Datasheet AD797 (Analog Devices) - 17

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Data Sheet AD797 Ultrasound/Sonar Imaging Preamp –30 100 ))
The AD600 variable gain amplifier provides the time-controlled
NCY E U
gain (TCG) function necessary for very wide dynamic range
–40 80 Q V E OUT NOISE R
sonar and low frequency ultrasound applications. Under some
F )
circumstances, it is necessary to buffer the input of the AD600
/µA –50 60 1V (0.1Hz
to preserve its low noise performance. To optimize dynamic
e R rms
range, this buffer should have a maximum of 6 dB of gain. The
V (dB T –60 40 (m
combination of low noise and low gain is difficult to achieve.
E OUV IS
The input buffer circuit shown in Figure 51 provides 1 nV/√Hz
O N
noise performance at a gain of 2 (dc to 1 MHz) by using 26.1 Ω
–70 20 E
resistors in its feedback path. Distortion is only −50 dBc at
LTAG O
1 MHz for a 2 V p-p output level and drops rapidly to better
V –80 0
52 than −70 dBc at an output level of 200 mV p-p. 0
100 1k 10k 100k 1M 10M 100M
6- 84
FREQUENCY (Hz)
00
26.1Ω
Figure 53. Total Integrated Voltage Noise and VOUT of Amorphous Detector Preamp
+VS * * Professional Audio Signal Processing—DAC Buffers 26.1Ω 2 7
The low noise and low distortion of the AD797 make it an ideal
V AD797 6 AD600 OUT
choice for professional audio signal processing. An ideal I-to-V
V 3
converter for a current output DAC would simply be a resistor
IN 4 * *
to ground, were it not for the fact that most DACs do not operate
–VS
linearly with voltage on their output. Standard practice is to
V
50
S = ±6Vdc
0 6- 84 operate an op amp as an I-to-V converter, creating a virtual
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00 ground at its inverting input. Normally, clock energy and current Figure 51. An Ultrasound Preamplifier Circuit steps must be absorbed by the op amp output stage. However, in
Amorphous (Photodiode) Detector
the configuration shown in Figure 54, Capacitor CF shunts high Large area photodiodes (CS ≥ 500 pF) and certain image frequency energy to ground while correctly reproducing the detectors (amorphous Si) have optimum performance when used desired output with extremely low THD and IMD. in conjunction with amplifiers with very low voltage (rather than
CF 82pF
very low current noise). Figure 52 shows the AD797 used with
100Ω
an amorphous Si (CS = 1000 pF) detector. The response is adjusted
3kΩ
for flatness using capacitor CL, and the noise is dominated by voltage noise amplified by the ac noise gain. The AD797’s excellent
+VS
input noise performance gives 27 μV rms total noise in a 1 MHz
*
bandwidth, as shown by Figure 53.
AD1862 2 7 DAC CL C1 2000pF AD797 6 V 100Ω 50pF OUT 3 4 10kΩ * –V
53
S
0
+V
6-
S
84
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00
*
Figure 54. A Professional Audio DAC Buffer
2 7 +VS I CS AD797 V S 6 OUT 1000pF 3 4 –IN 2 7 * AD797 6 VOUT
1
–VS
05 6-
5 +IN
84
3 *USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00
1
Figure 52. Amorphous Detector Preamp
4 20kΩ VOS ADJUST
5406-
–V
84
S
00 Figure 55. Offset Null Configuration Rev. K | Page 17 of 19 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION NOISE AND SOURCE IMPEDANCE CONSIDERATIONS LOW FREQUENCY NOISE WIDEBAND NOISE BYPASSING CONSIDERATIONS THE NONINVERTING CONFIGURATION THE INVERTING CONFIGURATION DRIVING CAPACITIVE LOADS SETTLING TIME DISTORTION REDUCTION Differential Line Receiver A General-Purpose ATE/Instrumentation I/O Driver Ultrasound/Sonar Imaging Preamp Amorphous (Photodiode) Detector Professional Audio Signal Processing—DAC Buffers OUTLINE DIMENSIONS ORDERING GUIDE