AD797
26.1Ω
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 53, Capacitor CF shunts high
frequency energy to ground while correctly reproducing the
desired output with extremely low THD and IMD.
+V
7
S
*
*
26.1Ω
2
3
V
OUT
6
*
AD600
AD797
V
4
IN
*
–V
S
V
= ±6Vdc
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 50. An Ultrasound Preamplifier Circuit
Amorphous (Photodiode) Detector
C
F
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 51 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 52.
82pF
100Ω
3kΩ
+V
S
*
AD1862
DAC
2
3
7
C1
2000pF
V
6
*
AD797
OUT
4
C
L
100Ω 50pF
–V
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
10kΩ
Figure 53. A Professional Audio DAC Buffer
+V
7
S
+V
S
*
2
3
–IN
+IN
2
3
7
C
S
V
OUT
AD797
6
*
I
S
V
AD797
6
OUT
1000pF
5
4
1
4
20kΩ
–V
S
V
ADJUST
OS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
–V
S
Figure 51. Amorphous Detector Preamp
Figure 54. Offset Null Configuration
–30
–40
–50
–60
–70
–80
100
80
60
40
20
0
V
OUT
NOISE
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
100M
Figure 52. Total Integrated Voltage Noise and VOUT
of Amorphous Detector Preamp
Rev. F | Page ꢀ7 of 20
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