Datasheet

Data Sheet AD797
Rev. I | Page 15 of 20
V
ER
R
O
R
×
5
–V
S
–V
S
+V
S
+V
S
V
IN
00846-043
TO TEKTRONIX
7A26
OSCILLOSCOPE
PREAMP INPUT
SECTION
(VIA LESS THAN 1FT
50Ω COAXIAL CABLE)
TEKTRONIX
CALIBRATION
FIXTURE
NOTES
USE CIRCUIT BOARD WITH GROUND PLANE.
2×
HP2835
2×
HP2835
1MΩ
226Ω
51pF
1kΩ 1kΩ
1kΩ
1kΩ
100Ω
250Ω
0.47µF
1µF
0.1µF
1µF
0.1µF
0.47µF
4.26kΩ
20pF
20pF
7
3
6
2
A2
AD829
A1
AD797
4
7
3
6
2
4
+
–
+
–
Figure 44. Settling Time Test Circuit
DISTORTION REDUCTION
The AD797 has distortion performance (THD < −120 dB,
@ 20 kHz, 3 V rms, R
L
= 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 distortion of the output stage
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 45).
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 45).
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.
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 8 and Figure 46 summarize the performance of the
AD797 with distortion cancellation and decompensation.
V
I
N
a.
b
.
AD797
00846-044
50pF
R1
R2
R2
C1, SEE TABLE
C2 = 50pF – C1
6
2
3
V
I
N
AD797
C2
C1
R1
6
2
3
8
8
V
O
U
T
Figure 45. Recommended Connections for Distortion Cancellation
and Bandwidth Enhancement
Table 8. Recommended External Compensation for
Distortion Cancellation and Bandwidth Enhancement
A/B A B
Gain
R1
(Ω)
R2
(Ω)
C1
(pF)
C2
(pF)
3 dB
BW
C1
(pF)
C2
(pF)
3 dB
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
00846-045
–80
300k
–120
300100
–110
–100
–90
100k30k10k3k1k
FREQUENCY (Hz)
THD (dB)
0.01
0.003
0.001
0.0003
0.0001
THD (%)
NOISE LIMIT, G = +1000
NOISE LIMIT, G = +100
G = +1000
R
L
= 600Ω
G = +1000
R
L
= 10kΩ
G = +10
R
L
= 600Ω
G = +100
R
L
= 600Ω
Figure 46. Total Harmonic Distortion (THD) vs. Frequency @ 3 V rms
for Figure 45b