ADC088S052CIMTX - National Semiconductor

ADC088S052

ADC088S052 8-Channel, 200 ksps to 500 ksps, 8-Bit A/D Converter

Literature Number: SNAS340E

ADC088S052

January 28, 2011

8-Channel, 200 ksps to 500 ksps, 8-Bit A/D Converter

General Description

The ADC088S052 is a low-power, eight-channel CMOS 8-bit

analog-to-digital converter specified for conversion through-

put rates of 200 ksps to 500 ksps. The converter is based on

a successive-approximation register architecture with an in-

ternal track-and-hold circuit. It can be configured to accept up

to eight input signals at inputs IN0 through IN7.

The output serial data is straight binary and is compatible with

several standards, such as SPI™, QSPI™, MICROWIRE™,

and many common DSP serial interfaces.

The ADC088S052 may be operated with independent analog

and digital supplies. The analog supply (VA) can range from

+2.7V to +5.25V, and the digital supply (VD) can range from

+2.7V to VA. Normal power consumption using a +3V or +5V

supply is 1.2 mW and 6.5 mW, respectively. The power-down

feature reduces the power consumption to 0.03 µW using a

+3V supply and 0.15 µW using a +5V supply.

The ADC088S052 is packaged in a 16-lead TSSOP package.

Operation over the extended industrial temperature range of

−40°C to +105°C is guaranteed.

Features

■Eight input channels

■Variable power management

■Independent analog and digital supplies

■SPI/QSPI/MICROWIRE/DSP compatible

■Packaged in 16-lead TSSOP

Key Specifications

■Conversion Rate 200 ksps to 500 ksps

■DNL (VA = VD = 2.7V to 5.0V) ±0.2 LSB (max)

■INL (VA = VD = 2.7V to 5.0V) ±0.2 LSB (max)

■Power Consumption

■—3V Supply 1.2 mW (typ)

—5V Supply 6.5 mW (typ)

Applications

■Automotive Navigation

■Portable Systems

■Medical Instruments

■Mobile Communications

■Instrumentation and Control Systems

Connection Diagram

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Ordering Information

Order Code Temperature Range Description

ADC088S052CIMT −40°C to +105°C 16-Lead TSSOP Package

ADC088S052CIMTX −40°C to +105°C 16-Lead TSSOP Package, Tape & Reel

ADC088S052EVAL Evaluation Board

SPI™ is a trademark of Motorola, Inc.

ADC088S052 8-Channel, 200 ksps to 500 ksps, 8-Bit A/D Converter

Block Diagram

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Pin Descriptions and Equivalent Circuits

Pin No. Symbol Equivalent Circuit Description

ANALOG I/O

4 - 11 IN0 to IN7 Analog inputs. These signals can range from 0V to VREF.

DIGITAL I/O

16 SCLK

Digital clock input. The guaranteed performance range of

frequencies for this input is 3.2 MHz to 8 MHz. This clock directly

controls the conversion and readout processes.

15 DOUT Digital data output. The output samples are clocked out of this pin

on the falling edges of the SCLK pin.

14 DIN Digital data input. The ADC088S052's Control Register is loaded

through this pin on rising edges of the SCLK pin.

1 CS Chip select. On the falling edge of CS, a conversion process

begins. Conversions continue as long as CS is held low.

POWER SUPPLY

2VA

Positive analog supply pin. This voltage is also used as the

reference voltage. This pin should be connected to a quiet +2.7V

to +5.25V source and bypassed to GND with 1 µF and 0.1 µF

monolithic ceramic capacitors located within 1 cm of the power pin.

13 VD

Positive digital supply pin. This pin should be connected to a +2.7V

to VA supply, and bypassed to GND with a 0.1 µF monolithic

ceramic capacitor located within 1 cm of the power pin.

3 AGND The ground return for the analog supply and signals.

12 DGND The ground return for the digital supply and signals.

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ADC088S052

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Analog Supply Voltage VA−0.3V to 6.5V

Digital Supply Voltage VD−0.3V to VA + 0.3V,

max 6.5V

Voltage on Any Pin to GND −0.3V to VA +0.3V

Input Current at Any Pin (Note 3) ±10 mA

Package Input Current(Note 3) ±20 mA

Power Dissipation at TA = 25°C See (Note 4)

ESD Susceptibility (Note 5)

Human Body Model

Machine Model

2500V

250V

For soldering specifications:

see product folder at www.national.com and

www.national.com/ms/MS/MS-SOLDERING.pdf

Junction Temperature +150°C

Storage Temperature −65°C to +150°C

Operating Ratings (Note 1, Note 2)

Operating Temperature −40°C ≤ TA ≤ +105°

VA Supply Voltage +2.7V to +5.25V

VD Supply Voltage +2.7V to VA

Digital Input Voltage 0V to VA

Analog Input Voltage 0V to VA

Clock Frequency 50 kHz to 16 MHz

Package Thermal Resistance

Package θJA

16-lead TSSOP on 4-

layer, 2 oz. PCB 96°C / W

ADC088S052 Converter Electrical Characteristics (Note 7)

The following specifications apply for VA = VD = +2.7V to +5.25V, AGND = DGND = 0V, fSCLK = 3.2 MHz to 8 MHz, fSAMPLE = 200

ksps to 500 ksps, and CL = 50pF, unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°

Symbol Parameter Conditions Typical Limits

(Note 6)Units

STATIC CONVERTER CHARACTERISTICS

Resolution with No Missing Codes 8Bits

INL Integral Non-Linearity (End Point

Method) ±0.05 ±0.2 LSB (max)

DNL Differential Non-Linearity ±0.06 ±0.2 LSB (min)

VOFF Offset Error +0.6 ±0.7 LSB (max)

OEM Offset Error Match ±0.02 ±0.2 LSB (max)

FSE Full Scale Error +0.5 ±0.6 LSB (max)

FSEM Full Scale Error Match ±0.01 ±0.2 LSB (max)

DYNAMIC CONVERTER CHARACTERISTICS

FPBW Full Power Bandwidth (−3dB) 8 MHz

SINAD Signal-to-Noise Plus Distortion Ratio fIN = 40.2 kHz, −0.02 dBFS 49.6 49.2 dB (min)

SNR Signal-to-Noise Ratio fIN = 40.2 kHz, −0.02 dBFS 49.6 49.3 dB (min)

THD Total Harmonic Distortion fIN = 40.2 kHz, −0.02 dBFS −70.6 −63.1 dB (max)

SFDR Spurious-Free Dynamic Range fIN = 40.2 kHz, −0.02 dBFS 68.1 64.8 dB (min)

ENOB Effective Number of Bits fIN = 40.2 kHz 7.95 7.88 Bits (min)

ISO Channel-to-Channel Isolation fIN = 20 kHz 67.3 dB

IMD

Intermodulation Distortion, Second

Order Terms fa = 19.5 kHz, fb = 20.5 kHz −75.0 dB

Intermodulation Distortion, Third Order

Terms fa = 19.5 kHz, fb = 20.5 kHz −69.4 dB

ANALOG INPUT CHARACTERISTICS

VIN Input Range 0 to VAV

IDCL DC Leakage Current ±1 µA (max)

CINA Input Capacitance Track Mode 33 pF

Hold Mode 3 pF

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ADC088S052

Symbol Parameter Conditions Typical Limits

(Note 6)Units

DIGITAL INPUT CHARACTERISTICS

VIH Input High Voltage VA = VD = +2.7V to +3.6V 2.1 V (min)

VA = VD = +4.75V to +5.25V 2.4 V (min)

VIL Input Low Voltage 0.8 V (max)

IIN Input Current VIN = 0V or VD±0.01 ±1 µA (max)

CIND Digital Input Capacitance 2 4pF (max)

DIGITAL OUTPUT CHARACTERISTICS

VOH Output High Voltage ISOURCE = 200 µA, VD − 0.5 V (min)

VOL Output Low Voltage ISINK = 200 µA to 1.0 mA, 0.4 V (max)

IOZH, IOZL Hi-Impedance Output Leakage Current ±1 µA (max)

COUT

Hi-Impedance Output Capacitance

(Note 7) 2 4pF (max)

Output Coding Straight (Natural) Binary

POWER SUPPLY CHARACTERISTICS (CL = 10 pF)

VA, VDAnalog and Digital Supply Voltages VA ≥ VD 2.7 V (min)

5.25 V (max)

IA + ID

Total Supply Current

Normal Mode ( CS low)

VA = VD = +2.7V to +3.6V,

fSAMPLE = 500 kSPS, fIN = 40 kHz 0.4 1.0 mA (max)

VA = VD = +4.75V to +5.25V,

fSAMPLE = 500 kSPS, fIN = 40 kHz 1.3 1.7 mA (max)

Total Supply Current

Shutdown Mode (CS high)

VA = VD = +2.7V to +3.6V,

fSCLK = 0 ksps 10 nA

VA = VD = +4.75V to +5.25V,

fSCLK = 0 ksps 30 nA

Power Consumption

Normal Mode ( CS low)

VA = VD = +3.0V

fSAMPLE = 500 kSPS, fIN = 40 kHz 1.2 3.0 mW (max)

VA = VD = +5.0V

fSAMPLE = 500 kSPS, fIN = 40 kHz 6.5 8.5 mW (max)

Power Consumption

Shutdown Mode (CS high)

VA = VD = +3.0V

fSCLK = 0 ksps 0.03 µW

VA = VD = +5.0V

fSCLK = 0 ksps 0.15 µW

AC ELECTRICAL CHARACTERISTICS

fSCLKMIN Minimum Clock Frequency 0.8 3.2 MHz (min)

fSCLK Maximum Clock Frequency 16 8MHz (max)

Sample Rate

Continuous Mode 50 200 ksps (min)

1000 500 ksps (max)

tCONVERT Conversion (Hold) Time 13 SCLK cycles

DC SCLK Duty Cycle 30 40 % (min)

70 60 % (max)

tACQ Acquisition (Track) Time 3SCLK cycles

Throughput Time Acquisition Time + Conversion Time 16 SCLK cycles

tAD Aperture Delay 4 ns

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ADC088S052

ADC088S052 Timing Specifications

The following specifications apply for VA = VD = +2.7V to +5.25V, AGND = DGND = 0V, fSCLK = 3.2 MHz to 8 MHz, fSAMPLE = 200

ksps to 500 ksps, and CL = 50pF. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°C.

Symbol Parameter Conditions Typical Limits

(Note 6)Units

tCSH CS Hold Time after SCLK Rising Edge 0 10 ns (min)

tCSS

CS Setup Time prior to SCLK Rising

Edge 5 10 ns (min)

tEN CS Falling Edge to DOUT enabled 5 30 ns (max)

tDACC

DOUT Access Time after SCLK Falling

Edge 17 27 ns (max)

tDHLD

DOUT Hold Time after SCLK Falling

Edge 4 ns (typ)

tDS

DIN Setup Time prior to SCLK Rising

Edge 3 10 ns (min)

tDH DIN Hold Time after SCLK Rising Edge 3 10 ns (min)

tCH SCLK High Time 0.4 x tSCLK ns (min)

tCL SCLK Low Time 0.4 x tSCLK ns (min)

tDIS

CS Rising Edge to DOUT High-

Impedance

DOUT falling 2.4 20 ns (max)

DOUT rising 0.9 20 ns (max)

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

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.

Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified.

Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND or VIN > VA or VD), the current at that pin should be limited to 10 mA.

The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 10 mA to two.

Note 4: The absolute maximum junction temperature (TJmax) for this device is 150°C. The maximum allowable power dissipation is dictated by TJmax, the

junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA)/θJA. In the 16-

pin TSSOP, θJA is 96°C/W, so PDMAX = 1,200 mW at 25°C and 625 mW at the maximum operating ambient temperature of 105°C. Note that the power consumption

of this device under normal operation is a maximum of 12 mW. The values for maximum power dissipation listed above will be reached only when the ADC088S052

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).

Obviously, such conditions should always be avoided.

Note 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO ohms

Note 6: Tested limits are guaranteed to National's AOQL (Average Outgoing Quality Level).

Note 7: The min/max specification limits are guaranteed by design, test, or statistical analysis.

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ADC088S052

Timing Diagrams

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FIGURE 1. ADC088S052 Operational Timing Diagram

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FIGURE 2. ADC088S052 Serial Timing Diagram

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FIGURE 3. SCLK and CS Timing Parameters

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ADC088S052

Specification Definitions

ACQUISITION TIME is the time required for the ADC to ac-

quire the input voltage. During this time, the hold capacitor is

charged by the input voltage.

APERTURE DELAY is the time between the fourth falling

edge of SCLK and the time when the input signal is internally

acquired or held for conversion.

CONVERSION TIME is the time required, after the input volt-

age is acquired, for the ADC to convert the input voltage to a

digital word.

CHANNEL-TO-CHANNEL ISOLATION is resistance to cou-

pling of energy from one channel into another channel.

CROSSTALK is the coupling of energy from one channel into

another channel. This is similar to Channel-to-Channel Isola-

tion, except for the sign of the data.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

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

DUTY CYCLE is the ratio of the time that a repetitive digital

waveform is high to the total time of one period. The specifi-

cation here refers to the SCLK.

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE

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

Distortion 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.

FULL SCALE ERROR (FSE) is a measure of how far the last

code transition is from the ideal 1½ LSB below VREF+ and is

defined as:

VFSE = Vmax + 1.5 LSB – VREF+

where Vmax is the voltage at which the transition to the maxi-

mum code occurs. FSE can be expressed in Volts, LSB or

percent of full scale range.

GAIN ERROR is the deviation of the last code transition

(111...110) to (111...111) from the ideal (VREF - 1.5 LSB), after

adjusting for offset error.

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

viation of each individual code from a line drawn from negative

full scale (½ LSB below the first code transition) through pos-

itive full scale (½ LSB above the last code transition). 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 an individual ADC input at the

same time. It is defined as the ratio of the power in the second

and third order intermodulation products to the power in one

of the original frequencies. Second order products are fa ±

fb, where fa and fb are the two sine wave input frequencies.

Third order products are (2fa ± fb ) and (fa ± 2fb). IMD is usually

expressed in dB.

MISSING CODES are those output codes that will never ap-

pear at the ADC outputs. These codes cannot be reached with

any input value. The ADC088S052 is guaranteed not to have

any missing codes.

OFFSET ERROR is the deviation of the first code transition

(000...000) to (000...001) from the ideal (i.e. GND + 0.5 LSB).

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 d.c.

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, including harmonics but excluding d.c.

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

pressed in dBc, of the rms total of the first five harmonic

components at the output to the rms level of the input signal

frequency as seen at the output. THD is calculated as

where Af1 is the RMS power of the input frequency at the out-

put and Af2 through Af6 are the RMS power in the first 5

harmonic frequencies.

THROUGHPUT TIME is the minimum time required between

the start of two successive conversions. It is the acquisition

time plus the conversion and read out times. In the case of

the ADC088S052, this is 16 SCLK periods.

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ADC088S052

Typical Performance Characteristics VA = VD = +5.0V, TA = +25°C, fSAMPLE = 500 ksps, fSCLK = 8 MHz,

fIN = 40.2 kHz unless otherwise stated.

DNL

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DNL

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INL

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INL

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DNL vs. Supply

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INL vs. Supply

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ADC088S052

SNR vs. Supply

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THD vs. Supply

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ENOB vs. Supply

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DNL vs. VD with VA = 5.0 V

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INL vs. VD with VA = 5.0 V

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DNL vs. SCLK Duty Cycle

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ADC088S052

INL vs. SCLK Duty Cycle

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SNR vs. SCLK Duty Cycle

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THD vs. SCLK Duty Cycle

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ENOB vs. SCLK Duty Cycle

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DNL vs. SCLK

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INL vs. SCLK

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ADC088S052

SNR vs. SCLK

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THD vs. SCLK

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ENOB vs. SCLK

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DNL vs. Temperature

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INL vs. Temperature

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SNR vs. Temperature

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ADC088S052

THD vs. Temperature

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ENOB vs. Temperature

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SNR vs. Input Frequency

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THD vs. Input Frequency

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ENOB vs. Input Frequency

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Power Consumption vs. SCLK

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ADC088S052

1.0 Functional Description

The ADC088S052 is a successive-approximation analog-to-

digital converter designed around a charge-redistribution dig-

ital-to-analog converter.

1.1 ADC088S052 OPERATION

Simplified block diagrams of the ADC088S052 in both track

and hold operation are shown in Figure 4 and Figure 5 re-

spectively. In Figure 4, the ADC088S052 is in track mode:

switch SW1 connects the sampling capacitor to one of eight

analog input channels through the multiplexer, and SW2 bal-

ances the comparator inputs. The ADC088S052 is in this

state for the first three SCLK cycles after CS is brought low.

Figure 5 shows the ADC088S052 in hold mode: switch SW1

connects the sampling capacitor to ground, maintaining the

sampled voltage, and switch SW2 unbalances the compara-

tor. The control logic then instructs the charge-redistribution

DAC to add or subtract fixed amounts of charge to or from the

sampling capacitor until the comparator is balanced. When

the comparator is balanced, the digital word supplied to the

DAC is the digital representation of the analog input voltage.

The ADC088S052 is in this state for the last thirteen SCLK

cycles after CS is brought low.

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FIGURE 4. ADC088S052 in Track Mode

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FIGURE 5. ADC088S052 in Hold Mode

1.2 SERIAL INTERFACE

An operational timing diagram and a serial interface timing

diagram for the ADC088S052 are shown in The Timing Dia-

grams section. CS, chip select, initiates conversions and

frames the serial data transfers. SCLK (serial clock) controls

both the conversion process and the timing of serial data.

DOUT is the serial data output pin, where a conversion result

is sent as a serial data stream, MSB first. Data to be written

to the ADC088S052's Control Register is placed at DIN, the

serial data input pin. New data is written to the ADC at DIN

with each conversion.

A serial frame is initiated on the falling edge of CS and ends

on the rising edge of CS. Each frame must contain an integer

multiple of 16 rising SCLK edges. The ADC's DOUT pin is in

a high impedance state when CS is high and is active when

CS is low. Thus, CS acts as an output enable. Similarly, SCLK

is internally gated off when CS is brought high.

During the first 3 cycles of SCLK, the ADC is in the track

mode, acquiring the input voltage. For the next 13 SCLK cy-

cles the conversion is accomplished and the data is clocked

out. SCLK falling edges 1 through 4 clock out leading zeros,

falling edges 5 through 12 clock out the conversion result,

MSB first, and falling edges 13 through 16 clock out trailing

zeros. If there is more than one conversion in a frame (con-

tinuous conversion mode), the ADC will re-enter the track

mode on the falling edge of SCLK after the N*16th rising edge

of SCLK and re-enter the hold/convert mode on the N*16+4th

falling edge of SCLK. "N" is an integer value.

The ADC088S052 enters track mode under three different

conditions. In Figure 1, CS goes low with SCLK high and the

ADC enters track mode on the first falling edge of SCLK. In

the second condition, CS goes low with SCLK low. Under this

condition, the ADC automatically enters track mode and the

falling edge of CS is seen as the first falling edge of SCLK. In

the third condition, CS and SCLK go low within the times de-

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ADC088S052

fined by tCSU and tCLH. While there is no timing restriction with

respect to the rising edges of CS and SCLK, it is best to strictly

observe the minimum tCSU and TCLH times given in the Timing

Specifications to prevent uncertainty as to when date begins

clocking in at DIN.

While a conversion is in progress, the address of the next

input for conversion is clocked into a control register through

the DIN pin on the first 8 rising edges of SCLK after the fall of

CS. See Tables 1, 2, 3. That is, the address for the NEXT

conversion to be run is loaded at the beginning of the current

conversion.

There is no need to incorporate a power-up delay or dummy

conversion as the ADC088S052 is able to acquire the input

signal to full resolution in the first conversion immediately fol-

lowing power-up. The first conversion result after power-up

will be that of IN0.

TABLE 1. Control Register Bits

Bit 7 (MSB) Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

DONTC DONTC ADD2 ADD1 ADD0 DONTC DONTC DONTC

TABLE 2. Control Register Bit Descriptions

Bit #: Symbol: Description

7, 6, 2, 1, 0 DONTC Don't care. The values of these bits do not affect the device.

5 ADD2 These three bits determine which input channel will be sampled and converted

at the next conversion cycle. The mapping between codes and channels is

shown in Table 3.

4 ADD1

3 ADD0

TABLE 3. Input Channel Selection

ADD2 ADD1 ADD0 Input Channel

0 0 0 IN0 (Default)

0 0 1 IN1

0 1 0 IN2

0 1 1 IN3

1 0 0 IN4

1 0 1 IN5

1 1 0 IN6

1 1 1 IN7

1.3 ADC088S052 TRANSFER FUNCTION

The output format of the ADC088S052 is straight binary.

Code transitions occur midway between successive integer

LSB values. The LSB width for the ADC088S052 is VA / 256.

The ideal transfer characteristic is shown in Figure 6. The

transition from an output code of 0000 0000 to a code of 0000

0001 is at 1/2 LSB, or a voltage of VA / 512. Other code tran-

sitions occur at steps of one LSB.

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FIGURE 6. Ideal Transfer Characteristic

1.4 ANALOG INPUTS

An equivalent circuit for one of the ADC088S052's input chan-

nels is shown in Figure 7. Diodes D1 and D2 provide ESD

protection for the analog inputs. The operating range for the

analog inputs is 0 V to VA. Going beyond this range will cause

the ESD diodes to conduct and result in erratic operation. For

this reason, these ESD diodes should NOT be used to clamp

the input signal.

The capacitor C1 in Figure 7 has a typical value of 3 pF and

is mainly the package pin capacitance. Resistor R1 is the on

resistance of the multiplexer and track / hold switch and is

typically 500 ohms. Capacitor C2 is the ADC088S052 sam-

pling capacitor, and is typically 30 pF. The ADC088S052 will

deliver best performance when driven by a low-impedance

source (less than 100 ohms). This is especially important

when using the ADC088S052 to sample dynamic signals. Al-

so important when sampling dynamic signals is a band-pass

or low-pass filter which reduces harmonics and noise in the

input.

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FIGURE 7. Equivalent Input Circuit

1.5 DIGITAL INPUTS AND OUTPUTS

The ADC088S052's digital inputs (SCLK, CS, and DIN) have

an operating range of 0 V to VA. They are not prone to latch-

up and may be asserted before the digital supply (VD) without

any risk. The digital output (DOUT) operating range is con-

trolled by VD. The output high voltage is VD - 0.5V (min) while

the output low voltage is 0.4V (max).

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ADC088S052

2.0 Applications Information

2.1 TYPICAL APPLICATION CIRCUIT

A typical application is shown in Figure 8. The split analog and

digital supply pins are both powered, in this example, by the

National LP2950 low-dropout voltage regulator. The analog

supply is bypassed with a capacitor network located close to

the ADC088S052. The digital supply is separated from the

analog supply by an isolation resistor and bypassed with ad-

ditional capacitors. The ADC088S052 uses the analog supply

(VA) as its reference voltage, so it is very important that VA be

kept as clean as possible. Due to the low power requirements

of the ADC088S052, it is also possible to use a precision ref-

erence as a power supply.

To minimize the error caused by the changing input capaci-

tance of the ADC088S052, a capacitor is connected from

each input pin to ground. The capacitor, which is much larger

than the input capacitance of the ADC088S052 when in track

mode, provides the current to quickly charge the sampling

capacitor of the ADC088S052. An isolation resistor is added

to isolate the load capacitance from the input source.

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FIGURE 8. Typical Application Circuit

2.2 POWER SUPPLY CONSIDERATIONS

There are three major power supply concerns with this prod-

uct: power supply sequencing, power management, and the

effect of digital supply noise on the analog supply.

2.2.1 Power Supply Sequence

The ADC088S052 is a dual-supply device. The two supply

pins share ESD resources, so care must be exercised to en-

sure that the power is applied in the correct sequence. To

avoid turning on the ESD diodes, the digital supply (VD) can-

not exceed the analog supply (VA) by more than 300 mV, not

even on a transient basis. Therefore, VA must ramp up before

or concurrently with VD.

2.2.2 Power Management

The ADC088S052 is fully powered-up whenever CS is low

and fully powered-down whenever CS is high, with one ex-

ception. If operating in continuous conversion mode, the AD-

C088S052 automatically enters power-down mode between

SCLK's 16th falling edge of a conversion and the SCLK's 1st

falling edge of the subsequent conversion (see Figure 1).

In continuous conversion mode, the ADC088S052 can per-

form multiple conversions back to back. Each conversion

requires 16 SCLK cycles and the ADC088S052 will perform

conversions continuously as long as CS is held low. Contin-

uous mode offers maximum throughput.

In burst mode, the user may trade off throughput for power

consumption by performing fewer conversions per unit time.

This means spending more time in power-down mode and

less time in normal mode. By utilizing this technique, the user

can achieve very low sample rates while still utilizing an SCLK

frequency within the electrical specifications. The Power Con-

sumption vs. SCLK curve in the Typical Performance Curves

section shows the typical power consumption of the AD-

C088S022 in the continuous conversion mode. Burst mode

power is calculated as shown below, where PC is average

power consumption, (tN) is the time in the normal (continuous

conversion) mode, tS is the time in shut down mode, PN is the

power consumption in the normal mode and PS is the power

consumption in the shutdown mode.

20166615

FIGURE 9. Power Consumption Equation

2.2.3 Power Supply Noise Considerations

The charging of any output load capacitance requires current

from the digital supply, VD. The current pulses required from

the supply to charge the output capacitance will cause voltage

variations on the digital supply. If these variations are large

enough, they could degrade SNR and SINAD performance of

the ADC. Furthermore, if the analog and digital supplies are

tied directly together, the noise in the on-chip digital supply

will be coupled directly into the analog supply, causing greater

performance degradation than would noise on the digital sup-

ply alone. Similarly, discharging the output capacitance when

the digital output goes from a logic high to a logic low will dump

current into the die substrate. Load discharge currents will

cause "ground bounce" noise in the substrate that will de-

grade noise performance if that current is large enough. The

larger the output capacitance, the more current flows through

the die substrate and the greater the noise coupled into the

analog channel.

15 www.national.com

ADC088S052

The first solution to keeping digital noise out of the analog

supply is to decouple the analog and digital supplies from

each other or use separate supplies for them. To keep noise

out of the digital supply, keep the output load capacitance as

small as practical. If the load capacitance is greater than 50

pF, use a 100 Ω series resistor at the ADC output, located as

close to the ADC output pin as practical. This will limit the

charge and discharge current of the output capacitance and

maintain noise performance. Since the series resistor and the

load capacitance form a low frequency pole, verify signal in-

tegrity once the series resistor has been added.

2.3 LAYOUT AND GROUNDING

Capacitive coupling between the noisy digital circuitry and the

sensitive analog circuitry can lead to poor performance. The

solution is to keep the analog circuitry separated from the

digital circuitry and keep the clock line as short as possible.

Digital circuits create substantial supply and ground current

transients. The logic noise generated could have significant

impact upon system noise performance. To avoid perfor-

mance degradation of the ADC088S052 due to supply noise,

do not use the same supply for the ADC088S052 that is used

for digital logic.

Generally, analog and digital lines should cross each other at

90° to avoid crosstalk. However, to maximize accuracy in

mixed signal systems, avoid crossing analog and digital lines

altogether. It is important to keep clock lines as short as pos-

sible and isolated from ALL other lines, including other digital

lines. In addition, the clock line should also be treated as a

transmission line and be properly terminated.

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.

We recommend the use of a single, uniform ground plane and

the use of split power planes. The power planes should be

located within the same board layer. All analog circuitry (input

amplifiers, filters, reference components, etc.) should be

placed over the analog power plane. All digital circuitry and I/

O lines should be placed over the digital power plane. Fur-

thermore, all components in the reference circuitry and the

input signal chain that are connected to ground should be

connected together with short traces and enter the analog

ground plane at a single, quiet point.

www.national.com 16

ADC088S052

Physical Dimensions inches (millimeters) unless otherwise noted

16-Lead TSSOP

Order Number ADC088S052CIMT, ADC088S052CIMTX

NS Package Number MTC16

17 www.national.com

ADC088S052

Notes

ADC088S052 8-Channel, 200 ksps to 500 ksps, 8-Bit A/D Converter

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