AD811

REV. D

–10–

Driving Capacitive Loads

The feedback and gain resistor values in Table I will result in

very flat closed-loop responses in applications where the load

capacitances are below 10 pF. Capacitances greater than this

will result in increased peaking and overshoot, although not

necessarily in a sustained oscillation.

There are at least two very effective ways to compensate for this

effect. One way is to increase the magnitude of the feedback

resistor, which lowers the 3 dB frequency. The other method is

to include a small resistor in series with the output of the ampli-

fier to isolate it from the load capacitance. The results of these

two techniques are illustrated in Figure 32. Using a 1.5 k

Ω

feedback resistor, the output ripple is less than 0.5 dB when driv-

ing 100 pF. The main disadvantage of this method is that it

sacrifices a little bit of gain flatness for increased capacitive load

drive capability. With the second method, using a series resistor,

the loss of flatness does not occur.

0.1 F

0.1 F

AD811

Figure 31. Recommended Connection for Driving a Large

Capacitive Load

12

–6

3

–3

0

1M

9

6

10M

100M

FREQUENCY – Hz

G = +2

Figure 32. Performance Comparison of Two Methods for

Driving a Capacitive Load

100

0

1000

30

10

100

20

10

60

40

50

70

80

90

LOAD CAPACITANCE – pF

G = +2

V =

15V

R VALUE SPECIFIED

IS FOR FLATTEST

FREQUENCY RESPONSE

S

S

Figure 33. Recommended Value of Series Resistor vs. the

Amount of Capacitive Load

Figure 33 shows recommended resistor values for different load

capacitances. Refer again to Figure 32 for an example of the

results of this method. Note that it may be necessary to adjust

the load resistance.

Applications which require driving a large load capacitance at a

high slew rate are often limited by the output current available

from the driving amplifier. For example, an amplifier limited to

25 mA output current cannot drive a 500 pF load at a slew rate

greater than 50 V/

µs. However, because of the AD811’s 100 mA

output current, a slew rate of 200 V/

µs is achievable when driv-

ing this same 500 pF capacitor (see Figure 34).

10

90

100

0%

5V

2V

100ns

Figure 34. Output Waveform of an AD811 Driving a

Ω