AD7951BSTZ datasheet(21/32 Pages) AD

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Data Sheet

AD7951

Rev. A | Page 21 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7951 is easy to drive, the driver amplifier must

meet the following requirements:



For multichannel, multiplexed applications, the driver

amplifier and the AD7951 analog input circuit must be

able to settle for a full-scale step of the capacitor array at a

14-bit level (0.006%). For the amplifier, settling at 0.1% to

0.01% is more commonly specified. This differs significantly

from the settling time at a 14-bit level and should be

verified prior to driver selection. The AD8021 op amp com-

bines ultralow noise and high gain bandwidth and meets

this settling time requirement even when used with gains

of up to 13.



The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7951. The noise coming from

the driver is filtered by the external one-pole low-pass filter

as shown in Figure 27. The SNR degradation due to the

amplifier is

 





















log

NADC

LOSS

SNR

where:

VNADC is the noise of the ADC, which is:

SNR

INp-p

NADC



f–3dB is the cutoff frequency of the input filter (3.9 MHz).

N is the noise factor of the amplifier (+1 in buffer

configuration).

eN is the equivalent input voltage noise density of the op

amp, in nV/√Hz.



The driver needs to have a THD performance suitable to

that of the AD7951. Figure 15 shows the THD vs. frequency

that the driver should exceed.

The AD8021 meets these requirements and is appropriate for

almost all applications. The AD8021 needs a 10 pF external

compensation capacitor that should have good linearity as an

NPO ceramic or mica type. Moreover, the use of a noninverting

+1 gain arrangement is recommended and helps to obtain the

best signal-to-noise ratio.

The AD8022 can also be used when a dual version is needed

and a gain of 1 is present. The AD829 is an alternative in applica-

tions where high frequency (above 100 kHz) performance is not

required. In applications with a gain of 1, an 82 pF compensation

capacitor is required. The AD8610 is an option when low bias

current is needed in low frequency applications.

Since the AD7951 uses a large geometry, high voltage input

switch, the best linearity performance is obtained when using

the amplifier at its maximum full power bandwidth. Gaining

the amplifier to make use of the more dynamic range of the

ADC results in increased linearity errors. For applications

requiring more resolution, the use of an additional amplifier

with gain should precede a unity follower driving the AD7951.

See Table 8 for a list of recommended op amps.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-x

12 V supply, very low noise, low distortion,

low power, low frequency

AD829

±15 V supplies, very low noise, low frequency

AD8021

±12 V supplies, very low noise, high frequency

AD8022

±12 V supplies, very low noise, high

frequency, dual

AD8610/

AD8620

±13 V supplies, low bias current, low

frequency, single/dual

VOLTAGE REFERENCE INPUT/OUTPUT

The AD7951 allows the choice of either a very low temperature

drift internal voltage reference, an external reference, or an

external buffered reference.

The internal reference of the AD7951 provides excellent perform-

ance and can be used in almost all applications. However, the

linearity performance is guaranteed only with an external reference.

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