AD797
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.
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.
20pF
1kΩ
1kΩ
+V
S
**
50pF*
7
DIFFERENTIAL
INPUT
2
3
8
4
AD797
6
22pF
**
–V
S
R2
1kΩ
1kΩ
2kΩ
+V
7
S
*
20pF
+V
7
S
2
3
*
*OPTIONAL
**USE THE POWER SUPPLY BYPASSING
SHOWN IN FIGURE 35.
3
AD797
6
*
1kΩ
Figure 46. Differential Line Receiver
V
IN
4
AD811
6
*
V
OUT
16
14
12
10
8
2
4
–V
S
–V
S
649Ω
649Ω
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
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.
6
10
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 47. Output Voltage Noise Spectral Density
for Differential Line Receiver
–90
–100
–110
–120
–130
0.003
WITHOUT
OPTIONAL
50pF C
0.001
N
MEASUREMENT
0.0003
0.0001
LIMIT
WITH
OPTIONAL
50pF C
N
100
300
1k
3k
10k
30k
100k
300k
FREQUENCY (Hz)
Figure 48. Total Harmonic Distortion (THD) vs. Frequency
for Differential Line Receiver
Rev. F | Page ꢀ6 of 20
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