AD797
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.
TO TEKTRONIX
7A26
OSCILLOSCOPE
PREAMP INPUT
SECTION
1MΩ
20pF
Table 6 and Figure 45 summarize the performance of the
226Ω
4.26kΩ
AD797 with distortion cancellation and decompensation.
(VIA LESS THAN 1FT
50Ω COAXIAL CABLE)
R1
2
3
–
A2
250Ω
6
AD829
+
50pF
7
2×
HP2835
4
R2
2×
HP2835
0.47µF
2
3
8
0.47µF
6
AD797
+V
S
–V
S
1kΩ
1kΩ
100Ω
1kΩ
TEKTRONIX
CALIBRATION
FIXTURE
R1
V
20pF
IN
1kΩ
C2
C1
2
3
–
A1
6
AD797
+
4
R2
7
2
3
8
51pF
6
AD797
0.1µF
1µF
+V
S
1µF
0.1µF
–V
C1, SEE TABLE
C2 = 50pF – C1
S
NOTES
Figure 44. Recommended Connections for Distortion Cancellation
and Bandwidth Enhancement
USE CIRCUIT BOARD WITH GROUND PLANE.
Figure 43. Settling Time Test Circuit
Table 6. Recommended External Compensation for
Distortion Cancellation and Bandwidth Enhancement
DISTORTION REDUCTION
The AD797 has distortion performance (THD < −120 dB,
@ 20 kHz, 3 V rms, RL = 600 Ω) unequaled by most voltage
feedback amplifiers.
A/B
R2
(Ω) (pF) (pF) BW
A
B
R1
C1
C2
3 dB
C1
C2
3 dB
Gain (Ω)
(pF) (pF) BW
ꢀ0
ꢀ00
909 ꢀ00
ꢀ k ꢀ0
0
0
0
±0
±0
±0
6 MHz
ꢀ MHz
ꢀꢀ0 kHz 33
0
ꢀ±
±0
33
ꢀ±
6 MHz
ꢀ.± MHz
4±0 kHz
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.
ꢀ000 ꢀ0 k ꢀ0
0.01
–80
G = +1000
R
= 600Ω
L
0.003
–90
NOISE LIMIT, G = +1000
NOISE LIMIT, G = +100
G = +1000
R
= 10kΩ
L
0.001
–100
–110
–120
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.
G = +100
= 600Ω
R
L
0.0003
0.0001
G = +10
= 600Ω
R
L
100
300
1k
3k
10k
30k
100k
300k
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.
FREQUENCY (Hz)
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 ꢀ± of 20
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