AD797ARZ datasheet(16/20 Pages) AD

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AD797

Rev. F | Page 16 of 20

extraction of a low level signal in the presence of common-

mode noise. As shown in Figure 47, the AD797 provides this

function with only 9 nV/√Hz noise at the output. Figure 48

shows the AD797 20-bit THD performance over the audio band

and the 16-bit accuracy to 250 kHz.

AD797

1kΩ

DIFFERENTIAL

INPUT

1kΩ

20pF

50pF*

20pF

–VS

+VS

VOUT

OPTIONAL

USE THE POWER SUPPLY BYPASSING

SHOWN IN FIGURE 35.

Figure 46. Differential Line Receiver

10M

100

100k

10k

FREQUENCY (Hz)

Figure 47. Output Voltage Noise Spectral Density

for Differential Line Receiver

FREQUENCY (Hz)

–90

300k

–120

–130

300

100

–110

–100

100k

30k

10k

0.003

0.0003

0.001

0.0001

WITHOUT

OPTIONAL

50pF CN

WITH

OPTIONAL

50pF CN

MEASUREMENT

LIMIT

Figure 48. Total Harmonic Distortion (THD) vs. Frequency

for Differential Line Receiver

A General-Purpose ATE/Instrumentation I/O Driver

The ultralow noise and distortion of the AD797 can be

combined with the wide bandwidth, slew rate, and load drive

of a current feedback amplifier to yield a very wide dynamic

range general-purpose driver. The circuit shown in Figure 49

combines the AD797 with the AD811 in just such an

application. Using the component values shown, this circuit is

capable of better than −90 dB THD with a ±5 V, 500 kHz output

signal. The circuit is, therefore, suitable for driving a high

resolution ADC as an output driver in automatic test equipment

(ATE) systems. Using a 100 kHz sine wave, the circuit drives a

600 Ω load to a level of 7 V rms with less than −109 dB THD

and a 10 kΩ load at less than −117 dB THD.

+VS

–VS

AD797

22pF

2kΩ

649Ω

1kΩ

AD811

–VS

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

VOUT

VIN

Figure 49. A General-Purpose ATE/Instrumentation I/O Driver

Ultrasound/Sonar Imaging Preamp

The AD600 variable gain amplifier provides the time-controlled

gain (TCG) function necessary for very wide dynamic range

sonar and low frequency ultrasound applications. Under some

circumstances, it is necessary to buffer the input of the AD600

to preserve its low noise performance. To optimize dynamic

range, this buffer should have a maximum of 6 dB of gain. The

combination of low noise and low gain is difficult to achieve.

The input buffer circuit shown in Figure 50 provides 1 nV/√Hz

noise performance at a gain of 2 (dc to 1 MHz) by using

26.1 Ω resistors in its feedback path. Distortion is only −50 dBc

at 1 MHz for a 2 V p-p output level and drops rapidly to better

than −70 dBc at an output level of 200 mV p-p.

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