Data Sheet

AD797

Rev. K | Page 15 of 19

VERROR × 5

–VS

–VS

+VS

+VS

VIN

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,

at 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 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.

VIN

a.

b.

AD797

50pF

R1

R2

R2

C1, SEE TABLE

C2 = 50pF – C1

6

2

3

VIN

AD797

C2

C1

R1

6

2

3

8

8

VOUT

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

–80

300k

–120

300

100

–110

–100

–90

100k

30k

10k

3k

1k

FREQUENCY (Hz)

0.01

0.003

0.001

0.0003

0.0001

NOISE LIMIT, G = +1000

NOISE LIMIT, G = +100

G = +1000

RL = 600Ω

G = +1000

RL = 10kΩ

G = +10

RL = 600Ω

G = +100

RL = 600Ω

Figure 46. Total Harmonic Distortion (THD) vs. Frequency at 3 V rms

for Figure 45b