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.
AD8031/AD8032
1000
VS = 5V
200mV STEP
WITH 30% OVERSHOOT
100
RS = 5Ω
RS = 0Ω
RS = 20Ω
RS = 20Ω
10
RS = 0Ω, 5Ω
RG
RF
RS
VOUT
CL
1
0
1
2
3
4
5
CLOSED-LOOP GAIN (V/V)
Figure 46. Capacitive Load Drive vs. Closed-Loop Gain
Rev. C | Page 15 of 20