The AD863xA can directly drive 1nF in unity-gain without oscillation. The

unity-gain follower (buffer) is the most sensitive configuration to capacitive loading.

Application Notes

Direct capacitive loading reduces the phase margin of amplifiers and this results in

ringing or even oscillation. Applications that require greater capacitive drive capability

should use an isolation resistor between the output and the capacitive load like the

circuit in Figure 3. The isolation resistor RISO and the load capacitor CL form a zero to

increase stability. The bigger the RISO resistor value, the more stable VOUT will be.

Note that this method results in a loss of gain accuracy because RISO forms a voltage

divider with the RL.

Figure 3. Indirectly Driving Heavy Capacitive Load

An improvement circuit is shown in Figure 4. It provides DC accuracy as well as

AC stability. The RF provides the DC accuracy by connecting the inverting signal with

the output.

The CF and RISO serve to counteract the loss of phase margin by feeding the high

frequency component of the output signal back to the amplifier’s inverting input,

thereby preserving phase margin in the overall feedback loop.

Figure 4. Indirectly Driving Heavy Capacitive Load with DC Accuracy

For no-buffer configuration, there are two other ways to increase the phase

margin: (a) by increasing the amplifier’s gain, or (b) by placing a capacitor in parallel

with the feedback resistor to counteract the parasitic capacitance associated with

inverting node.

480

μA, 6.5MHz, RRIO CMOS Operational Amplifiers

UMW

AD8631,AD8632,AD8634

R

www.umw-ic.com

9

UTD Semiconductor Co.,Limited