AD8021 datasheet(19/20 Pages) AD

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REV. D

AD8021

–19–

Driving Capacitive Loads

When the AD8021 drives a capacitive load, the high frequency

response may show excessive peaking before it rolls off. Two

techniques can be used to improve stability at high frequency and

reduce peaking. The first technique is to increase the compensa-

tion capacitor, CC, which reduces the peaking while maintaining

gain flatness at low frequencies. The second technique is to add a

resistor, RSNUB, in series between the output pin of the AD8021

and the capacitive load, CL. Figure 15 shows the response of the

AD8021 when both CC and RSNUB are used to reduce peaking.

For a given CL, Figure 16 can be used to determine the value of

RSNUB that maintains 2 dB of peaking in the frequency response.

Note, however, that using RSNUB attenuates the low frequency

output by a factor of RLOAD/(RSNUB + RLOAD).

CC = 7pF;

RSNUB = 0

CC = 8pF;

RSNUB = 0

0.1

1000

100

FREQUENCY – MHz

CC = 8pF;

RSNUB = 17.4

499

49.9

33pF

FET

PROBE

–VS

RSNUB

+VS

1.0

Figure 15. Peaking vs. RSNUB and CC for CL = 33 pF

CAPACITIVE LOAD – pF

050

510

15202530354045

Figure 16. Relationship of RSNUB vs. CL for 2 dB

Peaking at a Gain of +2

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