1nF

Arbitrary Drive Fully Differential Drive

Bipolar Drive Unipolar Drive

Figure 6c. IN+/IN– = –1dBFS 2kHz Fully Differential Sine, Common

Mode = 2.5V, 32k Point FFT, fSMPL = 400ksps. Circuit Shown in

Figure 6a with LT6237 Amplifiers, RFILT = 24.9Ω, CFILT = 1nF

Figure 6d. IN+ = –1dBFS 2kHz Bipolar Sine, IN– = 2.5V, 32k

Point FFT, fSMPL = 400ksps. Circuit Shown in Figure 6a with

LT6237 Amplifiers, RFILT = 24.9Ω, CFILT = 1nF

Figure 6e. IN+ = –1dBFS 2kHz Unipolar Sine, IN– = 0V, 32k Point

FFT, fSMPL = 400ksps. Circuit Shown in Figure 6a with LT6237

Amplifiers, RFILT = 24.9Ω, CFILT = 1nF

SFDR = 118dB

SNR = 95.3dB

±4.096V RANGE

ARBITRARY DRIVE

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 F06b

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

SNR = 95.3dB

THD = –111dB

SINAD = 95.2dB

SFDR = 113dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 F06c

±2.048V RANGE

BIPOLAR DRIVE (IN

–

= 2.5V)

SNR = 89.4dB

THD = –112dB

SINAD = 89.4dB

SFDR = 114dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 F06d

0V TO 4.096V RANGE

UNIPOLAR DRIVE (IN

–

= 0V)

SNR = 89.4dB

THD = –109dB

SINAD = 89.3dB

SFDR = 111dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 F06e

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

The two-tone test shown in Figure 6b demonstrates the

arbitrary input drive capability of the LTC2344-18. This test

simultaneously drives IN+ with a −7dBFS 2kHz single-ended

sine wave and IN− with a −7dBFS 3.1kHz single-ended sine

wave. Together, these signals sweep the analog inputs

across a wide range of common mode and differential

mode voltage combinations, similar to the more general

arbitrary input signal case. They also have a simple spec-

tral representation. An ideal differential converter with no

common-mode sensitivity will digitize this signal as two

−7dBFS spectral tones, one at each sine wave frequency.

The FFT plot in Figure 6b demonstrates the LTC2344-18

response approaches this ideal, with 118dB of SFDR

limited by the converter's second harmonic distortion

response to the 2kHz sine wave on IN+.

The ability of the LTC2344-18 to accept arbitrary signal

swings over a wide input common mode range with high

CMRR can simplify application solutions. Figure 7 depicts

one way of using the LTC2344-18 to digitize signals of

this type. Two channels of the LTC2344-18 simultaneously

sense the voltage on and bidirectional current through a

sense resistor over a wide common mode range. In many

applications of this type, the impedance of the external

circuitry is low enough that the ADC sampling network

can fully settle without buffering.

The common mode input range of the LTC2344-18 includes

VDD, allowing the circuit shown in Figure 8a to amplify and

measure a load current (ILOAD) from a single 5V supply.

Figure 8b shows a measured transient supply current

step of an LTC3207 LED driver load. Note the LTC6252

supplies limit the usable current sense range of this circuit

to 50mA to 450mA.

Figure 9a illustrates a more general method of amplifying

an input signal. The amplifier stage provides a differential

gain of approximately 10V/V to the desired sensor signal

while the unwanted common mode signal is attenuated

by the ADC CMRR. Figure 9b shows measured CMRR

performance of this solution, which is competitive with

the best commercially available instrumentation amplifiers.

APPLICATIONS INFORMATION

Figure 7. Simultaneously Sense Voltage (CH0) and Current

(CH1) Over a Wide Common Mode Range

Figure 8a. Sense 50mA to 450mA Current from Single 5V

Supply with Amplification

Figure 8b. Transient Supply Current Step Measured Using

Circuit in Figure 8a Loaded with LTC3207 LED Driver

LTC2344-18

234418 F07

ONLY CHANNELS 0 AND 1 SHOWN FOR CLARITY

0.1µF

47µF

0V ≤ VS1 ≤ 5V

0V ≤ V

≤ 5V

VS1 – VS2

SENSE

ISENSE =

IN0+

IN0–

IN1+

IN1–

REFINREFBUF

ISENSE

SENSE

VS2

VS1

0V TO 4.096V RANGE

TIME (µs)

100

120

140

160

180

200

LOAD

(mA)

234418 F08b

LTC2344-18

234418 F08a

ONLY CHANNEL 0 SHOWN FOR CLARITY

0.1µF

47µF

IN0+

IN0–

VDD

REFINREFBUF

LTC6252

LOAD

2.49k

274Ω

–

1Ω

LOAD

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

Buffering Single-Ended Analog Input Signals

While the circuit shown in Figure 6a is capable of buffering

single-ended input signals, the circuit shown in Figure 10

is preferable when the single-ended signal reference level

is inherently low impedance and doesn't require buffering.

This circuit eliminates one driver and lowpass filter, reduc-

ing part count, power dissipation, and SNR degradation

due to driver noise. Using the recommended driver and

filter combinations in Table 2, the performance of this

circuit with single-ended input signals is on par with the

performance of the circuit in Figure 6a.

ADC REFERENCE

As shown previously in Table 1b, the LTC2344-18 supports

three reference configurations. The first uses both the in-

ternal bandgap reference and reference buffer. The second

externally overdrives the internal reference but retains the

internal buffer, which isolates the external reference from

ADC conversion transients. This configuration is ideal

for sharing a single precision external reference across

multiple ADCs. The third disables the internal buffer and

overdrives the REFBUF pin externally.

Internal Reference with Internal Buffer

The LTC2344-18 has an on-chip, low noise, low drift

(20ppm/°C maximum), temperature compensated band-

gap reference that is factory trimmed to 2.048V. The

reference output connects through a 20kΩ resistor to

APPLICATIONS INFORMATION

Figure 9b. CMRR vs Input Frequency. Circuit Shown in Figure 9a

Figure 9a. Digitize Differential Signals with High CMRR

Figure 10. Buffering Single-Ended Input Signals. See Table 2 For Recommended

Amplifier and Filter Combinations

= IN

–

= 5V

SINE

±4.096V RANGE

FREQUENCY (Hz)

100

10k

100k

100

110

120

130

140

150

CMRR (dB)

234418 F09b

–2V

LTC2344-18

234418 F09a

ONLY CHANNEL 0 SHOWN FOR CLARITY

–

0.1µF

47µF

IN0+

IN0–

½ LT6237

REFINREFBUF

LOWPASS FILTERS

BW ~ 6.4MHz

1nF

24.9Ω

549Ω

–

2.49k

LTC2344-18

234418 F10

ONLY CHANNEL 0 SHOWN FOR CLARITY

–

0.1µF

47µF

IN0+

IN0–

AMPLIFIER

REFINREFBUF

OPTIONAL

LOWPASS FILTER

CFILT

RFILT

–2V

IN+

IN–

UNIPOLAR

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

the REFIN pin, which serves as the input to the on-chip

reference buffer, as shown in Figure 11a. When employing

the internal bandgap reference, the REFIN pin should be

bypassed to GND (Pin 11) close to the pin with a 0.1μF

ceramic capacitor to filter wideband noise. The reference

buffer amplifies VREFIN to create the converter master

reference voltage VREFBUF = 2•VREFIN on the REFBUF pin,

nominally 4.096V when using the internal bandgap refer-

ence. Bypass REFBUF to GND (Pin 11) close to the pin with

at least a 47μF ceramic capacitor (X7R, 10V, 1210 size or

X5R, 10V, 0805 size) to compensate the reference buffer,

absorb transient conversion currents, and minimize noise.

External Reference with Internal Buffer

If more accuracy and/or lower drift is desired, REFIN can

be easily overdriven by an external reference since 20kΩ

of resistance separates the internal bandgap reference

output from the REFIN pin, as shown in Figure 11b. The

valid range of external reference voltage overdrive on the

REFIN pin is 1.25V to 2.2V, resulting in converter mas-

ter reference voltages VREFBUF between 2.5V and 4.4V,

respectively. Linear Technology offers a portfolio of high

performance references designed to meet the needs of

many applications. With its small size, low power, and high

accuracy, the LTC6655-2.048 is well suited for use with the

LTC2344-18 when overdriving the internal reference. The

LTC6655-2.048 offers 0.025% (maximum) initial accuracy

and 2ppm/°C (maximum) temperature coefficient for high

precision applications. The LTC6655-2.048 is fully speci-

fied over the H-grade temperature range, complementing

the extended temperature range of the LTC2344-18 up to

125°C. Bypassing the LTC6655-2.048 with a 2.7µF to 100µF

ceramic capacitor close to the REFIN pin is recommended.

External Reference with Disabled Internal Buffer

The internal reference buffer supports VREFBUF = 4.4V

maximum. By grounding REFIN, the internal buffer may

be disabled allowing REFBUF to be overdriven with an

external reference voltage between 2.5V and 5V, as shown

APPLICATIONS INFORMATION

234418 F11a

47µF 6.5k

20k

LTC2344-18

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

0.1µF

234418 F11b

47µF 6.5k

20k

LTC2344-18

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

2.7µF

LTC6655-2.048

Figure 11a. Internal Reference with Internal Buffer Configuration

Figure 11b. External Reference with Internal Buffer Configuration

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

in Figure 11c. Maximum input signal swing and SNR are

achieved by overdriving REFBUF using an external 5V

reference. The buffer feedback resistors load the REFBUF

pin with 13kΩ even when the reference buffer is disabled.

The LTC6655-5 offers the same small size, accuracy, drift,

and extended temperature range as the LTC6655-2.048,

and achieves a typical SNR of 96.5dB when paired with

the LTC2344-18. Bypass the LTC6655-5 to GND (Pin 11)

close to the REFBUF pin with at least a 47μF ceramic ca-

pacitor (X7R, 10V, 1210 size or X5R, 10V, 0805 size) to

absorb transient conversion currents and minimize noise.

APPLICATIONS INFORMATION

234418 F11c

47µF 6.5k

20k

LTC2344-18

REFERENCE

BUFFER

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

Figure 11c. External Reference with Disabled Internal

Buffer Configuration

CNV

IDLE

PERIOD

IDLE

PERIOD

234418 F12

Figure 12. CNV Waveform Showing Burst Sampling

The LTC2344-18 converter draws a charge (QCONV) from

the REFBUF pin during each conversion cycle. On short

time scales most of this charge is supplied by the external

REFBUF bypass capacitor, but on longer time scales all of

the charge is supplied by either the reference buffer, or

when the internal reference buffer is disabled, the external

reference. This charge draw corresponds to a DC current

equivalent of IREFBUF = QCONV•fSMPL, which is proportional

to sample rate. In applications where a burst of samples

is taken after idling for long periods of time, as shown in

Figure 12, IREFBUF quickly transitions from approximately

0.4mA to 1.5mA (VREFBUF = 5V, fSMPL=400kHz). This

current step triggers a transient response in the external

reference that must be considered, since any deviation

in VREFBUF affects converter accuracy. If an external

reference is used to overdrive REFBUF, the fast settling

LTC6655 family of references is recommended.

Internal Reference Buffer Transient Response

For optimum performance in applications employing burst

sampling, the external reference with internal reference

buffer configuration should be used. The internal reference

buffer incorporates a proprietary design that minimizes

movements in VREFBUF when responding to a burst of

LTC2344-18

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conversions following an idle period. Figure 13 compares

the burst conversion response of the LTC2344-18 with an

input near full scale for two reference configurations. The

first configuration employs the internal reference buffer

with REFIN externally overdriven by an LTC6655-2.048,

while the second configuration disables the internal ref-

erence buffer and overdrives REFBUF with an external

LTC6655-4.096. In both cases REFBUF is bypassed to

GND with a 47µF ceramic capacitor.

DYNAMIC PERFORMANCE

Fast Fourier transform (FFT) techniques are used to test

the ADC’s frequency response, distortion, and noise at the

rated throughput. By applying a low distortion sine wave

and analyzing the digital output using an FFT algorithm,

the ADC’s spectral content can be examined for frequen-

cies outside the fundamental. The LTC2344-18 provides

guaranteed tested limits for both AC distortion and noise

measurements.

Signal-to-Noise and Distortion Ratio (SINAD)

The signal-to-noise and distortion ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency and the RMS amplitude of all other frequency

components at the A/D output. The output is band-limited

APPLICATIONS INFORMATION

Figure 13. Burst Conversion Response of the LTC2344-18,

fSMPL=400ksps

to frequencies below half the sampling frequency, exclud-

ing DC. Figure 14 shows that the LTC2344-18 achieves a

typical SINAD of 95.2dB in the ±4.096V range at a 400kHz

sampling rate with a fully differential 2kHz input signal.

Figure 14. 32k Point FFT fSMPL = 400ksps, fIN = 2kHz

Signal-to-Noise Ratio (SNR)

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency and

the RMS amplitude of all other frequency components

except the first five harmonics and DC. Figure 14 shows

that the LTC2344-18 achieves a typical SNR of 95.3dB in

the ±4.096V range at a 400kHz sampling rate with a fully

differential 2kHz input signal.

Total Harmonic Distortion (THD)

Total harmonic distortion (THD) is the ratio of the RMS sum

of all harmonics of the input signal to the fundamental itself.

The out-of-band harmonics alias into the frequency band

between DC and half the sampling frequency (fSMPL/2).

THD is expressed as:

THD =20log V2

2+V3

2+V4

2...VN

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the second

through Nth harmonics, respectively. Figure 14 shows

INTERNAL REFERENCE BUFFER

EXTERNAL REFERENCE ON REFBUF

±4.096V SOFTSPAN

= 4V

–

= 0V

TIME (µs)

100

200

300

400

500

–5

DEVIATION FROM FINAL VALUE (LSB)

234418 F13

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

SNR = 95.3dB

THD = –112dB

SINAD = 95.2dB

SFDR = 114dB

FREQUENCY (kHz)

120

160

200

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

234418 F14

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

that the LTC2344-18 achieves a typical THD of –112dB

(N=6) in the ±4.096V range at a 400kHz sampling rate

with a fully differential 2kHz input signal.

POWER CONSIDERATIONS

The LTC2344-18 provides two power supply pins: the 5V

core power supply (VDD) and the digital input/output (I/O)

interface power supply (OVDD). The flexible OVDD supply

allows the LTC2344-18 to communicate with CMOS logic

operating between 1.8V and 5V, including 2.5V and 3.3V

systems. When using LVDS I/O mode, the range of OVDD

is 2.375V to 5.25V.

Power Supply Sequencing

The LTC2344-18 does not have any specific power supply

sequencing requirements. Care should be taken to adhere

to the maximum voltage relationships described in the

Absolute Maximum Ratings section. The LTC2344-18 has

an internal power-on-reset (POR) circuit which resets the

converter on initial power-up and whenever VDD drops

below 2V. Once the supply voltage re-enters the nominal

supply voltage range, the POR reinitializes the ADC. No

conversions should be initiated until at least 10ms after

a POR event to ensure the initialization period has ended.

When employing the internal reference buffer, allow 200ms

for the buffer to power up and recharge the REFBUF bypass

capacitor. Any conversion initiated before these times will

produce invalid results.

TIMING AND CONTROL

CNV Timing

The LTC2344-18 sampling and conversion is controlled by

CNV. A rising edge on CNV transitions all channels’ S/H

circuits from track mode to hold mode, simultaneously

sampling the input signals on all channels and initiating

a conversion. Once a conversion has been started, it

cannot be terminated early except by resetting the ADC,

as discussed in the Reset Timing section. For optimum

performance, drive CNV with a clean, low jitter signal and

avoid transitions on data I/O lines leading up to the rising

edge of CNV. Additionally, to minimize channel-to-channel

crosstalk, avoid high slew rates on the analog inputs for

100ns before and after the rising edge of CNV. Converter

status is indicated by the BUSY output, which transitions

low-to-high at the start of each conversion and stays high

until the conversion is complete. Once CNV is brought high

to begin a conversion, it should be returned low between

40ns and 60ns later or after the falling edge of BUSY to

minimize external disturbances during the internal conver-

sion process. If CNV is returned low after the falling edge

of BUSY, it should be held low for at least 390ns before

bringing it high again, since the converter acquisition

time (tACQ) is set by the CNV low time (tCNVL) in this case.

Internal Conversion Clock

The LTC2344-18 has an internal clock that is trimmed to

achieve a maximum conversion time of 525 • N – 20ns

with N channels enabled. With a minimum acquisition time

of 390ns when converting four channels simultaneously,

throughput performance of 400ksps is guaranteed without

any external adjustments.

Power Down Mode

When PD is brought high, the LTC2344-18 is powered

down and subsequent conversion requests are ignored. If

this occurs during a conversion, the device powers down

once the conversion completes. In this mode, the device

draws only a small regulator standby current resulting in a

typical power dissipation of 0.33mW. To exit power down

mode, bring the PD pin low and wait at least 10ms before

initiating a conversion. When employing the internal refer-

ence buffer, allow 200ms for the buffer to power up and

recharge the REFBUF bypass capacitor. Any conversion

initiated before these times will produce invalid results.

Reset Timing

A global reset of the LTC2344-18, equivalent to a power-

on-reset event, may be executed without needing to cycle

the supplies. This feature is useful when recovering from

system-level events that require the state of the entire sys-

APPLICATIONS INFORMATION

LTC2344-18

234418f

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tem to be reset to a known synchronized value. To initiate

a global reset, bring PD high twice without an intervening

conversion, as shown in Figure 15. The reset event is trig-

gered on the second rising edge of PD, and asynchronously

ends based on an internal timer. Reset clears all serial data

output registers and restores the internal SoftSpan configu-

ration register default state of all channels in SoftSpan7.

If reset is triggered during a conversion, the conversion

is immediately halted. The normal power down behavior

associated with PD going high is not affected by reset. Once

PD is brought low, wait at least 10ms before initiating a

conversion. When employing the internal reference buffer,

allow 200ms for the buffer to power up and recharge the

REFBUF bypass capacitor. Any conversion initiated before

these times will produce invalid results.

Auto Nap Mode

The LTC2344-18 automatically enters nap mode after a

conversion has finished and completely powers up once a

new conversion is initiated on the rising edge of CNV. Auto

nap mode causes the power dissipation of the LTC2344-

18 to decrease as the sampling frequency is reduced,

as shown in Figure 16. This decrease in average power

dissipation occurs because a portion of the LTC2344-18

circuitry is turned off during nap mode, and the fraction

of the conversion cycle (tCYC) spent napping increases as

the sampling frequency (fSMPL) is decreased.

APPLICATIONS INFORMATION

DIGITAL INTERFACE

The LTC2344-18 features CMOS and LVDS serial interfaces,

selectable using the LVDS/CMOS pin. The flexible OVDD

supply allows the LTC2344-18 to communicate with any

CMOS logic operating between 1.8V and 5V, including

2.5V and 3.3V systems, while the LVDS interface supports

low noise digital designs. In CMOS mode, applications

may employ between one and four lanes of serial data

output, allowing the user to optimize bus width and data

Figure 16. Power Dissipation of the LTC2344-18

Decreases with Decreasing Sampling Frequency

CNV

tCONV

tCNVH

tPDH

tPDL

BUSY

RESET RESET TIME

SET INTERNALLY

SECOND RISING EDGE OF

PD TRIGGERS RESET

PD tWAKE

234418 F15

Figure 15. Reset Timing for the LTC2344-18

OVDD

VDD

SAMPLING FREQUENCY (kHz)

100

200

300

400

SUPPLY CURRENT (mA)

234418 F16

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

APPLICATIONS INFORMATION

throughput. Together, these I/O interface options enable

the LTC2344-18 to communicate equally well with legacy

microcontrollers and modern FPGAs.

Serial CMOS I/O Mode

As shown in Figure 17, in CMOS I/O mode the serial data

bus consists of a serial clock input, SCKI, serial data

input, SDI, serial clock output, SCKO, and four lanes of

serial data output, SDO0 to SDO3. Communication with

the LTC2344-18 across this bus occurs during predefined

data transaction windows. Within a window, the device

accepts 12-bit SoftSpan configuration words for the next

conversion on SDI and outputs 24-bit packets containing

conversion results and channel configuration information

from the previous conversion on SDO0 to SDO3. New

data transaction windows open 10ms after powering up

or resetting the LTC2344-18, and at the end of each con-

version on the falling edge of BUSY. In the recommended

use case, the data transaction should be completed with

a minimum tQUIET time of 20ns prior to the start of the

next conversion, as shown in Figure 17. New SoftSpan

configuration words are only accepted within this recom-

mended data transaction window, but SoftSpan changes

take effect immediately with no additional analog input

settling time required before starting the next conversion.

It is still possible to read conversion data after starting the

next conversion, but this will degrade conversion accuracy

and therefore is not recommended.

Just prior to the falling edge of BUSY and the opening of

a new data transaction window, SCKO is forced low and

SDO0 to SDO3 are updated with the latest conversion

results from analog input channels 0 to 3, respectively.

Rising edges on SCKI serially clock conversion results

and analog input channel configuration information out

on SDO0 to SDO3 and trigger transitions on SCKO that are

skew-matched to the data on SDO0 to SDO3. The resulting

0 C1 C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17

CNV

CS = PD = 0

DON’T CARE

tACQ

BUSY

SDO3

SCKO

SDO0

SCKI

SDI

DON’T CARE

SAMPLE N

RECOMMENDED DATA TRANSACTION WINDOW

SAMPLE N + 1

SOFTSPAN CONFIGURATION WORD FOR CONVERSION N + 1

CHANNEL 0

24-BIT PACKET

CONVERSION N

CHANNEL 1

24-BIT PACKET

CONVERSION N

CHANNEL 3

24-BIT PACKET

CONVERSION N

CHANNEL 0

24-BIT PACKET

CONVERSION N

234418 TD01

tHSDISCKI

CONVERSION RESULT CHAN ID SOFTSPAN CONVERSION RESULT

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17DON’T CARE

CONVERSION RESULT CHAN ID SOFTSPAN CONVERSION RESULT

• • •

tSCKI tSCKIH

tBUSYLH

tSCKIL

tHSDOSCKI

tDSDOSCKI

tSSDISCKI

tQUIET

tDSDOBUSYL

tCNVH

tSKEW

tCONV

tCNVL

tCYC

Figure 17. Serial CMOS I/O Mode

LTC2344-18

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SCKO frequency is half that of SCKI. SCKI rising edges

also latch SoftSpan configuration words provided on SDI,

which are used to program the internal 12-bit SoftSpan

configuration register. See the section Programming the

SoftSpan Configuration Register in CMOS I/O Mode for

further details. SCKI is allowed to idle either high or low

in CMOS I/O mode. As shown in Figure 18, the CMOS

bus is enabled when CS is low and is disabled and Hi-Z

when CS is high, allowing the bus to be shared across

multiple devices.

The data on SDO0 to SDO3 are grouped into 24-bit packets

consisting of an 18-bit conversion result, followed by a zero,

2-bit analog channel ID, and 3-bit SoftSpan code, all pre-

sented MSB first. As suggested in Figures 17 and 18, each

SDO lane outputs these packets for all analog input chan-

nels in a sequential, circular manner. For example, the first

24-bit packet output on SDO0 corresponds to analog input

channel 0, followed by the packets for channels 1, 2 and 3.

The data output on SDO0 then wraps back to channel

0, and this pattern repeats indefinitely. Other SDO lanes

follow a similar circular pattern, except the first packet

presented on each lane corresponds to its associated

analog input channel.

When interfacing the LTC2344-18 with a standard SPI

bus, capture output data at the receiver on rising edges of

SCKI. SCKO is not used in this case. Multiple SDO lanes

are also usually not useful in this case. In other applica-

tions, such as interfacing the LTC2344-18 with an FPGA

or CPLD, rising and falling edges of SCKO may be used

to capture serial output data on SDO0 to SDO3 in double

data rate (DDR) fashion. Capturing data using SCKO adds

robustness to delay variations over temperature and supply.

Full Four Lane Serial CMOS Output Data Capture

As shown in Table 3, full 400ksps per channel throughput

can be achieved with a 65MHz SCKI frequency by capturing

the first packet (24 SCKI cycles total) from all four serial

data output lanes SDO0 to SDO3. This configuration also

allows conversion results from all channels to be captured

using as few as 18 SCKI cycles if the 2-bit analog channel

ID and 3-bit SoftSpan code are not needed. Multi-lane data

capture is usually best suited for use with FPGA or CPLD

capture hardware, but may be useful in other application-

specific cases.

Fewer Than Four Lane Serial CMOS Output Data Capture

Applications that cannot accommodate the full four

lanes of serial data capture may employ fewer lanes

without reconfiguring the LTC2344-18. For example,

capturing the first two packets (48 SCKI cycles total)

from SDO0 and SDO2 provides data for analog input

channels 0 and 1, and 2 and 3, respectively, using two

APPLICATIONS INFORMATION

Figure 18. Internal SoftSpan Configuration Register Behavior. Serial CMOS Bus Response to CS

PD = 0

DON’T CARE

BUSY

SDO3

SCKO

SDO0

SCKI

SDI

DON’T CARE

234418 F18

Hi-Z

Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET

(PARTIAL)

Hi-Z

CHANNEL 3 PACKET CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET

(PARTIAL)

• • •

NEW SoftSpan CONFIGURATION WORD

(OVERWRITES INTERNAL CONFIG REGISTER)

FIVE ALL-ZERO WORDS AND ONE PARTIAL WORD

(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)

tEN tDIS

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

output lanes. If only one lane can be accommodated,

capturing the first four packets (96 SCKI cycles total)

from SDO0 provides data for all analog input channels.

As shown in Table 3, full 400ksps per channel throughput

can be achieved with a 65MHz SCKI frequency in the four

lane case, but the maximum CMOS SCKI frequency of

100MHz limits the throughput to less than 400ksps per

channel in the two lane and one lane cases. Finally, note

that in choosing the number of lanes and which lanes

to use for data capture, the user is not restricted to the

specific cases mentioned above. Other choices may be

more optimal in particular applications.

Programming the SoftSpan Configuration Register in

CMOS I/O Mode

The internal 12-bit SoftSpan configuration register con-

trols the SoftSpan range for all analog input channels of

the LTC2344-18. The default state of this register after

power-up or resetting the device is all ones, configuring

each channel to convert in SoftSpan 7, the ± VREFBUF range

(see Table 1a). The state of this register may be modified

by providing a new 12-bit SoftSpan configuration word on

SDI during the data transaction window shown in Figure

17. New SoftSpan configuration words are only accepted

within this recommended data transaction window, but

SoftSpan changes take effect immediately with no addi-

tional analog input settling time required before starting

the next conversion. Setting a channel’s SoftSpan code to

SS[2:0] = 000 immediately disables the channel, resulting

in a corresponding reduction in tCONV on the next conver-

sion. Similarly, enabling a previously disabled channel

requires no additional analog input settling time before

starting the next conversion. The mapping between the

serial SoftSpan configuration word, the internal SoftSpan

configuration register, and each channel’s 3-bit SoftSpan

code is illustrated in Figure 19.

If fewer than 12 SCKI rising edges are provided during a

data transaction window, the partial word received on SDI

will be ignored and the SoftSpan configuration register will

not be updated. If exactly 12 SCKI rising edges are provided,

the SoftSpan configuration register will be updated to

match the received SoftSpan configuration word, S[11:0].

The one exception to this behavior occurs when S[11:0] is

all zeros. In this case, the SoftSpan configuration register

will not be updated, allowing applications to retain the

current SoftSpan configuration state by idling SDI low. If

more than 12 SCKI rising edges are provided during a data

transaction window, each complete 12-bit word received

on SDI will be interpreted as a new SoftSpan configuration

word and applied to the SoftSpan configuration register

as described above. Any partial words are ignored.

Typically, applications will update the SoftSpan configura-

tion register in the manner shown in Figures 17 and 18.

After the opening of a new data transaction window at the

falling edge of BUSY, the user supplies a 12-bit SoftSpan

configuration word on SDI during the first 12 SCKI cycles.

This new word overwrites the internal configuration

The user then holds SDI low for the remainder of the

APPLICATIONS INFORMATION

Table 3. Required SCKI Frequency to Achieve Various Throughputs in Common Output Bus Configurations with Four Channels

Enabled. Shaded Entries Denote Throughputs That Are Not Achievable In a Given Configuration. Calculated Using fSCKI = (Number of

SCKI Cycles)/(tACQ,MIN – tQUIET)

I/O MODE NUMBER OF SDO

LANES

NUMBER OF SCKI

CYCLES

REQUIRED fSCKI (MHz) TO ACHIEVE THROUGHPUT OF

400ksps/CHANNEL

(tACQ = 390ns)

200ksps/CHANNEL

(tACQ = 2890ns)

100ksps/CHANNEL

(tACQ = 7890ns)

CMOS

4 18 50 7 3

4 24 65 9 4

2 48 Not Achievable 17 7

1 96 Not Achievable 34 13

LVDS 1 48 130 (260Mbps) 17 (34Mbps) 7 (14Mbps)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

data transaction window causing the register to retain

its contents regardless of the number of additional SCKI

cycles applied. SoftSpan settings may be retained across

multiple conversions by holding SDI low for the entire

data transaction window, regardless of the number of

SCKI cycles applied.

Serial LVDS I/O Mode

In LVDS I/O mode, information is transmitted using posi-

tive and negative signal pairs (LVDS+/LVDS−) with bits

differentially encoded as (LVDS+ − LVDS−). These signals

are typically routed using differential transmission lines

with 100Ω characteristic impedance. Logical 1s and 0s

are nominally represented by differential +350mV and

−350mV, respectively. For clarity, all LVDS timing diagrams

and interface discussions adopt the logical rather than

physical convention.

As shown in Figure 20, in LVDS I/O mode the serial data

bus consists of a serial clock differential input, SCKI, serial

data differential input, SDI, serial clock differential output,

SCKO, and serial data differential output, SDO. Communi-

cation with the LTC2344-18 across this bus occurs during

predefined data transaction windows. Within a window,

the device accepts 12-bit SoftSpan configuration words

for the next conversion on SDI and outputs 24-bit packets

containing conversion results and channel configuration

information from the previous conversion on SDO. New

data transaction windows open 10ms after powering up

or resetting the LTC2344-18, and at the end of each con-

version on the falling edge of BUSY. In the recommended

use case, the data transaction should be completed with

a minimum tQUIET time of 20ns prior to the start of the

next conversion, as shown in Figure 20. New SoftSpan

configuration words are only accepted within this recom-

mended data transaction window, but SoftSpan changes

take effect immediately with no additional analog input

1 2 3 4 5 6 7 8 9 10 11 12

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0DON’T CARE

SCKI

SDI

SoftSpan CONFIGURATION WORD

CMOS I/O MODE

SoftSpan CONFIGURATION WORD

INTERNAL 12-BIT SoftSpan CONFIGURATION REGISTER

(SAME FOR CMOS AND LVDS)

LVDS I/O MODE

1234567891011 0

tSCKI

1 2 3 4 5 6 7 8 9 10 11 12

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0DON’T CARE

SCKI

(LVDS)

SDI

(LVDS)

tSCKI

CHANNEL 3 SoftSpan

CODE SS[2:0]

CHANNEL 2 SoftSpan

CODE SS[2:0]

CHANNEL 1 SoftSpan

CODE SS[2:0]

CHANNEL 0 SoftSpan

CODE SS[2:0]

234418 F19

tSCKIH

tSCKIL

tSSDISCKI

tHSDISCKI

tSCKIH

tSSDISCKI

tHSDISCKI

tSCKIL

APPLICATIONS INFORMATION

Figure 19. Mapping Between Serial SoftSpan Configuration Word, Internal SoftSpan

Configuration Register, and SoftSpan Code for Each Analog Input Channel

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

APPLICATIONS INFORMATION

Figure 20. Serial LVDS I/O Mode

S11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 2624

S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0

D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17 D16 D15

CNV

(CMOS)

CS = PD = 0

234418 F20

DON’T CARE

tACQ

BUSY

(CMOS)

SCKO

(LVDS)

SDO

(LVDS)

SCKI

(LVDS)

SDI

(LVDS)

DON’T CARE

SAMPLE N

tCNVL

tCYC

RECOMMENDED DATA TRANSACTION WINDOW

SAMPLE N + 1

SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1

CHANNEL 0

24-BIT PACKET

CONVERSION N

CHANNEL 1

24-BIT PACKET

CONVERSION N

CHANNEL 3

24-BIT PACKET

CONVERSION N

CONVERSION RESULT CHAN ID SoftSpan

9089 91 92 93 94 95 96

D0 SS1SS2 SS0 D17

CHANNEL 0

24-BIT PACKET

CONVERSION N

CONVERSION

RESULT

CHAN ID SoftSpan

tSKEW

tCNVH

tBUSYLH

tDSDOSCKI

tHSDOSCKI

tDSDOBUSYL

tHSDISCKI tSSDISCKI

tSSDISCKI tHSDISCKI

tQUIET

tCONV

0 C1 C00 C1 C0

tSCKI tSCKIH

tSCKIL

settling time required before starting the next conversion.

It is still possible to read conversion data after starting the

next conversion, but this will degrade conversion accuracy

and therefore is not recommended.

Just prior to the falling edge of BUSY and the opening

of a new data transaction window, SDO is updated with

the latest conversion results from analog input channel

0. Both rising and falling edges on SCKI serially clock

conversion results and analog input channel configuration

information out on SDO. SCKI is also echoed on SCKO,

skew-matched to the data on SDO. Whenever possible, it

is recommended that rising and falling edges of SCKO be

used to capture DDR serial output data on SDO, as this will

yield the best robustness to delay variations over supply

and temperature. SCKI rising and falling edges also latch

SoftSpan configuration words provided on SDI, which are

used to program the internal 12-bit SoftSpan configura-

tion register. See the section Programming the SoftSpan

Configuration Register in LVDS I/O Mode for further

details. As shown in Figure 21, the LVDS bus is enabled

when CS is low and is disabled and Hi-Z when CS is high,

allowing the bus to be shared across multiple devices.

Due to the high speeds involved in LVDS signaling, LVDS

bus sharing must be carefully considered. Transmission

line limitations imposed by the shared bus may limit the

maximum achievable bus clock speed. LVDS inputs are

internally terminated with a 100Ω differential resistor when

CS is low, while outputs must be differentially terminated

with a 100Ω resistor at the receiver (FPGA). SCKI must

idle in the low state in LVDS I/O mode, including when

transitioning CS.

The data on SDO are grouped into 24-bit packets consisting

of an 18-bit conversion result, followed by a zero, 2-bit

analog channel ID and 3-bit SoftSpan code, all presented

MSB first. As suggested in Figures 20 and 21, SDO outputs

these packets for all analog input channels in a sequential,

circular manner. For example, the first 24-bit packet output

on SDO corresponds to analog input channel 0, followed

by the packets for channels 1, 2 and 3. The data output

on SDO then wraps back to channel 0, and this pattern

repeats indefinitely.

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

Serial LVDS Output Data Capture

As shown in Table 3, full 400ksps per channel throughput

can be achieved with a 130MHz SCKI frequency by captur-

ing four packets (48 SCKI cycles total) of DDR data from

SDO. The LTC2344-18 supports LVDS SCKI frequencies

up to 250MHz.

Programming the SoftSpan Configuration Register in

LVDS I/O Mode

The internal 12-bit SoftSpan configuration register con-

trols the SoftSpan range for all analog input channels of

the LTC2344-18. The default state of this register after

power-up or resetting the device is all ones, configuring

each channel to convert in SoftSpan 7, the ± VREFBUF range

(see Table 1a). The state of this register may be modified

by providing a new 12-bit SoftSpan configuration word on

SDI during the data transaction window shown in Figure

20. New SoftSpan configuration words are only accepted

within this recommended data transaction window, but

SoftSpan changes take effect immediately with no addi-

tional analog input settling time required before starting

the next conversion. Setting a channel’s SoftSpan code to

SS[2:0] = 000 immediately disables the channel, resulting

in a corresponding reduction in tCONV on the next conver-

sion. Similarly, enabling a previously disabled channel

requires no additional analog input settling time before

starting the next conversion. The mapping between the

serial SoftSpan configuration word, the internal SoftSpan

configuration register, and each channel’s 3-bit SoftSpan

code is illustrated in Figure 19.

If fewer than 12 SCKI edges (rising plus falling) are

provided during a data transaction window, the partial

word received on SDI will be ignored and the SoftSpan

configuration register will not be updated. If exactly 12

SCKI edges are provided, the SoftSpan configuration

configuration word, S[11:0]. The one exception to this

behavior occurs when S[11:0] is all zeros. In this case,

the SoftSpan configuration register will not be updated,

allowing applications to retain the current SoftSpan con-

figuration state by idling SDI low. If more than 12 SCKI

edges are provided during a data transaction window, each

complete 12-bit word received on SDI will be interpreted

as a new SoftSpan configuration word and applied to the

SoftSpan configuration register as described above. Any

partial words are ignored.

Typically, applications will update the SoftSpan configura-

tion register in the manner shown in Figures 20 and 21.

After the opening of a new data transaction window at

the falling edge of BUSY, the user supplies a 12-bit DDR

SoftSpan configuration word on SDI during the first 12

SCKI cycles. This new word overwrites the internal con-

figuration register contents following the 6th SCKI falling

edge. The user then holds SDI low for the remainder of

the data transaction window causing the register to retain

its contents regardless of the number of additional SCKI

cycles applied. SoftSpan settings may be retained across

multiple conversions by holding SDI low for the entire

data transaction window, regardless of the number of

SCKI cycles applied.

APPLICATIONS INFORMATION

Figure 21. Internal SoftSpan Configuration Register Behavior. Serial LVDS Bus Response to CS

(CMOS)

PD = 0

DON’T CARE

BUSY

(CMOS)

SCKO

(LVDS)

SDO

(LVDS)

SCKI

(LVDS)

SDI

(LVDS)

DON’T CARE

234418 F21

Hi-Z

Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET

(PARTIAL)

Hi-Z

FIVE ALL-ZERO WORDS AND ONE PARTIAL WORD

(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)

NEW SoftSpan CONFIGURATION WORD

tEN tDIS

(OVERWRITES INTERNAL CONFIG REGISTER)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

To obtain the best performance from the LTC2344-18, a

four-layer printed circuit board (PCB) is recommended.

Layout for the PCB should ensure the digital and analog

signal lines are separated as much as possible. In particu-

lar, care should be taken not to run any digital clocks or

signals alongside analog signals or underneath the ADC.

Also minimize the length of the REFBUF to GND (Pin 11)

bypass capacitor return loop, and avoid routing CNV near

signals which could potentially disturb its rising edge.

Supply bypass capacitors should be placed as close as

possible to the supply pins. Low impedance common re-

turns for these bypass capacitors are essential to the low

noise operation of the ADC. A single solid ground plane

is recommended for this purpose. When possible, screen

the analog input traces using ground.

Reference Design

For a detailed look at the reference design for this con-

verter, including schematics and PCB layout, please refer

to DC2520, the evaluation kit for the LTC2344-18.

BOARD LAYOUT

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/product/LTC2344-18#packaging for the most recent package drawings.

5.00 ±0.10

(4 SIDES)

NOTE:

1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE

M0-220 VARIATION WHHD-(X) (TO BE APPROVED)

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

PIN 1

TOP MARK

(NOTE 6)

0.40 ±0.10

BOTTOM VIEW—EXPOSED PAD

3.50 REF

(4-SIDES)

3.45 ±0.10

0.75 ±0.05 R = 0.115

TYP

0.25 ±0.05

(UH32) QFN 0406 REV D

0.50 BSC

0.200 REF

0.00 – 0.05

0.70 ±0.05

3.50 REF

(4 SIDES)

4.10 ±0.05

5.50 ±0.05

0.25 ±0.05

PACKAGE OUTLINE

0.50 BSC

RECOMMENDED SOLDER PAD LAYOUT

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

PIN 1 NOTCH R = 0.30 TYP

OR 0.35 × 45° CHAMFER

R = 0.05

TYP

3.45 ±0.05

UH Package

32-Lead Plastic QFN (5mm × 5mm)

(Reference LTC DWG # 05-08-1693 Rev D)

LTC2344-18

234418f

For more information www.linear.com/LTC2344-18

 LINEAR TECHNOLOGY CORPORATION 2016

LT 0816 • PRINTED IN USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC2344-18

RELATED PARTS

TYPICAL APPLICATION

PART NUMBER DESCRIPTION COMMENTS

ADCs

LTC2345-18/LTC2345-16 18-Bit/16-Bit, 200ksps, 8-Channel Simultaneous

Sampling, ±5/±1.25LSB INL, Serial ADC

5V Supply, SoftSpan Inputs with Wide Common Mode Range, 92/91dB

SNR, Serial CMOS and LVDS I/O, 7mm × 7mm QFN-48 Package

LTC2348-18/LTC2348-16 18-/16-Bit, 200ksps, 8-Channel Simultaneous

Sampling, ±3/±1LSB INL, Serial ADC ±10.24V SoftSpan Inputs with Wide Common Mode Range, 97/94dB SNR,

Serial CMOS and LVDS I/O, 7mm × 7mm LQFP-48 Package

LTC2335-18/LTC2335-16 18-Bit/16-Bit, 1Msps, 8-Channel Multiplexed,

±3/±1LSB INL, Serial ADC

±10.24V SoftSpan Inputs with Wide Common Mode Range, 97/94dB SNR,

Serial CMOS and LVDS I/O, 7mm × 7mm LQFP-48 Package

LTC2378-20/LTC2377-20/

LTC2376-20

20-Bit, 1Msps/500ksps/250ksps,

±0.5ppm INL Serial, Low Power ADC

2.5V Supply, ±5V Fully Differential Input, 104dB SNR, MSOP-16 and

4mm × 3mm DFN-16 Packages

LTC2338-18/LTC2337-18/

LTC2336-18

18-Bit, 1Msps/500ksps/250ksps, Serial,

Low Power ADC

5V Supply, ±10.24V Fully Differential Input, 100dB SNR, MSOP-16 Package

LTC2328-18/LTC2327-18/

LTC2326-18

18-Bit, 1Msps/500ksps/250ksps, Serial,

Low Power ADC

5V Supply, ±10.24V Pseudo-Differential Input, 95dB SNR,

MSOP-16 Package

LTC2373-18/LTC2372-18 18-Bit, 1Msps/500ksps, 8-Channel, Serial ADC 5V Supply, 8 Channel Multiplexed, Configurable Input Range, 100dB SNR,

DGC, 5mm × 5mm QFN-32 Package

LTC2379-18/LTC2378-18/

LTC2377-18/LTC2376-18

18-Bit,1.6Msps/1Msps/500ksps/250ksps, Serial,

Low Power ADC

2.5V Supply, Differential Input, 101.2dB SNR, ±5V Input Range, DGC, Pin

Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages

LTC2380-16/LTC2378-16/

LTC2377-16/LTC2376-16

16-Bit, 2Msps/1Msps/500ksps/250ksps, Serial,

Low Power ADC

2.5V Supply, Differential Input, 96.2dB SNR, ±5V Input Range, DGC, Pin

Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages

LTC2389-18/LTC2389-16 18-Bit/16-Bit, 2.5Msps, Parallel/Serial ADC 5V Supply, Pin-Configurable Input Range, 99.8dB/96dB SNR, Parallel or

Serial I/O 7mm × 7mm LQFP-48 and QFN-48 Packages

LTC2173-14/LTC2174-14/

LTC2175-14

14-Bit, 80Msps/105Msps/125Msps 1.8V Quad

ADC, Ultralow Power

376mW/450mW/558mW, 73.1dB SNR, 88dB SFDR, Serial LVDS Outputs,

7mm × 8mm QFN-52

LTC1859/LTC1858/

LTC1857

16-/14-/12-Bit, 8-Channel, 100ksps, Serial ADC ±10V, SoftSpan, Single-Ended or Differential Inputs, Single 5V Supply,

SSOP-28 Package

LTC1609 16-Bit, 200ksps Serial ADC ±10V, Configurable Unipolar/Bipolar Input, SSOP-28 and SO-20 Packages

LTC1606/LTC1605 16-Bit, 250ksps/100ksps, Parallel ADC ±10V Input, 5V Supply, 75mW/55mW, SSOP-28 and SO-28 Packages

DACs

LTC2756/LTC2757 18-Bit, Serial/Parallel IOUT SoftSpan DAC ±1LSB INL/DNL, Software-Selectable Ranges, SSOP-28/7mm × 7mm

LQFP-48 Package

LTC2668 16-Channel 16-/12-Bit ±10V VOUT SoftSpan DACs ±4LSB INL, Precision Reference 10ppm/°C Max, 6mm × 6mm QFN-40 Package

References

LTC6657 Low Drift Low Noise Buffered Reference 5V/3V/2.5V, 1.5ppm/°C, 0.5ppm Peak-to-Peak Noise, MSOP-8 Package

LTC6655 Precision Low Drift Low Noise Buffered Reference 5V/2.5V/2.048V/1.25V, 2ppm/°C, 0.25ppm Peak-to-Peak Noise, MSOP-8 Package

LTC6652 Precision Low Drift Low Noise Buffered Reference 5V/2.5V/2.048V/1.25V, 5ppm/°C, 2.1ppm Peak-to-Peak Noise, MSOP-8 Package

Amplifiers

LT6236/LT6237/LT6238 Single/Dual/Quad Operational Amplifier with

Low Wideband Noise

215MHz, 3.5mA/Amplifier, 1.1nV/√Hz

LT6233/LT6234/LT6235 Single/Dual/Quad Low Noise Rail-to-Rail Output

Op Amps

60MHz,1.2mA,1.2nV/√Hz,15V/μs,0.5mV

LTC6252/LTC6253/

LTC6254

720MHz, 3.5mA Power Efficient Rail-to-Rail I/O

Op Amp

720MHz GBW, Unity Gain Stable, Low Noise

Sense Current from Rail with Amplification

LTC2344-18

234418 TA02

ONLY CHANNEL 0 SHOWN FOR CLARITY

0.1µF

47µF

IN0+

IN0–

VDD

REFINREFBUF

LTC6252

LOAD

2.49k

274Ω

–

1Ω

LOAD