150kHz, 5.5μA, Rail-to-Rail I/O

SGM8521/2/4

CMOS Operational Amplifiers

7

SG Micro Corp

www.sg-micro.com

APPLICATION NOTES

Driving Capacitive Loads

The SGM8521/2/4 can directly drive 250pF in unity-gain without

oscillation. The unity-gain follower (buffer) is the most sensitive

configuration to capacitive loading. Direct capacitive loading

reduces the phase margin of amplifiers and this results in

ringing or even oscillation. Applications that require greater

capacitive driving capability should use an isolation resistor

between the output and the capacitive load like the circuit in

SGM8521

Figure 1. Indirectly Driving Heavy Capacitive Load

An improved circuit is shown in Figure 2. It provides DC

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.

SGM8521

Figure 2. Indirectly Driving Heavy Capacitive Load with DC

Accuracy

For non-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.

Power-Supply Bypassing and Layout

The SGM8521/2/4 family operates from either a single +2.1V to

+5.5V supply or dual ±1.05V to ±2.75V supplies. For

0.1µF ceramic capacitor which should be placed close to the

supplies should be bypassed to ground with separate 0.1µF

ceramic capacitors. 2.2µF tantalum capacitor can be added for

better performance.

SGM8521

Vn

Vp

SGM8521

Vn

Vp

10

μF

0.1

μF

10

μF

0.1

μF

10

μF

0.1

μF

Figure 3. Amplifier with Bypass Capacitors