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AD7902

Data Sheet

Rev. B | Page 16 of 28

DRIVER AMPLIFIER CHOICE

Although the AD7902 is easy to drive, the driver amplifier must

meet the following requirements:



The noise generated by the driver amplifier must be kept

as low as possible to preserve the SNR and transition noise

performance of the AD7902. The noise from the driver is

filtered by the one-pole, low-pass filter of the AD7902 analog

input circuit, made by RIN and CIN or by the external filter,

if one is used. Because the typical noise of the AD7902 is

56 μV rms, the SNR degradation due to the amplifier is



















)

(

47.3

log

3dB

LOSS

SNR

where:

f−3dB is the input bandwidth, in megahertz, of the AD7902

(10 MHz) or the cutoff frequency of the input filter, if one

is used.

N is the noise gain of the amplifier (for example, gain = 1

in buffer configuration; see Figure 35).

eN is the equivalent input noise voltage of the op amp, in

nV/√Hz.



For ac applications, the driver must have a THD performance

that is commensurate with the AD7902.



For multichannel, multiplexed applications, the driver

amplifier and the AD7902 analog input circuit must settle

for a full-scale step onto the capacitor array at a 16-bit level

(0.0015%, 15 ppm). In the amplifier data sheet, settling at

0.1% to 0.01% is more commonly specified. This may differ

significantly from the settling time at a 16-bit level. Be sure

to verify the settling time prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-1/

ADA4841-2

Very low noise, small, and low power

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8655

5 V single supply, low noise

AD8605, AD8615

5 V single supply, low power

VOLTAGE REFERENCE INPUT

The AD7902 voltage reference input, REF, has a dynamic input

impedance and must therefore be driven by a low impedance

source with efficient decoupling between the REFx and GND

pins, as explained in the Layout section.

When REF is driven by a very low impedance source (for example,

a reference buffer using the AD8031 or the AD8605), a 10 μF

(X5R, 0805 size) ceramic chip capacitor is appropriate for

optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR430, ADR431,

ADR433, ADR434, or ADR435 reference.

If desired, a reference decoupling capacitor with values as small

as 2.2 μF can be used with a minimal impact on performance,

especially DNL.

Regardless, there is no need for an additional lower value ceramic

decoupling capacitor (for example, 100 nF) between the REFx

and GND pins.

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