AD797B datasheet(17/19 Pages) AD

17 / 19 page

Data Sheet

AD797

Rev. K | Page 17 of 19

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 51 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.

+VS

VS = ±6Vdc

VOUT

AD797

26.1Ω

–VS

AD600

VIN

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 51. An Ultrasound Preamplifier Circuit

Amorphous (Photodiode) Detector

Large area photodiodes (CS ≥ 500 pF) and certain image

detectors (amorphous Si) have optimum performance when used

in conjunction with amplifiers with very low voltage (rather than

very low current noise). Figure 52 shows the AD797 used with

an amorphous Si (CS = 1000 pF) detector. The response is adjusted

for flatness using capacitor CL, and the noise is dominated by

voltage noise amplified by the ac noise gain. The AD797’s excellent

input noise performance gives 27 μV rms total noise in a 1 MHz

bandwidth, as shown by Figure 53.

+VS

AD797

10kΩ

100Ω

50pF

1000pF

–VS

VOUT

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 52. Amorphous Detector Preamp

100M

100

10M

100k

10k

FREQUENCY (Hz)

–80

–30

–50

–70

–60

–40

NOISE

VOUT

Figure 53. Total Integrated Voltage Noise and VOUT

of Amorphous Detector Preamp

Professional Audio Signal Processing—DAC Buffers

The low noise and low distortion of the AD797 make it an ideal

choice for professional audio signal processing. An ideal I-to-V

converter for a current output DAC would simply be a resistor

to ground, were it not for the fact that most DACs do not operate

linearly with voltage on their output. Standard practice is to

operate an op amp as an I-to-V converter, creating a virtual

ground at its inverting input. Normally, clock energy and current

steps must be absorbed by the op amp output stage. However, in

the configuration shown in Figure 54, Capacitor CF shunts high

frequency energy to ground while correctly reproducing the

desired output with extremely low THD and IMD.

+VS

–VS

AD797

82pF

2000pF

100Ω

3kΩ

AD1862

DAC

VOUT

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 54. A Professional Audio DAC Buffer

VOUT

+VS

–VS

AD797

VOS ADJUST

–IN

+IN

20kΩ

Figure 55. Offset Null Configuration

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