AD7634BSTZ datasheet(21/32 Pages) AD

21 / 32 page

Data Sheet

AD7634

Rev. B | Page 21 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7634 is easy to drive, the driver amplifier must

meet the following requirements:



For multichannel, multiplexed applications, the driver ampli-

fier and the AD7634 analog input circuit must be able to

settle for a full-scale step of the capacitor array at a 18-bit

level (0.0004%). For the amplifier, settling at 0.1% to 0.01%

is more commonly specified. This differs significantly from

the settling time at a 18-bit level and should be verified

prior to driver selection. The AD8021 op amp combines ultra-

low 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 AD7634. 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

































)

(

)

(

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+ 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 AD7634. 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 appli-

cations 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.

Because the AD7634 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 AD7634.

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 AD7634 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–

AD7634

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 R1/R2 for the

different input ranges.

Table 10. R1/R2 Configuration

Input Range



Common-Mode Voltage

5 V

2.5 kΩ

2.5 V

10 V

2.5 kΩ

Open

5 V

±5 V, ±10 V

100 Ω

0 V

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.

AD7634BSTZ Datasheet(PDF) 21 Page - Analog Devices

AD7634BSTZ Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL