AD797ARZ-REEL71 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD797ARZ-REEL71 Datasheet PDF : 20 Pages
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AD797
2×
HP2835
TO TEKTRONIX
7A26
OSCILLOSCOPE
PREAMP INPUT
SECTION
1MΩ
20pF
226Ω
4.26kΩ
(VIA LESS THAN 1FT
50Ω COAXIAL CABLE)
2 – A2
AD829
3+
7
4
0.47µF
+VS
–VS
250Ω
6
0.47µF
VERROR × 5
2×
HP2835
The benefits of adding C1 are evident for closed-loop gains
of ≥100. A maximum value of ≈33 pF at gains of ≥1000 is
recommended. At a gain of 1000, the bandwidth is 450 kHz.
Table 6 and Figure 45 summarize the performance of the
AD797 with distortion cancellation and decompensation.
R1
R2
VIN
50pF
2
8
AD797 6
3
1kΩ
1kΩ
TEKTRONIX
CALIBRATION
FIXTURE
VIN
1kΩ
100Ω 1kΩ
20pF
2 – A1
AD797 6
3+
7
4
51pF
1µF
1µF
0.1µF
+VS
–VS
0.1µF
NOTES
USE CIRCUIT BOARD WITH GROUND PLANE.
Figure 43. Settling Time Test Circuit
DISTORTION REDUCTION
The AD797 has distortion performance (THD < −120 dB,
@ 20 kHz, 3 V rms, RL = 600 Ω) unequaled by most voltage
feedback amplifiers.
At higher gains and higher frequencies, THD increases due to a
reduction in loop gain. However, in contrast to most conven-
tional voltage feedback amplifiers, the AD797 provides two
effective means of reducing distortion as gain and frequency
are increased: cancellation of the output stage’s distortion and
gain bandwidth enhancement by decompensation. By applying
these techniques, gain bandwidth can be increased to 450 MHz
at G = 1000, and distortion can be held to −100 dB at 20 kHz for
G = 100.
The unique design of the AD797 provides cancellation of the
output stage’s distortion. To achieve this, a capacitance equal to
the effective compensation capacitance, usually 50 pF, is
connected between Pin 8 and the output (see C2 in Figure 44).
Use of this feature improves distortion performance when the
closed-loop gain is more than 10 or when frequencies of interest
are greater than 30 kHz.
Bandwidth enhancement via decompensation is achieved by
connecting a capacitor from Pin 8 to ground (see C1 in Figure 44).
Adding C1 results in subtracting from the value of the internal
compensation capacitance (50 pF), yielding a smaller effective
compensation capacitance and therefore a larger bandwidth.
a.
R1
C2
R2
C1
2
8
AD797 6
VOUT
VIN
3
C1, SEE TABLE
C2 = 50pF – C1
b.
Figure 44. Recommended Connections for Distortion Cancellation
and Bandwidth Enhancement
Table 6. Recommended External Compensation for
Distortion Cancellation and Bandwidth Enhancement
A/B
A
B
R1 R2 C1 C2 3 dB
Gain (Ω) (Ω) (pF) (pF) BW
C1 C2 3 dB
(pF) (pF) BW
10 909 100 0 50 6 MHz 0 50 6 MHz
100 1 k 10 0 50 1 MHz 15 33 1.5 MHz
1000 10 k 10 0 50 110 kHz 33 15 450 kHz
–80
G = +1000
RL = 600Ω
–90
NOISE LIMIT, G = +1000
–100
–110
G = +1000
RL = 10kΩ
NOISE LIMIT, G = +100
G = +100
RL = 600Ω
0.01
0.003
0.001
0.0003
–120
G = +10
RL = 600Ω
0.0001
100
300
1k
3k
10k
30k
FREQUENCY (Hz)
100k 300k
Figure 45. Total Harmonic Distortion (THD) vs. Frequency @ 3 V rms
for Figure 44b
Differential Line Receiver
The differential receiver circuit of Figure 46 is useful for many
applications, from audio to MRI imaging. The circuit allows
Rev. F | Page 15 of 20
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