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AD811

Rev. E | Page 13 of 20

Driving Capacitive Loads

The feedback and gain resistor values in Table 3 result in very

flat closed-loop responses in applications where the load capaci-

tances are below 10 pF. Capacitances greater than this 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 39. Using a 1.5 kΩ

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

driving 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.

AD811

–

RS (OPTIONAL)

VOUT

–VS

+VS

RFB

VIN

0.1

µF

0.1

µF

Figure 38. Recommended Connection for Driving a Large Capacitive Load

–6

–3

FREQUENCY (MHz)

100

VS = ±15V

CL = 100pF

RL = 10kΩ

GAIN = +2

RFB = 1.5kΩ

RS = 0

RFB = 649Ω

RS = 30Ω

Figure 39. Performance Comparison of Two Methods

for Driving a Capacitive Load

100

LOAD CAPACITANCE (pF)

100

1000

GAIN = +2

VS = ±15V

RS VALUE SPECIFIED

IS FOR FLATTEST

FREQUENCY RESPONSE

Figure 40. Recommended Value of Series Resistor vs.

the Amount of Capacitive Load

Figure 40 shows recommended resistor values for different load

capacitances. Refer again to Figure 39 for an example of the

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

the gain setting resistor, RG, to correct for the attenuation which

results due to the divider formed by the series resistor, RS, and

the load resistance.

Applications that 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 the same 500 pF capacitor, as shown in Figure 41.

100

100ns

VIN

VOUT

Figure 41. Output Waveform of an AD811 Driving a 500 pF Load.

Gain = +2, RFB = 649 Ω, RS = 15 Ω, RS = 10 kΩ

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