ADC08200CIMT/NOPB - National Semiconductor

February 8, 2008

ADC08200

8-Bit, 20 Msps to 200 Msps, Low Power A/D Converter with

Internal Sample-and-Hold

General Description

The ADC08200 is a low-power, 8-bit, monolithic analog-to-

digital converter with an on-chip track-and-hold circuit. Opti-

mized for low cost, low power, small size and ease of use, this

product operates at conversion rates up to 230 Msps while

consuming just 1.05 mW per MHz of clock frequency, or 210

mW at 200 Msps. Raising the PD pin puts the ADC08200 into

a Power Down mode where it consumes about 1 mW.

The unique architecture achieves 7.3 Effective Bits with

50 MHz input frequency. The ADC08200 is resistant to latch-

up and the outputs are short-circuit proof. The top and bottom

of the ADC08200's reference ladder are available for con-

nections, enabling a wide range of input possibilities. The

digital outputs are TTL/CMOS compatible with a separate

output power supply pin to support interfacing with 3V or 2.5V

logic. The digital inputs (CLK and PD) are TTL/CMOS com-

patible. The output data format is straight binary.

The ADC08200 is offered in a 24-lead plastic package

(TSSOP) and, while specified over the industrial temperature

range of −40°C to +85°C, it will function over the to −40°C to

+105°C temperature range.

Features

■Single-ended input

■Internal sample-and-hold function

■Low voltage (single +3V) operation

■Small package

■Power-down feature

Key Specifications

■Resolution 8 Bits

■Maximum sampling frequency 200 Msps (min)

■DNL ±0.4 LSB (typ)

■ENOB (fIN= 50 MHz) 7.3 bits (typ)

■THD (fIN= 50 MHz) 61 dB (typ)

■Power Consumption

■—Operating 1.05 mW/Msps (typ)

—Power Down 1 mW (typ)

Applications

■Flat panel displays

■Projection systems

■Set-top boxes

■Battery-powered instruments

■Communications

■Medical imaging

■Astronomy

Pin Configuration

20017901

ADC08200 8-Bit, 20 Msps to 200 Msps, Low Power A/D Converter with Internal Sample-and-Hold

Ordering Information

Order Number Temperature Range Package

ADC08200CIMT −40°C ≤ TA ≤ +105°C TSSOP

ADC08200CIMTX −40°C ≤ TA ≤ +105°C TSSOP (tape and reel)

Use ADC08B200EB Evaluation Board

Block Diagram

20017902

Pin Descriptions and Equivalent Circuits

Pin No. Symbol Equivalent Circuit Description

6VIN Analog signal input. Conversion range is VRB to VRT.

3VRT

Analog Input that is the high (top) side of the reference ladder

of the ADC. Nominal range is 0.5V to VA. Voltage on VRT and

VRB inputs define the VIN conversion range. Bypass well. See

Section 2.0 for more information.

9VRM

Mid-point of the reference ladder. This pin should be

bypassed to a quiet point in the ground plane with a 0.1 µF

capacitor.

10 VRB

Analog Input that is the low side (bottom) of the reference

ladder of the ADC. Nominal range is 0.0V to (VRT – 0.5V).

Voltage on VRT and VRB inputs define the VIN conversion

range. Bypass well. See Section 2.0 for more information.

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ADC08200

Pin No. Symbol Equivalent Circuit Description

23 PD

Power Down input. When this pin is high, the converter is in

the Power Down mode and the data output pins hold the last

conversion result.

24 CLK CMOS/TTL compatible digital clock Input. VIN is sampled on

the rising edge of CLK input.

13 thru 16

and

19 thru 22

D0–D7

Conversion data digital Output pins. D0 is the LSB, D7 is the

MSB. Valid data is output just after the rising edge of the CLK

input.

7VIN GND Reference ground for the single-ended analog input, VIN.

1, 4, 12 VA

Positive analog supply pin. Connect to a quiet voltage source

of +3V. VA should be bypassed with a 0.1 µF ceramic chip

capacitor for each pin, plus one 10 µF capacitor. See Section

3.0 for more information.

18 VDR Power supply for the output drivers. If connected to VA,

decouple well from VA.

17 DR GND The ground return for the output driver supply.

2, 5, 8, 11 AGND The ground return for the analog supply.

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ADC08200

Absolute Maximum Ratings

(Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltage (VA)3.8V

Driver Supply Voltage (VDR) VA +0.3V

Voltage on Any Input or Output Pin −0.3V to VA

Reference Voltage (VRT, VRB) VA to AGND

CLK, PD Voltage Range −0.05V to

(VA + 0.05V)

Input Current at Any Pin (Note 3) ±25 mA

Package Input Current (Note 3) ±50 mA

Power Dissipation at TA = 25°C See (Note 5)

ESD Susceptibility (Note 6)

Human Body Model

Machine Model

2500V

200V

Soldering Temperature, Infrared,

10 seconds (Note 7) 235°C

Storage Temperature −65°C to +150°C

Operating Ratings (Notes 1, 2)

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

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

Driver Supply Voltage (VDR) +2.4V to VA

Ground Difference |GND - DR GND| 0V to 300 mV

Upper Reference Voltage (VRT) 0.5V to (VA −0.3V)

Lower Reference Voltage (VRB) 0V to (VRT −0.5V)

VIN Voltage Range VRB to VRT

Package Thermal Resistance

Package θJA

24-Lead TSSOP 92°C/W

Converter Electrical Characteristics

The following specifications apply for VA = VDR = +3.0VDC, VRT = +1.9V, VRB = 0.3V, CL = 5 pF, fCLK = 200 MHz at 50% duty cycle.

Boldface limits apply for TJ = TMIN to TMAX: all other limits TJ = 25°C (Notes 4, 8, 9)

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

DC ACCURACY

INL Integral Non-Linearity +1.0

−0.3

+1.9

−1.2

LSB (max)

LSB (min)

DNL Differential Non-Linearity ±0.4 ±0.95 LSB (max)

Missing Codes 0(max)

FSE Full Scale Error 36 50 mV (max)

VOFF Zero Scale Offset Error 46 60 mV (max)

ANALOG INPUT AND REFERENCE CHARACTERISTICS

VIN Input Voltage 1.6 VRB V (min)

VRT V (max)

CIN VIN Input Capacitance VIN = 0.75V +0.5 Vrms (CLK LOW) 3 pF

(CLK HIGH) 4 pF

RIN RIN Input Resistance >1 MΩ

BW Full Power Bandwidth 500 MHz

VRT Top Reference Voltage 1.9 VAV (max)

0.5 V (min)

VRB Bottom Reference Voltage 0.3 VRT − 0.5 V (max)

0V (min)

VRT - VRB Reference Voltage Delta 1.6 1.0 V (min)

2.3 V (max)

RREF Reference Ladder Resistance VRT to VRB 160 120 Ω (min)

200 Ω (max)

CLK, PD DIGITAL INPUT CHARACTERISTICS

VIH Logical High Input Voltage VDR = VA = 3.6V 2.0 V (min)

VIL Logical Low Input Voltage VDR = VA = 2.7V 0.8 V (max)

IIH Logical High Input Current VIH = VDR = VA = 3.6V 10 nA

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ADC08200

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

IIL Logical Low Input Current VIL = 0V, VDR = VA = 2.7V −50 nA

CIN Logic Input Capacitance 3 pF

DIGITAL OUTPUT CHARACTERISTICS

VOH High Level Output Voltage VA = VDR = 2.7V, IOH = −400 µA 2.6 2.4 V (min)

VOL Low Level Output Voltage VA = VDR = 2.7V, IOL = 1.0 mA 0.4 0.5 V (max)

DYNAMIC PERFORMANCE

ENOB Effective Number of Bits

fIN = 4 MHz, VIN = FS − 0.25 dB 7.5 Bits

fIN = 20 MHz, VIN = FS − 0.25 dB 7.4 Bits

fIN = 50 MHz, VIN = FS − 0.25 dB 7.3 6.9 Bits (min)

fIN = 70 MHz, VIN = FS − 0.25 dB 7.2 Bits

fIN = 100 MHz, VIN = FS − 0.25 dB 7.0 Bits

SINAD Signal-to-Noise & Distortion

fIN = 4 MHz, VIN = FS − 0.25 dB 47 dB

fIN = 20 MHz, VIN = FS − 0.25 dB 46 dB

fIN = 50 MHz, VIN = FS − 0.25 dB 46 43.3 dB (min)

fIN = 70 MHz, VIN = FS − 0.25 dB 45 dB

fIN = 100 MHz, VIN = FS − 0.25 dB 44 dB

SNR Signal-to-Noise Ratio

fIN = 4 MHz, VIN = FS − 0.25 dB 47 dB

fIN = 20 MHz, VIN = FS − 0.25 dB 46 dB

fIN = 50 MHz, VIN = FS − 0.25 dB 46 43.4 dB (min)

fIN = 70 MHz, VIN = FS − 0.25 dB 45 dB

fIN = 100 MHz, VIN = FS − 0.25 dB 44 dB

SFDR Spurious Free Dynamic Range

fIN = 4 MHz, VIN = FS − 0.25 dB 60 dBc

fIN = 20 MHz, VIN = FS − 0.25 dB 58 dBc

fIN = 50 MHz, VIN = FS − 0.25 dB 60 dBc

fIN = 70 MHz, VIN = FS − 0.25 dB 57 dBc

fIN = 100 MHz, VIN = FS − 0.25 dB 54 dBc

THD Total Harmonic Distortion

fIN = 4 MHz, VIN = FS − 0.25 dB −60 dBc

fIN = 20 MHz, VIN = FS − 0.25 dB −58 dBc

fIN = 50 MHz, VIN = FS − 0.25 dB −60 dBc

fIN = 70 MHz, VIN = FS − 0.25 dB -56 dBc

fIN = 100 MHz, VIN = FS − 0.25 dB −53 dBc

HD2 2nd Harmonic Distortion

fIN = 4 MHz, VIN = FS − 0.25 dB −66 dBc

fIN = 20 MHz, VIN = FS − 0.25 dB -68 dBc

fIN = 50 MHz, VIN = FS − 0.25 dB −66 dBc

fIN = 70 MHz, VIN = FS − 0.25 dB -60 dBc

fIN = 100 MHz, VIN = FS − 0.25 dB −55 dBc

HD3 3rd Harmonic Distortion

fIN = 4 MHz, VIN = FS − 0.25 dB −72 dBc

fIN = 20 MHz, VIN = FS − 0.25 dB −58 dBc

fIN = 50 MHz, VIN = FS − 0.25 dB −72 dBc

fIN = 70 MHz, VIN = FS − 0.25 dB -58 dBc

fIN = 100 MHz, VIN = FS − 0.25 dB −60 dBc

IMD Intermodulation Distortion f1 = 11 MHz, VIN = FS − 6.25 dB

f2 = 12 MHz, VIN = FS − 6.25 dB -55 dBc

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ADC08200

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

POWER SUPPLY CHARACTERISTICS

IAAnalog Supply Current DC Input 69.75 86 mA (max)

fIN = 10 MHz, VIN = FS − 3 dB 69.75 mA

IDR Output Driver Supply Current DC Input, PD = Low 0.25 0.6 mA (max)

IA + IDR Total Operating Current DC Input, PD = Low 70 86.6 mA (max)

CLK Low, PD = Hi 0.3 mA

PC Power Consumption DC Input, Excluding Reference 210 260 mW (max)

CLK Low, PD = Hi 1 mW

PSRR1Power Supply Rejection Ratio FSE change with 2.7V to 3.3V change in

54 dB

PSRR2Power Supply Rejection Ratio SNR reduction with 200 mV at 1MHz on

supply 45 dB

AC ELECTRICAL CHARACTERISTICS

fC1 Maximum Conversion Rate 230 200 MHz (min)

fC2 Minimum Conversion Rate 10 MHz

tCL Minimum Clock Low Time 0.87 1.0 ns (min)

tCH Minimum Clock High Time 0.65 0.75 ns (min)

tOH

Output Hold Time, Output Falling

(Note 11)

CLK to Data Invalid, VA = 3.3V to 3.6V,

tA = −40°C to +105°C, CL = 8 pF 2.4 3.3 ns (max)

Output Hold Time, Output Rising

(Note 11)

CLK to Data Invalid, VA = 3.3V to 3.6V,

tA = −40°C to +105°C, CL = 8 pF 1.9 2.5 ns (max)

tOD

Output Delay, Output Falling

(Note 11)

CLK to Data Transition, VA = 3.3V to

3.6V, VA = −40°C to +105°C, CL = 8 pF 3.9 2.4 ns (min)

5.1 ns (max)

Output Delay, Output Rising

(Note 11)

CLK to Data Transition, VA = 3.3V to

3.6V, tA = −40°C to +105°C, CL = 8 pF 3.3 2.4 ns (min)

4.0 ns (max)

tSLEW Output Slew Rate

Output Falling, VA = 3.3V, CL = 8 pF,

tA = −40°C to +105°C 0.73 V/ns

Output Rising, VA = 3.3V, CL = 8 pF,

tA = −40°C to +105°C 0.88 V/ns

Pipeline Delay (Latency) 6 Clock Cycles

tAD Sampling (Aperture) Delay CLK Rise to Acquisition of Data 2.6 ns

tAJ Aperture Jitter 2 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

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 = AGND = DR GND = 0V, unless otherwise specified.

Note 3: When the input voltage at any pin exceeds the power supplies (that is, less than AGND or DR GND, or greater than VA or VDR), the current at that pin

should be limited to 25 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 25 mA to two.

Note 4: The Electrical characteristics tables list guaranteed specifications under the listed Recommended Conditions except as otherwise modified or specified

by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations for room temperature only and are not guaranteed.

Note 5: 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. The values

for maximum power dissipation listed above will be reached only when this 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). Obviously, such conditions should always be avoided.

Note 6: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO Ohms.

Note 7: See AN-450, “Surface Mounting Methods and Their Effect on Product Reliability”.

Note 8: The analog inputs are protected as shown below. Input voltage magnitudes up to VA + 300 mV or to 300 mV below GND will not damage this device.

However, errors in the A/D conversion can occur if the input goes above VDR or below GND by more than 100 mV. For example, if VA is 2.7VDC the full-scale

input voltage must be ≤2.8VDC to ensure accurate conversions.

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ADC08200

20017907

Note 9: To guarantee accuracy, it is required that VA and VDR be well bypassed. Each supply pin must be decoupled with separate bypass capacitors.

Note 10: Typical figures are at TJ = 25°C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing Quality

Level).

Note 11: These specifications are guaranteed by design and not tested.

Note 12: Typical output slew rate is based upon the maximum tOD and tOH figures.

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ADC08200

Specification Definitions

APERTURE (SAMPLING) DELAY is that time required after

the rise of the clock input for the sampling switch to open. The

Sample/Hold circuit effectively stops capturing the input sig-

nal and goes into the “hold” mode tAD after the clock goes

high.

APERTURE JITTER is the variation in aperture delay from

sample to sample. Aperture jitter shows up as input noise.

CLOCK DUTY CYCLE is the ratio of the time that the clock

wave form is at a logic high to the total time of one clock pe-

riod.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

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

Measured at 200 Msps with a ramp input.

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 per-

fect 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 is a measure of how far the last code

transition is from the ideal 1½ LSB below VRT and is defined

as:

Vmax + 1.5 LSB – VRT

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

mum (full scale) code occurs.

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

viation of each individual code from a line drawn from zero

scale (½ LSB below the first code transition) through positive

full scale (½ LSB above the last code transition). The devia-

tion of any given code from this straight line is measured from

the center of that code value. The end point test method is

used. Measured at 200 Msps with a ramp input.

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 second and third

order intermodulation products to the power in one of the

original frequencies. IMD is usually expressed in dBFS.

MISSING CODES are those output codes that are skipped

and will never appear at the ADC outputs. These codes can-

not be reached with any input value.

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

input clock before the data update is present at the output

pins.

OUTPUT HOLD TIME is the length of time that the output data

is valid after the rise of the input clock.

PIPELINE DELAY (LATENCY) is the number of clock cycles

between initiation of conversion and when that data is pre-

sented to the output driver stage. New data is available at

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

Pipeline Delay plus the Output Delay.

POWER SUPPLY REJECTION RATIO (PSRR) is a measure

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

voltage. For the ADC08200, PSRR1 is the ratio of the change

in Full-Scale Error that results from a change in the DC power

supply voltage, expressed in dB. PSRR2 is a measure of how

well an AC signal riding upon the power supply is rejected

from the output and is here defined as

where SNR0 is the SNR measured with no noise or signal on

the supply line and SNR1 is the SNR measured with a

1 MHz, 200 mVP-P signal riding upon the supply lines.

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

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

value of the sum of all other spectral components below one-

half the sampling 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 at the output to the rms value of all of the other

spectral components below half the clock frequency, includ-

ing 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 at the output 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 nine harmonic levels

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

is calculated as

where Af1 is the RMS power of the fundamental (output) fre-

quency and Af2 through Af10 are the RMS power of the first 9

harmonic frequencies in the output spectrum

ZERO SCALE OFFSET ERROR is the error in the input volt-

age required to cause the first code transition. It is defined as

VOFF = VZT − VRB

where VZT is the first code transition input voltage.

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ADC08200

Timing Diagram

20017931

FIGURE 1. ADC08200 Timing Diagram

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ADC08200

Typical Performance Characteristics VA = VDR = 3V, fCLK = 200 MHz, fIN = 50 MHz, unless otherwise

stated

INL

20017908

INL vs. Temperature

20017914

INL vs. Supply Voltage

20017915

INL vs. Sample Rate

20017910

DNL

20017909

DNL vs. Temperature

20017917

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ADC08200

DNL vs. Supply Voltage

20017918

DNL vs. Sample Rate

20017911

SNR vs. Temperature

20017920

SNR vs. Supply Voltage

20017921

SNR vs. Sample Rate

20017912

SNR vs. Input Frequency

20017923

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ADC08200

SNR vs. Clock Duty Cycle

20017924

Distortion vs. Temperature

20017925

Distortion vs. Supply Voltage

20017926

Distortion vs. Sample Rate

20017913

Distortion vs. Input Frequency

20017928

Distortion vs. Clock Duty Cycle

20017929

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ADC08200

SINAD/ENOB vs. Temperature

20017930

SINAD/ENOB vs. Supply Voltage

20017938

SINAD/ENOB vs. Sample Rate

20017916

SINAD/ENOB vs. Input Frequency

20017939

SINAD/ENOB vs. Clock Duty Cycle

20017940

Power Consumption vs. Sample Rate

20017919

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ADC08200

Spectral Response @ fIN = 50 MHz

20017946

Spectral Response @ fIN = 76 MHz

20017947

Spectral Response @ fIN = 99 MHz

20017948

Intermodulation Distortion

20017943

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ADC08200

Functional Description

The ADC08200 uses a new, unique architecture that

achieves over 7 effective bits at input frequencies up to and

beyond 100 MHz.

The analog input signal that is within the voltage range set by

VRT and VRB is digitized to eight bits. Input voltages below

VRB will cause the output word to consist of all zeroes. Input

voltages above VRT will cause the output word to consist of

all ones.

Incorporating a switched capacitor bandgap, the ADC08200

exhibits a power consumption that is proportional to frequen-

cy, limiting power consumption to what is needed at the clock

rate that is used. This and its excellent performance over a

wide range of clock frequencies makes it an ideal choice as

a single ADC for many 8-bit needs.

Data is acquired at the rising edge of the clock and the digital

equivalent of that data is available at the digital outputs 6 clock

cycles plus tOD later. The ADC08200 will convert as long as

the clock signal is present. The output coding is straight bi-

nary.

The device is in the active state when the Power Down pin

(PD) is low. When the PD pin is high, the device is in the power

down mode, where the output pins hold the last conversion

before the PD pin went high and the device consumes just

1.4 mW . Holding the clock input low will further reduce the

power consumption in the power down mode to about 1 mW.

Applications Information

1.0 REFERENCE INPUTS

The reference inputs VRT and VRB are the top and bottom of

the reference ladder, respectively. Input signals between

these two voltages will be digitized to 8 bits. External voltages

applied to the reference input pins should be within the range

specified in the Operating Ratings table. Any device used to

drive the reference pins should be able to source sufficient

current into the VRT pin and sink sufficient current from the

VRB pin to maintain the desired voltages.

20017932

FIGURE 2. Simple, low component count reference biasing. Because of the ladder and external resistor tolerances, the

reference voltage of this circuit can vary too much for some applications.

The reference bias circuit of Figure 2 is very simple and the

performance is adequate for many applications. However,

circuit tolerances will lead to a wide reference voltage range.

Better reference stability can be achieved by driving the ref-

erence pins with low impedance sources.

The circuit of Figure 3 will allow a more accurate setting of the

reference voltages. The upper amplifier must be able to

source the reference current as determined by the value of

the reference resistor and the value of (VRT − VRB). The lower

amplifier must be able to sink this reference current. Both

amplifiers should be stable with a capacitive load. The

LM8272 was chosen because of its rail-to-rail input and output

capability, its high current output and its ability to drive large

capacitive loads.

The divider resistors at the inputs to the amplifiers could be

changed to suit the application reference voltage needs, or

the divider can be replaced with potentiometers or DACs for

precise settings. The bottom of the ladder (VRB) may be re-

turned to ground if the minimum input signal excursion is 0V.

VRT should always be more positive than VRB by the minimum

VRT - VRB difference in the Electrical Characteristics table to

minimize noise. While VRT may be as high as the VA supply

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ADC08200

voltage and VRB may be as low as ground, the difference be-

tween these two voltages (VRT − VRB) should not exceed 2.3V

to prevent waveform distortion.

The VRM pin is the center of the reference ladder and should

be bypassed to a quiet point in the ground plane with a 0.1 µF

capacitor. DO NOT leave this pin open and DO NOT load this

pin with more than 10µA.

20017933

FIGURE 3. Driving the reference to force desired values requires driving with a low impedance source.

2.0 THE ANALOG INPUT

The analog input of the ADC08200 is a switch followed by an

integrator. The input capacitance changes with the clock lev-

el, appearing as 3 pF when the clock is low, and 4 pF when

the clock is high. The sampling nature of the analog input

causes current spikes at the input that result in voltage spikes

there. Any amplifier used to drive the analog input must be

able to settle within the clock high time. The LMH6702 and

the LMH6628 have been found to be good amplifiers to drive

the ADC08200.

Figure 4 shows an example of an input circuit using the

LMH6702. Any input amplifier should incorporate some gain

as operational amplifiers exhibit better phase margin and

transient response with gains above 2 or 3 than with unity

gain. If an overall gain of less than 3 is required, attenuate the

input and operate the amplifier at a higher gain, as shown in

Figure 4.

The RC at the amplifier output filters the clock rate energy that

comes out of the analog input due to the input sampling circuit.

The optimum time constant for this circuit depends not only

upon the amplifier and ADC, but also on the circuit layout and

board material. A resistor value should be chosen between

18Ω and 47Ω and the capacitor value chose according to the

formula

The value of "C" in the formula above should include the ADC

input capacitance when the clock is high

This will provide optimum SNR performance for Nyquist ap-

plications. Best THD performance is realized when the ca-

pacitor and resistor values are both zero, but this would

compromise SNR and SINAD performance. Generally, the

capacitor should not be added for undersampling applica-

tions.

The circuit of Figure 4 has both gain and offset adjustments.

If you eliminate these adjustments normal circuit tolerances

may result in signal clipping unless care is exercised in the

worst case analysis of component tolerances and the input

signal excursion is appropriately limited to account for the

worst case conditions.

Full scale and offset adjustments may also be made by ad-

justing VRT and VRB, perhaps with the aid of a pair of DACs.

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ADC08200

20017934

FIGURE 4. The input amplifier should incorporate some gain for best performance (see text).

3.0 POWER SUPPLY CONSIDERATIONS

A/D converters draw sufficient transient current to corrupt

their own power supplies if not adequately bypassed. A

10 µF tantalum or aluminum electrolytic capacitor should be

placed within an inch (2.5 cm) of the A/D power pins, with a

0.1 µF ceramic chip capacitor placed within one centimeter of

the converter's power supply pins. Leadless chip capacitors

are preferred because they have low lead inductance.

While a single voltage source is recommended for the VA and

VDR supplies of the ADC08200, these supply pins should be

well isolated from each other to prevent any digital noise from

being coupled into the analog portions of the ADC. A choke

or 27Ω resistor is recommended between these supply lines

with adequate bypass capacitors close to the supply pins.

As is the case with all high speed converters, the ADC08200

should be assumed to have little power supply rejection. None

of the supplies for the converter should be the supply that is

used for other digital circuitry in any system with a lot of digital

power being consumed. The ADC supplies should be the

same supply used for other analog circuitry.

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

supply voltage or below ground by more than 300 mV, not

even on a transient basis. This can be a problem upon appli-

cation of power and power shut-down. Be sure that the sup-

plies to circuits driving any of the input pins, analog or digital,

do not come up any faster than does the voltage at the

ADC08200 power pins.

4.0 THE DIGITAL INPUT PINS

The ADC08200 has two digital input pins: The PD pin and the

Clock pin.

4.1 The PD Pin

The Power Down (PD) pin, when high, puts the ADC08200

into a low power mode where power consumption is reduced

to about 1.4 mW with the clock running, or to about 1 mW with

the clock held low. Output data is valid and accurate about 1

microsecond after the PD pin is brought low.

The digital output pins retain the last conversion output code

when either the clock is stopped or the PD pin is high.

4.2 The ADC08200 Clock

Although the ADC08200 is tested and its performance is

guaranteed with a 200 MHz clock, it typically will function well

with clock frequencies from 10 MHz to 230 MHz.

The low and high times of the clock signal can affect the per-

formance of any A/D Converter. Because achieving a precise

duty cycle is difficult, the ADC08200 is designed to maintain

performance over a range of duty cycles. While it is specified

and performance is guaranteed with a 50% clock duty cycle

and 200 Msps, ADC08200 performance is typically main-

tained with clock high and low times of 0.65 ns and 0.87 ns,

respectively, corresponding to a clock duty cycle range of

13% to 82.5% with a 200 MHz clock. Note that minimum low

and high times may not be simultaneously asserted.

The CLOCK line should be series terminated at the clock

source in the characteristic impedance of that line if the clock

line is longer than

17 www.national.com

ADC08200

where tr is the clock rise time and tprop is the propagation rate

of the signal along the trace. Typical tprop is about 150 ps/inch

(59 ps/cm) on FR-4 board material.

If the clock source is used to drive more than just the

ADC08200, the CLOCK pin should be a.c. terminated with a

series RC to ground such that the resistor value is equal to

the characteristic impedance of the clock line and the capac-

itor 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 located as close

as possible to, but within one centimeter of, the ADC08200

clock pin. Further, this termination should be close to but be-

yond the ADC08200 clock pin as seen from the clock source.

Typical tprop is about 150 ps/inch on FR-4 board material. For

FR-4 board material, the value of C becomes

Where L is the length of the clock line in inches.

This termination should be located as close as possible to,

but within one centimeter of, the ADC08200 clock pin.

5.0 LAYOUT AND GROUNDING

Proper grounding and proper routing of all signals are essen-

tial to ensure accurate conversion. A combined analog and

digital ground plane should be used.

Coupling between the typically noisy digital circuitry and the

sensitive analog circuitry can lead to poor performance that

may seem impossible to isolate and remedy. The solution is

to keep all lines separated from each other by at least six

times the height above the reference plane, and to keep the

analog circuitry well separated from the digital circuitry.

The DR GND connection to the ground plane should not use

the same feedthrough used by other ground connections.

High power digital components should not be located on or

near a straight line between the ADC or any linear component

and the power supply area as the resulting common return

current path could cause fluctuation in the analog input

“ground” return of the ADC.

Generally, analog and digital lines should cross each other at

90° to avoid getting digital noise into the analog path. In high

frequency systems, however, avoid crossing analog and dig-

ital lines altogether. Clock lines should be isolated from ALL

other lines, analog AND digital. Even the generally accepted

90° crossing should be avoided as even a little coupling can

cause problems at high frequencies. Best performance at

high frequencies is obtained with a straight signal path.

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 and ground should be connected to a

very clean point in the ground plane.

20017936

FIGURE 5. Layout Example

Figure 5 gives an example of a suitable layout. All analog cir-

cuitry (input amplifiers, filters, reference components, etc.)

should be placed together away from any digital components.

6.0 DYNAMIC PERFORMANCE

The ADC08200 is a.c. tested and its dynamic performance is

guaranteed. To meet the published specifications, the clock

source driving the CLK input must exhibit less than 2 ps (rms)

of jitter. For best a.c. performance, isolating the ADC clock

from any digital circuitry should be done with adequate

buffers, as with a clock tree. See Figure 6.

It is good practice to keep the ADC clock line as short as pos-

sible and to keep it well away from any other signals. Other

signals can introduce jitter into the clock signal. The clock

signal can also introduce noise into the analog path.

20017937

FIGURE 6. Isolating the ADC Clock from Digital Circuitry

7.0 COMMON APPLICATION PITFALLS

Driving the inputs (analog or digital) beyond the power

supply rails. For proper operation, all inputs should not go

more than 300 mV below the ground pins or 300 mV above

the supply pins. Exceeding these limits on even a transient

basis may cause faulty or erratic operation. It is not uncom-

mon for high speed digital circuits (e.g., 74F and 74AC de-

vices) to exhibit undershoot that goes more than a volt below

ground. A 51Ω resistor in series with the offending digital input

will usually eliminate the problem.

www.national.com 18

ADC08200

Care should be taken not to overdrive the inputs of the

ADC08200. Such practice may lead to conversion inaccura-

cies and even to device damage.

Attempting to drive a high capacitance digital data bus.

The more capacitance the output drivers must charge for

each conversion, the more instantaneous digital current is re-

quired from VDR and DR GND. These large charging current

spikes can couple into the analog section, degrading dynamic

performance. Buffering the digital data outputs (with a

74AF541, for example) may be necessary if the data bus ca-

pacitance exceeds 5 pF. Dynamic performance can also be

improved by adding 47Ω to 56Ω series resistors at each digital

output, reducing the energy coupled back into the converter

input pins.

Using an inadequate amplifier to drive the analog input.

As explained in Section 2.0, the capacitance seen at the input

alternates between 3 pF and 4 pF with the clock. This dynamic

capacitance is more difficult to drive than is a fixed capaci-

tance, and should be considered when choosing a driving

device.

Driving the VRT pin or the VRB pin with devices that can

not source or sink the current required by the ladder. As

mentioned in Section 1.0, care should be taken to see that

any driving devices can source sufficient current into the

VRT pin and sink sufficient current from the VRB pin. If these

pins are not driven with devices than can handle the required

current, these reference pins will not be stable, resulting in a

reduction of dynamic performance.

Using a clock source with excessive jitter, using an ex-

cessively long clock signal trace, or having other signals

coupled to the clock signal trace. This will cause the sam-

pling interval to vary, causing excessive output noise and a

reduction in SNR performance. The use of simple gates with

RC timing is generally inadequate as a clock source.

19 www.national.com

ADC08200

Physical Dimensions inches (millimeters) unless otherwise noted

NOTES: UNLESS OTHERWISE SPECIFIED

REFERENCE JEDEC REGISTRATION mo-153, VARIATION AD, DATED 7/93.

24-Lead Package TC

Order Number ADC08200CIMT

NS Package Number MTC24

www.national.com 20

ADC08200

Notes

21 www.national.com

ADC08200

Notes

ADC08200 8-Bit, 20 Msps to 200 Msps, Low Power A/D Converter with Internal Sample-and-Hold

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