ADC12DC105CISQE - Texas Instruments High-Performance Analog

ADC12DC105

ADC12DC105 Dual 12-Bit, 105 MSPS A/D Converter with CMOS Outputs

Literature Number: SNAS469A

October 23, 2008

ADC12DC105

Dual 12-Bit, 105 MSPS A/D Converter with CMOS Outputs

General Description

The ADC12DC105 is a high-performance CMOS analog-to-

digital converter capable of converting two analog input sig-

nals into 12-bit digital words at rates up to 105 Mega Samples

Per Second (MSPS). These converters use a differential,

pipelined architecture with digital error correction and an on-

chip sample-and-hold circuit to minimize power consumption

and the external component count, while providing excellent

dynamic performance. A unique sample-and-hold stage

yields a full-power bandwidth of 1 GHz. The AD-

C12DC080/105 may be operated from a single +3.0V or

+3.3V power supply. A power-down feature reduces the pow-

er consumption to very low levels while still allowing fast

wake-up time to full operation. The differential inputs provide

a 2V full scale differential input swing. A stable 1.2V internal

voltage reference is provided, or the ADC12DC105 can be

operated with an external 1.2V reference. Output data format

(offset binary versus 2's complement) and duty cycle stabi-

lizer are pin-selectable. The duty cycle stabilizer maintains

performance over a wide range of clock duty cycles.

The ADC12DC105 is available in a 60-lead LLP package and

operates over the industrial temperature range of −40°C to

+85°C.

Features

■Internal sample-and-hold circuit and precision reference

■Low power consumption

■Clock Duty Cycle Stabilizer

■Single +3.0V or +3.3V supply operation

■Power-down mode

■Offset binary or 2's complement output data format

■60-pin LLP package, (9x9x0.8mm, 0.5mm pin-pitch)

Key Specifications

■Resolution 12 Bits

■Conversion Rate 105 MSPS

■SNR (fIN = 170 MHz) 69 dBFS (typ)

■SFDR (fIN = 170 MHz) 83 dBFS (typ)

■Full Power Bandwidth 1 GHz (typ)

■Power Consumption 690 mW (typ), VA=3.0V

■800 mW (typ), VA=3.3V

Applications

■High IF Sampling Receivers

■Wireless Base Station Receivers

■Test and Measurement Equipment

■Communications Instrumentation

■Portable Instrumentation

Block Diagram

30073902

ADC12DC105 Dual 12-Bit, 105 MSPS A/D Converter with CMOS Outputs

Connection Diagram

30073901

Ordering Information

Industrial (−40°C ≤ TA ≤ +85°C) Package

ADC12DC105CISQ 60 Pin LLP

ADC12DC105CISQE 60 Pin LLP,

250 pc. Tape and Reel

ADC12DC105LFEB Evaluation Board

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ADC12DC105

Pin Descriptions and Equivalent Circuits

Pin No. Symbol Equivalent Circuit Description

ANALOG I/O

VINA+

VINB+

Differential analog input pins. The differential full-scale input signal

level is 2VP-P with each input pin signal centered on a common

mode voltage, VCM.

VINA-

VINB-

VRPA

VRPB

These pins should each be bypassed to AGND with a low ESL

(equivalent series inductance) 0.1 µF capacitor placed very close

to the pin to minimize stray inductance. An 0201 size 0.1 µF

capacitor should be placed between VRP and VRN as close to the

pins as possible, and a 1 µF capacitor should be placed in parallel.

VRP and VRN should not be loaded. VCMO may be loaded to 1mA

for use as a temperature stable 1.5V reference.

It is recommended to use VCMO to provide the common mode

voltage, VCM, for the differential analog inputs.

VCMOA

VCMOB

VRNA

VRNB

59 VREF

Reference Voltage. This device provides an internally developed

1.2V reference. When using the internal reference, VREF should be

decoupled to AGND with a 0.1 µF and a 1µF, low equivalent series

inductance (ESL) capacitor.

This pin may be driven with an external 1.2V reference voltage.

This pin should not be used to source or sink current when the

internal reference is used.

DIGITAL I/O

19 OF/DCS

This is a four-state pin controlling the input clock mode and output

data format.

OF/DCS = VA, output data format is 2's complement without duty

cycle stabilization applied to the input clock.

OF/DCS = AGND, output data format is offset binary, without duty

cycle stabilization applied to the input clock.

OF/DCS = (2/3)*VA, output data is 2's complement with duty cycle

stabilization applied to the input clock.

OF/DCS = (1/3)*VA, output data is offset binary with duty cycle

stabilization applied to the input clock.

18 CLK The clock input pin.

The analog inputs are sampled on the rising edge of the clock input.

PD_A

PD_B

This is a two-state input controlling Power Down.

PD = VA, Power Down is enabled and power dissipation is reduced.

PD = AGND, Normal operation.

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ADC12DC105

Pin No. Symbol Equivalent Circuit Description

42-49,

52-55

DA0-DA7,

DA8-DA11

Digital data output pins that make up the 12-bit conversion result

for Channel A. DA0 (pin 42) is the LSB, while DA11 (pin 55) is the

MSB of the output word. Output levels are CMOS compatible.

23-24,

27-36

DB0-DB1,

DB3-DB11

Digital data output pins that make up the 12-bit conversion result

for Channel B. DB0 (pin 23) is the LSB, while DB11 (pin 36) is the

MSB of the output word. Output levels are CMOS compatible.

39 DRDY

Data Ready Strobe. The data output transition is synchronized with

the falling edge of this signal. This signal switches at the same

frequency as the CLK input.

ANALOG POWER

8, 16, 17, 58,

60 VA

Positive analog supply pins. These pins should be connected to a

quiet source and be bypassed to AGND with 0.1 µF capacitors

located close to the power pins.

1, 4, 12, 15,

Exposed Pad AGND

The ground return for the analog supply.

The exposed pad on back of package must be soldered to ground

plane to ensure rated performance.

DIGITAL POWER

26, 38,50 VDR

Positive driver supply pin for the output drivers. This pin should be

connected to a quiet voltage source and be bypassed to DRGND

with a 0.1 µF capacitor located close to the power pin.

25, 37, 51 DRGND

The ground return for the digital output driver supply. This pins

should be connected to the system digital ground, but not be

connected in close proximity to the ADC's AGND pins.

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ADC12DC105

Absolute Maximum Ratings (Notes 1, 3)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltage (VA, VDR) −0.3V to 4.2V

Voltage on Any Pin

(Not to exceed 4.2V)

−0.3V to (VA +0.3V)

Input Current at Any Pin other

than Supply Pins (Note 4)

±5 mA

Package Input Current (Note 4) ±50 mA

Max Junction Temp (TJ) +150°C

Thermal Resistance (θJA)30°C/W

ESD Rating

Human Body Model (Note 6) 2500V

Machine Model (Note 6) 250V

Storage Temperature −65°C to +150°C

Soldering process must comply with National

Semiconductor's Reflow Temperature Profile

specifications. Refer to www.national.com/packaging.

(Note 7)

Operating Ratings (Notes 1, 3)

Operating Temperature −40°C ≤ TA ≤ +85°C

Supply Voltage (VA) +2.7V to +3.6V

Output Driver Supply (VDR) +2.4V to VA

Clock Duty Cycle

(DCS Enabled) 30/70 %

(DCS Disabled) 45/55 %

VCM 1.4V to 1.6V

|AGND-DRGND| ≤100mV

Converter Electrical Characteristics

Unless otherwise specified, the following specifications apply: AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF =

+1.2V, fCLK = 105 MHz, VCM = VCMO, CL = 5 pF/pin. Typical values are for TA = 25°C. Boldface limits apply for TMIN ≤ TA ≤

TMAX. All other limits apply for TA = 25°C (Notes 8, 9)

Symbol Parameter Conditions Typical

(Note 10) Limits Units

(Limits)

STATIC CONVERTER CHARACTERISTICS

Resolution with No Missing Codes 12 Bits (min)

INL Integral Non Linearity (Note 11) ±0.5 1.1 LSB (max)

-1.1 LSB (min)

DNL Differential Non Linearity ±0.2 0.55 LSB (max)

-0.55 LSB (min)

PGE Positive Gain Error -0.1 ±1 %FS (max)

NGE Negative Gain Error 0.18 ±1 %FS (max)

TC PGE Positive Gain Error Tempco −40°C ≤ TA ≤ +85°C -3 ppm/°C

TC NGE Negative Gain Error Tempco −40°C ≤ TA ≤ +85°C -7 ppm/°C

VOFF Offset Error 0.01 ±0.55 %FS (max)

TC VOFF Offset Error Tempco −40°C ≤ TA ≤ +85°C -4 ppm/°C

Under Range Output Code 0 0

Over Range Output Code 4095 4095

REFERENCE AND ANALOG INPUT CHARACTERISTICS

VCMO Common Mode Output Voltage 1.5 1.45

1.56

V (min)

V (max)

VCM Analog Input Common Mode Voltage 1.5 1.4

1.6

V (min)

V (max)

CIN

VIN Input Capacitance (each pin to GND)

(Note 12)

VIN = 1.5 Vdc

± 0.5 V

(CLK LOW) 8.5 pF

(CLK HIGH) 3.5 pF

VREF Internal Reference Voltage 1.2 1.176

1.224

V (min)

V (max)

TC VREF Internal Reference Voltage Tempco −40°C ≤ TA ≤ +85°C 18 ppm/°C

VRP Internal Reference Top 2 V

VRN Internal Reference Bottom 1 V

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ADC12DC105

Symbol Parameter Conditions Typical

(Note 10) Limits Units

(Limits)

Internal Reference Accuracy (VRP-VRN)10.89

1.06

V (Min)

V (max)

EXTVREF External Reference Voltage 1.20 1.176

1.224

V (min)

V (max)

Dynamic Converter Electrical Characteristics

Unless otherwise specified, the following specifications apply: AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF =

+1.2V, fCLK = 105 MHz, VCM = VCMO, CL = 5 pF/pin, . Typical values are for TA = 25°C. Boldface limits apply for TMIN ≤ TA ≤

TMAX. All other limits apply for TA = 25°C (Notes 3, 1)

Symbol Parameter Conditions Typical

(Note 10) Limits

Units

(Limits)

(Note 2)

DYNAMIC CONVERTER CHARACTERISTICS, AIN = -1dBFS

FPBW Full Power Bandwidth -1 dBFS Input, −3 dB Corner 1.0 GHz

SNR Signal-to-Noise Ratio

fIN = 10 MHz 71 dBFS

fIN = 70 MHz 70.5 dBFS

fIN =170 MHz 69.1 68 dBFS

fIN = 240 MHz 68.5 dBFS

SFDR Spurious Free Dynamic Range

fIN = 10 MHz 90 dBFS

fIN = 70 MHz 86 dBFS

fIN = 170 MHz 83 78 dBFS

fIN = 240 MHz 81 dBFS

ENOB Effective Number of Bits

fIN = 10 MHz 11.5 Bits

fIN = 70 MHz 11.4 Bits

fIN = 170 MHz 11.2 10.9 Bits

fIN = 240 MHz 11 dBFS

THD Total Harmonic Disortion

fIN = 10 MHz −86 dBFS

fIN = 70 MHz −85 dBFS

fIN = 170 MHz −84 -76.5 dBFS

fIN = 240 MHz -80 dBFS

H2 Second Harmonic Distortion

fIN = 10 MHz −95 dBFS

fIN = 70 MHz −90 dBFS

fIN = 170 MHz −83 -78 dBFS

fIN = 240 MHz -84 dBFS

H3 Third Harmonic Distortion

fIN = 10 MHz −90 dBFS

fIN = 70 MHz −86 dBFS

fIN = 170 MHz −83 -78 dBFS

fIN = 240 MHz -81 dBFS

SINAD Signal-to-Noise and Distortion Ratio

fIN = 10 MHz 70.9 dBFS

fIN = 70 MHz 70.3 dBFS

fIN = 170 MHz 69 67.4 dBFS

fIN = 240 MHz 68.2 dBFS

IMD Intermodulation Distortion fIN = 20 MHz and 21 MHz, each -7dBFS -84 dBFS

Crosstalk 0 MHz tested channel, fIN = 10 MHz at

-1dBFS other channel -100 dBFS

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ADC12DC105

Logic and Power Supply Electrical Characteristics

Unless otherwise specified, the following specifications apply: AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF =

+1.2V, fCLK = 105 MHz, VCM = VCMO, CL = 5 pF/pin. Typical values are for TA = 25°C. Boldface limits apply for TMIN ≤ TA ≤

TMAX. All other limits apply for TA = 25°C (Notes 8, 9)

Symbol Parameter Conditions Typical

(Note 10) Limits Units

(Limits)

DIGITAL INPUT CHARACTERISTICS (CLK, PD_A,PD_B)

VIN(1) Logical “1” Input Voltage VD = 3.3V 2.0 V (min)

VIN(0) Logical “0” Input Voltage VD = 3.0V 0.8 V (max)

IIN(1) Logical “1” Input Current VIN = 3.3V 10 µA

IIN(0) Logical “0” Input Current VIN = 0V −10 µA

CIN Digital Input Capacitance 5 pF

DIGITAL OUTPUT CHARACTERISTICS (DA0-DA11,DB0-DB11,DRDY)

VOUT(1) Logical “1” Output Voltage IOUT = −0.5 mA , VDR = 2.4V 2.0 V (min)

VOUT(0) Logical “0” Output Voltage IOUT = 1.6 mA, VDR = 2.4V 0.4 V (max)

+ISC Output Short Circuit Source Current VOUT = 0V −10 mA

−ISC Output Short Circuit Sink Current VOUT = VDR 10 mA

COUT Digital Output Capacitance 5 pF

POWER SUPPLY CHARACTERISTICS

IAAnalog Supply Current Full Operation 242 273 mA (max)

IDR Digital Output Supply Current Full Operation (Note 13) 32 mA

Power Consumption Excludes IDR (Note 13) 800 900 mW (max)

Power Down Power Consumption PD_A=PD_B=VA33 mW

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ADC12DC105

Timing and AC Characteristics

Unless otherwise specified, the following specifications apply: AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF =

+1.2V, fCLK = 105 MHz, VCM = VCMO, CL = 5 pF/pin. Typical values are for TA = 25°C. Timing measurements are taken at 50% of

the signal amplitude. Boldface limits apply for TMIN ≤ TA ≤ TMAX. All other limits apply for TA = 25°C (Notes 8, 9)

Symb Parameter Conditions Typical

(Note 10) Limits Units

(Limits)

Maximum Clock Frequency 105 MHz (max)

Minimum Clock Frequency 20 MHz (min)

tCH Clock High Time 4 ns

tCL Clock Low Time 4 ns

tCONV Conversion Latency 7Clock Cycles

tOD Output Delay of CLK to DATA Relative to rising edge of CLK 6.7 4.6

8.8

ns (min)

ns (max)

tSU Data Output Setup Time Relative to DRDY 4 3ns (min)

tHData Output Hold Time Relative to DRDY 5.5 3.8 ns (min)

tAD Aperture Delay 0.6 ns

tAJ Aperture Jitter 0.1 ps rms

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

guaranteed to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under

the listed test conditions. Operation of the device beyond the maximum Operating Ratings is not recommended.

Note 2: This parameter is specified in units of dBFS - indicating the value that would be attained with a full-scale input signal.

Note 3: All voltages are measured with respect to GND = AGND = DRGND = 0V, unless otherwise specified.

Note 4: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND, or VIN > VA), the current at that pin should be limited to ±5 mA. The

±50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of ±5 mA to 10.

Note 5: The maximum allowable power dissipation is dictated by TJ,max, the junction-to-ambient thermal resistance, (θJA), and the ambient temperature, (TA), and

can be calculated using the formula PD,max = (TJ,max - TA )/θJA. The values for maximum power dissipation listed above will be reached only when the device is

operated in a severe fault condition (e.g. when input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed). Such

conditions should always be avoided.

Note 6: Human Body Model is 100 pF discharged through a 1.5 kΩ resistor. Machine Model is 220 pF discharged through 0 Ω.

Note 7: Reflow temperature profiles are different for lead-free and non-lead-free packages.

Note 8: The inputs are protected as shown below. Input voltage magnitudes above VA or below GND will not damage this device, provided current is limited per

(Note 4). However, errors in the A/D conversion can occur if the input goes above 2.6V or below GND as described in the Operating Ratings section.

30073911

Note 9: With a full scale differential input of 2VP-P , the 12-bit LSB is 488 µV.

Note 10: Typical figures are at TA = 25°C and represent most likely parametric norms at the time of product characterization. The typical specifications are not

guaranteed.

Note 11: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive and negative

full-scale.

Note 12: The input capacitance is the sum of the package/pin capacitance and the sample and hold circuit capacitance.

Note 13: IDR is the current consumed by the switching of the output drivers and is primarily determined by load capacitance on the output pins, the supply voltage,

VDR, and the rate at which the outputs are switching (which is signal dependent). IDR=VDR(C0 x f0 + C1 x f1 +....C11 x f11) where VDR is the output driver power

supply voltage, Cn is total capacitance on the output pin, and fn is the average frequency at which that pin is toggling.

Note 14: This parameter is guaranteed by design and/or characterization and is not tested in production.

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ADC12DC105

Specification Definitions

APERTURE DELAY is the time after the falling edge of the

clock to when the input signal is acquired or held for conver-

sion.

APERTURE JITTER (APERTURE UNCERTAINTY) is the

variation in aperture delay from sample to sample. Aperture

jitter manifests itself as noise in the output.

CLOCK DUTY CYCLE is the ratio of the time during one cycle

that a repetitive digital waveform is high to the total time of

one period. The specification here refers to the ADC clock

input signal.

COMMON MODE VOLTAGE (VCM) is the common DC volt-

age applied to both input terminals of the ADC.

CONVERSION LATENCY is the number of clock cycles be-

tween initiation of conversion and when that data is presented

to the output driver stage. Data for any given sample is avail-

able at the output pins the Pipeline Delay plus the Output

Delay after the sample is taken. New data is available at every

clock cycle, but the data lags the conversion by the pipeline

delay.

CROSSTALK is coupling of energy from one channel into the

other channel.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

the maximum deviation from the ideal step size of 1 LSB.

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE

BITS) is another method of specifying Signal-to-Noise and

Distortion Ratio or SINAD. ENOB is defined as (SINAD -

1.76) / 6.02 and says that the converter is equivalent to a

perfect ADC of this (ENOB) number of bits.

FULL POWER BANDWIDTH is a measure of the frequency

at which the reconstructed output fundamental drops 3 dB

below its low frequency value for a full scale input.

GAIN ERROR is the deviation from the ideal slope of the

transfer function. It can be calculated as:

Gain Error = Positive Full Scale Error − Negative Full Scale

Error

It can also be expressed as Positive Gain Error and Negative

Gain Error, which are calculated as:

PGE = Positive Full Scale Error - Offset Error

NGE = Offset Error - Negative Full Scale Error

INTEGRAL NON LINEARITY (INL) is a measure of the de-

viation of each individual code from a best fit straight line. The

deviation of any given code from this straight line is measured

from the center of that code value.

INTERMODULATION DISTORTION (IMD) is the creation of

additional spectral components as a result of two sinusoidal

frequencies being applied to the ADC input at the same time.

It is defined as the ratio of the power in the intermodulation

products to the total power in the original frequencies. IMD is

usually expressed in dBFS.

LSB (LEAST SIGNIFICANT BIT) is the bit that has the small-

est value or weight of all bits. This value is VFS/2n, where

“VFS” is the full scale input voltage and “n” is the ADC reso-

lution in bits.

MISSING CODES are those output codes that will never ap-

pear at the ADC outputs. The ADC is guaranteed not to have

any missing codes.

MSB (MOST SIGNIFICANT BIT) is the bit that has the largest

value or weight. Its value is one half of full scale.

NEGATIVE FULL SCALE ERROR is the difference between

the actual first code transition and its ideal value of ½ LSB

above negative full scale.

OFFSET ERROR is the difference between the two input

voltages [(VIN+) – (VIN-)] required to cause a transition from

code 2047 to 2048.

OUTPUT DELAY is the time delay after the falling edge of the

clock before the data update is presented at the output pins.

PIPELINE DELAY (LATENCY) See CONVERSION LATEN-

CY.

POSITIVE FULL SCALE ERROR is the difference between

the actual last code transition and its ideal value of 1½ LSB

below positive full scale.

POWER SUPPLY REJECTION RATIO (PSRR) is a measure

of how well the ADC rejects a change in the power supply

voltage. PSRR is the ratio of the Full-Scale output of the ADC

with the supply at the minimum DC supply limit to the Full-

Scale output of the ADC with the supply at the maximum DC

supply limit, expressed in dB.

SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in

dB, of the rms value of the input signal to the rms value of the

sum of all other spectral components below one-half the sam-

pling frequency, not including harmonics or DC.

SIGNAL TO NOISE PLUS DISTORTION (S/N+D or

SINAD) Is the ratio, expressed in dB, of the rms value of the

input signal to the rms value of all of the other spectral com-

ponents below half the clock frequency, including harmonics

but excluding d.c.

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-

ence, expressed in dB, between the rms values of the input

signal and the peak spurious signal, where a spurious signal

is any signal present in the output spectrum that is not present

at the input.

TOTAL HARMONIC DISTORTION (THD) is the ratio, ex-

pressed in dB, of the rms total of the first six harmonic levels

at the output to the level of the fundamental at the output. THD

is calculated as:

where f1 is the RMS power of the fundamental (output) fre-

quency and f2 through f7 are the RMS power of the first 6

harmonic frequencies in the output spectrum.

SECOND HARMONIC DISTORTION (2ND HARM) is the dif-

ference expressed in dB, between the RMS power in the input

frequency at the output and the power in its 2nd harmonic

level at the output.

THIRD HARMONIC DISTORTION (3RD HARM) is the dif-

ference, expressed in dB, between the RMS power in the

input frequency at the output and the power in its 3rd harmonic

level at the output.

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ADC12DC105

Timing Diagrams

30073909

FIGURE 1. Output Timing

Transfer Characteristic

30073910

FIGURE 2. Transfer Characteristic

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ADC12DC105

Typical Performance Characteristics DNL, INL Unless otherwise specified, the following

specifications apply: AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF = +1.2V, fCLK = 105 MHz, 50% Duty Cycle,

DCS disabled, VCM = VCMO, TA = 25°C.

DNL

30073941

INL

30073942

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ADC12DC105

Typical Performance Characteristics Unless otherwise specified, the following specifications apply:

AGND = DRGND = 0V, VA = +3.3V, VDR = +2.5V, Internal VREF = +1.2V, fCLK = 105 MHz, 50% Duty Cycle, DCS disabled, VCM =

VCMO, fIN = 170 MHz, TA = 25°C.

SNR, SINAD, SFDR vs. VA

30073951

Distortion vs. VA

30073952

SNR, SINAD, SFDR vs. Clock Duty Cycle, fIN=40 MHz

30073957

Distortion vs. Clock Duty Cycle, fIN=40 MHz

30073958

SNR, SINAD, SFDR vs. Clock Duty Cycle, DCS Enabled,

fIN=40 MHz

30073959

Distortion vs. Clock Duty Cycle, DCS Enabled, fIN=40 MHz

30073960

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ADC12DC105

SNR and SFDR vs. fIN

30073978

POWER vs. fCLK

30073976

Spectral Response @ 10 MHz Input

30073968

Spectral Response @ 70 MHz Input

30073969

Spectral Response @ 170 MHz Input

30073970

IMD, fIN1 = 20 MHz, fIN2 = 21 MHz

30073971

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ADC12DC105

Functional Description

Operating on a single +3.0V or +3.3V supply, the AD-

C12DC105 digitizes two differential analog input signals to 12

bits, using a differential pipelined architecture with error cor-

rection circuitry and an on-chip sample-and-hold circuit to

ensure maximum performance. The user has the choice of

using an internal 1.2V stable reference, or using an external

1.2V reference. Any external reference is buffered on-chip to

ease the task of driving that pin. Duty cycle stabilization and

output data format are selectable using the quad state func-

tion OF/DCS pin (pin 19). The output data can be set for offset

binary or two's complement.

Applications Information

1.0 OPERATING CONDITIONS

We recommend that the following conditions be observed for

operation of the ADC12DC105:

2.7V ≤ VA ≤ 3.6V

2.4V ≤ VDR ≤ VA

20 MHz ≤ fCLK ≤ 105 MHz

1.2V internal reference

VREF = 1.2V (for an external reference)

VCM = 1.5V (from VCMO)

2.0 ANALOG INPUTS

2.1 Signal Inputs

2.1.1 Differential Analog Input Pins

The ADC12DC105 has a pair of analog signal input pins for

each of two channels. VIN+ and VIN− form a differential input

pair. The input signal, VIN, is defined as:

VIN = (VIN+) – (VIN−)

Figure 3 shows the expected input signal range. Note that the

common mode input voltage, VCM, should be 1.5V. Using

VCMO (pins 7,9) for VCM will ensure the proper input common

mode level for the analog input signal. The positive peaks of

the individual input signals should each never exceed 2.6V.

Each analog input pin of the differential pair should have a

maximum peak-to-peak voltage of 1V, be 180° out of phase

with each other and be centered around VCM.The peak-to-

peak voltage swing at each analog input pin should not ex-

ceed the 1V or the output data will be clipped.

30073980

FIGURE 3. Expected Input Signal Range

For single frequency sine waves the full scale error in LSB

can be described as approximately:

EFS = 4096 ( 1 - sin (90° + dev))

Where dev is the angular difference in degrees between the

two signals having a 180° relative phase relationship to each

other (see Figure 4). For single frequency inputs, angular er-

rors result in a reduction of the effective full scale input. For

complex waveforms, however, angular errors will result in

distortion.

30073981

FIGURE 4. Angular Errors Between the Two Input Signals

Will Reduce the Output Level or Cause Distortion

It is recommended to drive the analog inputs with a source

impedance less than 100Ω. Matching the source impedance

for the differential inputs will improve even ordered harmonic

performance (particularly second harmonic).

Table 1 indicates the input to output relationship of the AD-

C12DC105.

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ADC12DC105

TABLE 1. Input to Output Relationship

VIN+VIN−Binary Output 2’s Complement Output

VCM − VREF/2 VCM + VREF/2 00 0000 0000 00 10 0000 0000 00 Negative Full-Scale

VCM − VREF/4 VCM + VREF/4 01 0000 0000 00 11 0000 0000 00

VCM VCM 10 0000 0000 00 00 0000 0000 00 Mid-Scale

VCM + VREF/4 VCM − VREF/4 11 0000 0000 00 01 0000 0000 00

VCM + VREF/2 VCM − VREF/2 11 1111 1111 11 01 1111 1111 11 Positive Full-Scale

2.1.2 Driving the Analog Inputs

The VIN+ and the VIN− inputs of the ADC12DC105 have an

internal sample-and-hold circuit which consists of an analog

switch followed by a switched-capacitor amplifier.

Figure 5 and Figure 6 show examples of single-ended to dif-

ferential conversion circuits. The circuit in Figure 5 works well

for input frequencies up to approximately 70MHz, while the

circuit inFigure 6 works well above 70MHz.

30073982

FIGURE 5. Low Input Frequency Transformer Drive Circuit

30073983

FIGURE 6. High Input Frequency Transformer Drive Circuit

One short-coming of using a transformer to achieve the sin-

gle-ended to differential conversion is that most RF trans-

formers have poor low frequency performance. A differential

amplifier can be used to drive the analog inputs for low fre-

quency applications. The amplifier must be fast enough to

settle from the charging glitches on the analog input resulting

from the sample-and-hold operation before the clock goes

high and the sample is passed to the ADC core.

2.1.3 Input Common Mode Voltage

The input common mode voltage, VCM, should be in the range

of 1.4V to 1.6V and be a value such that the peak excursions

of the analog signal do not go more negative than ground or

more positive than 2.6V. It is recommended to use VCMO (pins

7,9) as the input common mode voltage.

If the ADC12DC105 is operated with VA=3.6V, a resistor of

approximately 1KΩ should be used from the VCMO pin to AG-

ND. This will help maintain stability over the entire tempera-

ture range when using a high supply voltage.

2.2 Reference Pins

The ADC12DC105 is designed to operate with an internal or

external 1.2V reference. The internal 1.2 Volt reference is the

default condition when no external reference input is applied

to the VREF pin. If a voltage is applied to the VREF pin, then

that voltage is used for the reference. The VREF pin should

always be bypassed to ground with a 0.1 µF capacitor close

to the reference input pin. Do not load this pin when using the

internal reference.

It is important that all grounds associated with the reference

voltage and the analog input signal make connection to the

ground plane at a single, quiet point to minimize the effects of

noise currents in the ground path.

The Reference Bypass Pins (VRP, VCMO, and VRN) for chan-

nels A and B are made available for bypass purposes. These

pins should each be bypassed to AGND with a low ESL

(equivalent series inductance) 0.1 µF capacitor placed very

close to the pin to minimize stray inductance. A 0.1 µF ca-

pacitor should be placed between VRP and VRN as close to

the pins as possible, and a 1 µF capacitor should be placed

in parallel. This configuration is shown in Figure 7. It is nec-

15 www.national.com

ADC12DC105

essary to avoid reference oscillation, which could result in

reduced SFDR and/or SNR. VCMO may be loaded to 1mA for

use as a temperature stable 1.5V reference. The remaining

pins should not be loaded.

Smaller capacitor values than those specified will allow faster

recovery from the power down mode, but may result in de-

graded noise performance. Loading any of these pins, other

than VCMO may result in performance degradation.

The nominal voltages for the reference bypass pins are as

follows:

VCMO = 1.5 V

VRP = 2.0 V

VRN = 1.0 V

2.3 OF/DCS Pin

Duty cycle stabilization and output data format are selectable

using this quad state function pin. When enabled, duty cycle

stabilization can compensate for clock inputs with duty cycles

ranging from 30% to 70% and generate a stable internal clock,

improving the performance of the part. With OF/DCS = VA the

output data format is 2's complement and duty cycle stabi-

lization is not used. With OF/DCS = AGND the output data

format is offset binary and duty cycle stabilization is not used.

With OF/DCS = (2/3)*VA the output data format is 2's com-

plement and duty cycle stabilization is applied to the clock. If

OF/DCS is (1/3)*VA the output data format is offset binary and

duty cycle stabilization is applied to the clock. While the sense

of this pin may be changed "on the fly," doing this is not rec-

ommended as the output data could be erroneous for a few

clock cycles after this change is made.

Note: This signal has no effect when SPI_EN is high and the

serial control interface is enabled.

3.0 DIGITAL INPUTS

Digital CMOS compatible inputs consist of CLK, PD_A, and

PD_B.

3.1 Clock Input

The CLK controls the timing of the sampling process. To

achieve the optimum noise performance, the clock input

should be driven with a stable, low jitter clock signal in the

range indicated in the Electrical Table. The clock input signal

should also have a short transition region. This can be

achieved by passing a low-jitter sinusoidal clock source

through a high speed buffer gate. The trace carrying the clock

signal should be as short as possible and should not cross

any other signal line, analog or digital, not even at 90°.

The clock signal also drives an internal state machine. If the

clock is interrupted, or its frequency is too low, the charge on

the internal capacitors can dissipate to the point where the

accuracy of the output data will degrade. This is what limits

the minimum sample rate.

The clock line should be terminated at its source in the char-

acteristic impedance of that line. Take care to maintain a

constant clock line impedance throughout the length of the

line. Refer to Application Note AN-905 for information on set-

ting characteristic impedance.

It is highly desirable that the the source driving the ADC clock

pins only drive that pin. However, if that source is used to drive

other devices, then each driven pin should be AC terminated

with a series RC to ground, such that the resistor value is

equal to the characteristic impedance of the clock line and the

capacitor value is:

where tPD is the signal propagation rate down the clock line,

"L" is the line length and ZO is the characteristic impedance

of the clock line. This termination should be as close as pos-

sible to the ADC clock pin but beyond it as seen from the clock

source. Typical tPD is about 150 ps/inch (60 ps/cm) on FR-4

board material. The units of "L" and tPD should be the same

(inches or centimeters).

The duty cycle of the clock signal can affect the performance

of the A/D Converter. Because achieving a precise duty cycle

is difficult, the ADC12DC105 has a Duty Cycle Stabilizer.

4.0 DIGITAL OUTPUTS

Digital outputs consist of the CMOS signals DA0-DA11, DB0-

DB11, and DRDY.

The ADC12DC105 has 12 CMOS compatible data output pins

corresponding to the converted input value for each channel,

and a data ready (DRDY) signal that should be used to cap-

ture the output data. Valid data is present at these outputs

while the PD pin is low. Data should be captured and latched

with the rising edge of the DRDY signal.

Be very careful when driving a high capacitance bus. The

more capacitance the output drivers must charge for each

conversion, the more instantaneous digital current flows

through VDR and DRGND. These large charging current

spikes can cause on-chip ground noise and couple into the

analog circuitry, degrading dynamic performance. Adequate

bypassing, limiting output capacitance and careful attention

to the ground plane will reduce this problem. The result could

be an apparent reduction in dynamic performance.

www.national.com 16

ADC12DC105

30073985

FIGURE 7. Application Circuit

17 www.national.com

ADC12DC105

5.0 POWER SUPPLY CONSIDERATIONS

The power supply pins should be bypassed with a 0.1 µF ca-

pacitor and with a 100 pF ceramic chip capacitor close to each

power pin. Leadless chip capacitors are preferred because

they have low series inductance.

As is the case with all high-speed converters, the AD-

C12DC105 is sensitive to power supply noise. Accordingly,

the noise on the analog supply pin should be kept below 100

mVP-P.

No pin should ever have a voltage on it that is in excess of the

supply voltages, not even on a transient basis. Be especially

careful of this during power turn on and turn off.

6.0 LAYOUT AND GROUNDING

Proper grounding and proper routing of all signals are essen-

tial to ensure accurate conversion. Maintaining separate ana-

log and digital areas of the board, with the ADC12DC105

between these areas, is required to achieve specified perfor-

mance.

Capacitive coupling between the typically noisy digital circuit-

ry and the sensitive analog circuitry can lead to poor perfor-

mance. The solution is to keep the analog circuitry separated

from the digital circuitry, and to keep the clock line as short as

possible.

Since digital switching transients are composed largely of

high frequency components, total ground plane copper

weight will have little effect upon the logic-generated noise.

This is because of the skin effect. Total surface area is more

important than is total ground plane area.

Generally, analog and digital lines should cross each other at

90° to avoid crosstalk. To maximize accuracy in high speed,

high resolution systems, however, avoid crossing analog and

digital lines altogether. It is important to keep clock lines as

short as possible and isolated from ALL other lines, including

other digital lines. Even the generally accepted 90° crossing

should be avoided with the clock line as even a little coupling

can cause problems at high frequencies. This is because oth-

er lines can introduce jitter into the clock line, which can lead

to degradation of SNR. Also, the high speed clock can intro-

duce noise into the analog chain.

Best performance at high frequencies and at high resolution

is obtained with a straight signal path. That is, the signal path

through all components should form a straight line wherever

possible.

Be especially careful with the layout of inductors and trans-

formers. Mutual inductance can change the characteristics of

the circuit in which they are used. Inductors and transformers

should not be placed side by side, even with just a small part

of their bodies beside each other. For instance, place trans-

formers for the analog input and the clock input at 90° to one

another to avoid magnetic coupling.

The analog input should be isolated from noisy signal traces

to avoid coupling of spurious signals into the input. Any ex-

ternal component (e.g., a filter capacitor) connected between

the converter's input pins and ground or to the reference input

pin and ground should be connected to a very clean point in

the ground plane.

All analog circuitry (input amplifiers, filters, reference compo-

nents, etc.) should be placed in the analog area of the board.

All digital circuitry and dynamic I/O lines should be placed in

the digital area of the board. The ADC12DC105 should be

between these two areas. Furthermore, all components in the

reference circuitry and the input signal chain that are con-

nected to ground should be connected together with short

traces and enter the ground plane at a single, quiet point. All

ground connections should have a low inductance path to

ground.

7.0 DYNAMIC PERFORMANCE

To achieve the best dynamic performance, the clock source

driving the CLK input must have a sharp transition region and

be free of jitter. Isolate the ADC clock from any digital circuitry

with buffers, as with the clock tree shown in Figure 8. The

gates used in the clock tree must be capable of operating at

frequencies much higher than those used if added jitter is to

be prevented.

As mentioned in Section 3.1 Clock Input, it is good practice to

keep the ADC clock line as short as possible and to keep it

well away from any other signals. Other signals can introduce

jitter into the clock signal, which can lead to reduced SNR

performance, and the clock can introduce noise into other

lines. Even lines with 90° crossings have capacitive coupling,

so try to avoid even these 90° crossings of the clock line.

30073986

FIGURE 8. Isolating the ADC Clock from other Circuitry

with a Clock Tree

www.national.com 18

ADC12DC105

Physical Dimensions inches (millimeters) unless otherwise noted

TOP View...............................SIDE View...............................BOTTOM View

60-Lead LLP Package

Ordering Number:

ADC12DC105CISQ

NS Package Number SQA60A

19 www.national.com

ADC12DC105

Notes

ADC12DC105 Dual 12-Bit, 105 MSPS A/D Converter with CMOS Outputs

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