AD797

REV. C

–14–

100M

1k

100

100

0

60

20

40

80

10M

1M

100k

10k

FREQUENCY – Hz

–80

–30

–50

–70

–60

–40

NOISE

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 oper-

ate 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 cur-

rent steps must be absorbed by the op amp’s output stage.

shunts high frequency energy to ground, while correctly repro-

ducing the desired output with extremely low THD and IMD.

C1

2000pF

82pF

3k

Ω

AD797

**

2

7

3

4

6

**

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

100

Ω

AD1862

DAC

Figure 50. A Professional Audio DAC Buffer

Figure 51. Offset Null Configuration

Ultrasound/Sonar Imaging Preamp

The AD600 variable gain amplifier provides the time controlled

gain (TCG) function necessary for very wide dynamic range so-

nar and low frequency ultrasound applications. Under some cir-

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

preserve its low noise performance. To optimize dynamic range

this buffer should have at most 6 dB of gain. The combination

of low noise and low gain is difficult to achieve. The input

buffer circuit shown in Figure 47 provides 1 nV/

√Hz noise per-

formance at a gain of two (dc to 1 MHz) by using 26.1

Ω resistors

in its feedback path. Distortion is only –50 dBc @ 1 MHz at a

2 volt p-p output level and drops rapidly to better than

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

26.1Ω

INPUT

7

4

3

2

AD797

**

**

AD600

**

**

6

26.1Ω

V

OUT

* USE POWER SUPPLY

** BYPASSING SHOWN IN FIGURE 32.

Ω

Ω

Figure 47. An Ultrasound Preamplifier Circuit

Amorphous (Photodiode) Detector

≥ 500 pF and certain image detec-

tors (amorphous Si), have optimum performance when used in

conjunction with amplifiers with very low voltage rather than

very low current noise. Figure 48 shows the AD797 used with

nated by voltage noise amplified by the ac noise gain. The 797’s

excellent input noise performance gives 27

µV rms total noise in

a 1 MHz bandwidth, as shown by Figure 49.

50pF

1000pF

10k

Ω

AD797

**

2

7

3

4

6

**

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

100

Ω

Figure 48. Amorphous Detector Preamp