AD8021 datasheet(21/32 Pages) AD

21 / 32 page

AD7952

Rev. 0 | Page 21 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7952 is easy to drive, the driver amplifier must

meet the following requirements:

•

For multichannel, multiplexed applications, the driver

amplifier and the AD7952 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 AD7952. The noise coming from

the driver is filtered by the external 1-pole, low-pass filter,

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

amplifier is

⎟

⎠

⎞

⎜

⎝

⎛

−

3dB

)

(

)

(

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 the buffer

configuration).

eN+ and eN− are the equivalent input voltage noise densities

of the op amps connected to IN+ and IN−, in nV/√Hz.

When the resistances used around the amplifiers are small,

this approximation can be used. If larger resistances are

used, their noise contributions should also be root-sum

squared.

•

The driver needs to have a THD performance suitable to

that of the AD7952. 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 SNR.

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

applications where high frequency performance (above 100 kHz)

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.

Because the AD7952 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 AD7952.

See Table 8 for a list of recommended op amps.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

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

ADA4922-1

±12 V supplies, low noise, high frequency,

single-ended-to-differential driver

AD8610/

AD8620

±13 V supplies, low bias current, low

frequency, single/dual

Single-to-Differential Driver

For single-ended sources, a single-to-differential driver, such

as the ADA4922-1, can be used because the AD7952 needs to

be driven differentially. The 1-pole filter using R = 15 Ω and

C = 2.7 nF provides a corner frequency of 3.9 MHz.

ANALOG

INPUT

IN+

IN–

AD7952

REF

10µF

15Ω

100nF

2.7nF

ADA4922-1

OUT+

VCC

VEE

OUT–

REF

Figure 30. Single-to-Differential Driver Using the ADA4922-1

For unipolar 5 V and 10 V input ranges, the internal (or

external) reference source can be used to level shift U2 for

the correct input span. If using an external reference, the

values for R1/R2 can be lowered to reduce resistive Johnson

noise (1.29E − 10 × √R). For the bipolar ±5 V and ±10 V input

ranges, the reference connection is not required because the

common-mode voltage is 0 V. See Table 9 for R1/R2 for the

different input ranges.

Table 9. R1/R2 Configuration

Input Range (V)

R1 (Ω)

R2 (Ω)

Common-Mode Voltage (V)

2.5 k

2.5

2.5 k

Open

±5, ±10

100

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