1MHz, 60μA, Rail-to-Rail I/O

SGM321/SGM358/SGM324

CMOS Operational Amplifiers

7

SG Micro Corp

www.sg-micro.com

APPLICATION NOTES

Driving Capacitive Loads

The SGM321/SGM358/SGM324 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

SGM321

Figure 1. Indirectly Driving Heavy Capacitive Load

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

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.

SGM321

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 SGM321/SGM358/SGM324 can operate from either a

single +2.1V to +5.5V supply or dual ±1.05V to ±2.75V

supplies. For single-supply operation, bypass the power

with separate 0.1µF ceramic capacitors. 2.2µF tantalum

capacitor can be added for better performance.

SGM321

Vn

Vp

10µF

0.1µF

SGM321

Vn

Vp

10µF

0.1µF

10µF

0.1µF

Figure 3. Amplifier with Bypass Capacitors