AD7631BSTZ datasheet(21/32 Pages) AD

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AD7631

Rev. A | Page 21 of 32

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

configuration).

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

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

This approximation can be utilized when the resistances

used around the amplifiers are small. 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 AD7631. 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

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 AD7631 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 AD7631.

See Table 9 for a list of recommended op amps.

Table 9. 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 AD7631 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–

AD7631

REF

10µF

15Ω

100nF

2.7nF

ADA4922-1

OUT+

VCC

VEE

OUT–

REF

Figure 31. 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 10 for the different

input ranges for R1/R2.

Table 10.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

This circuit can also be made discretely, and thus more flexible,

using any of the recommended low noise amplifiers in Table 9.

Again, to preserve the SNR of the converter, the resistors RF and

RG should be kept low.

VOLTAGE REFERENCE INPUT/OUTPUT

The AD7631 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 AD7631 provides excellent

performance and can be used in almost all applications.

However, the linearity performance is guaranteed only with

an external reference.

Internal Reference (REF = 5 V)(PDREF = Low,

PDBUF = Low)

To use the internal reference, the PDREF and PDBUF inputs

must be low. This enables the on-chip band gap reference, buffer,

and TEMP sensor resulting in a 5.00 V reference on the REF pin.

The internal reference is temperature-compensated to 5.000 V

±35 mV. The reference is trimmed to provide a typical drift of

3 ppm/°C. This typical drift characteristic is shown in Figure 19.

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