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.
*
7
*
*
*
+V
S
V
S
= ±6Vdc
V
OUT
AD797
00846-050
26.1Ω
26.1Ω
6
3
4
2
–V
S
AD600
V
IN
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 51. An Ultrasound Preamplifier Circuit
Amorphous (Photodiode) Detector
Large area photodiodes (C
S
≥ 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 (C
S
= 1000 pF) detector. The response is adjusted
for flatness using capacitor C
L
, 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.
*
7
*
+V
S
I
S
AD797
00846-051
10kΩ
100Ω
C
L
50pF
C
S
1000pF
6
3
4
2
–V
S
V
OUT
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 52. Amorphous Detector Preamp
00846-052
100M1k100
100
0
60
20
40
80
10M1M100k10k
FREQUENCY (Hz)
VOLTAGE NOISE (mV rms (0.1Hz FREQUENCY))
V
OUT
(dB Re 1V/µA)
–80
30
–50
–70
–60
–40
NOISE
V
OUT
Figure 53. Total Integrated Voltage Noise and V
OUT
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 C
F
shunts high
frequency energy to ground while correctly reproducing the
desired output with extremely low THD and IMD.
7
*
*
+V
S
–V
S
AD797
00846-053
C
F
82pF
6
2
C1
2000pF
4
100Ω
3kΩ
AD1862
DAC
3
V
OUT
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 54. A Professional Audio DAC Buffer
V
OUT
7
+
S
–V
S
AD797
6
2
4
3
1
5
V
OS
ADJUST
00846-054
–IN
+IN
20kΩ
Figure 55. Offset Null Configuration
