AD797

REV. C

–12–

0.47µF

250Ω

0.47µF

4.26k

1kΩ

100Ω

1kΩ

2x

HP2835

(VIA LESS THAN 1FT

50

COAXIAL CABLE)

1kΩ

20pF

51pF

NOTE:

USE CIRCUIT

BOARD

WITH GROUND

PLANE

1kΩ

TEKTRONIX

CALIBRATION

FIXTURE

VIN

20pF

1M

TO TEKTRONIX

7A26

OSCILLOSCOPE

PREAMP INPUT

SECTION

A1

AD797

7

4

6

–VS

+VS

1µF

0.1µF

1µF

0.1µF

3

2

A2

AD829

7

4

6

–VS

+VS

3

2

2x

HP2835

226

Ω

Ω

Ω

Ω

Ω

ΩΩ

Ω

Ω

Ω

Figure 40. Settling Time Test Circuit

DISTORTION REDUCTION

The AD797 has distortion performance (THD < –120 dB, @

feedback amplifiers.

At higher gains and higher frequencies THD will increase due

to reduction in loop gain. However in contrast to most conven-

tional voltage feedback amplifiers the AD797 provides two effec-

tive 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 for cancellation of the

output stage’s distortion (patent pending). To achieve this a ca-

pacitance equal to the effective compensation capacitance, usu-

ally 50 pF, is connected between Pin 8 and the output (C2 in

Figure 41). Use of this feature will improve distortion perfor-

mance when the closed loop gain is more than 10 or when fre-

quencies of interest are greater than 30 kHz.

Bandwidth enhancement via decompensation is achieved by

connecting a capacitor from Pin 8 to ground (C1 in Figure 41)

effectively subtracting from the value of the internal compensa-

tion capacitance (50 pF), yielding a smaller effective compensa-

tion capacitance and, therefore, a larger bandwidth. The

benefits of this begin at closed loop gains of 100 and up. A

maximum value of

≈33 pF at gains of 1000 and up is recom-

mended. At a gain of 1000 the bandwidth is 450 kHz.

Table IV and Figure 42 summarize the performance of the

AD797 with distortion cancellation and decompensation.

R1

R2

50pF

AD797

2

8

3

6

a.

C1, SEE TABLE

C2 = 50pF – C1

R1

R2

AD797

2

8

3

6

C1

C2

b.

Figure 41. Recommended Connections for Distortion

Cancellation and Bandwidth Enhancement

Table IV. Recommended External Compensation

A/B

A

B

R1

R2

C1

C2

3 dB

C1

C2 3 dB

ΩΩ

(pF)

BW

(pF)

BW

G = 10

909 100

0

50

6 MHz

0

50 6 MHz

G = 100

1 k

10

0

50

1 MHz

15

33 1.5 MHz

G = 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

G=1000

G=10

G=100

Figure 42. Total Harmonic Distortion (THD) vs. Frequency

@ 3 V rms for Figure 41b