Datasheet

Data Sheet AD8031/AD8032

Rev. F | Page 15 of 20

DRIVING CAPACITIVE LOADS

Capacitive loads interact with an op amp’s output impedance to

create an extra delay in the feedback path. This reduces circuit

stability and can cause unwanted ringing and oscillation. A

given value of capacitance causes much less ringing when the

amplifier is used with a higher noise gain.

The capacitive load drive of the AD8031/AD8032 can be

increased by adding a low valued resistor in series with the

capacitive load. Introducing a series resistor tends to isolate the

capacitive load from the feedback loop, thereby diminishing its

influence. Figure 46 shows the effects of a series resistor on the

capacitive drive for varying voltage gains. As the closed-loop

gain is increased, the larger phase margin allows for larger

capacitive loads with less overshoot. Adding a series resistor at

lower closed-loop gains accomplishes the same effect. For large

capacitive loads, the frequency response of the amplifier is

dominated by the roll-off of the series resistor and capacitive load.

1000

100

CAPACITIVE LOAD (pF)

CLOSED-LOOP GAIN (V/V)

0 1 2 3 4 5

= 5Ω

= 0Ω

= 20Ω

= 0Ω, 5Ω

= 5V

200mV STEP

WITH 30% OVERSHOOT

OUT

01056-046

Figure 46. Capacitive Load Drive vs. Closed-Loop Gain