OP270GSZ2 datasheet(15/20 Pages) AD

15 / 20 page

OP270

Rev. E | Page 15 of 20

CAPACITIVE LOAD DRIVING AND POWER SUPPLY

CONSIDERATIONS

The OP270 is unity-gain stable and capable of driving large

capacitive loads without oscillating. Nonetheless, good supply

bypassing is highly recommended. Proper supply bypassing

reduces problems caused by supply line noise and improves the

capacitive load driving capability of the OP270.

In the standard feedback amplifier, the output resistance of the

op amp combines with the load capacitance to form a low-pass

filter that adds phase shift in the feedback network and reduces

stability. A simple circuit to eliminate this effect is shown in

Figure 39. The components C1 and R3 decouple the amplifier

from the load capacitance and provide additional stability. The

values of C1 and R3 shown in Figure 39 are for a load capacitance

of up to 1000 pF when used with the OP270.

200pF

VIN

VOUT

PLACE SUPPLY DECOUPLING

CAPACITOR AT OP270

OP270

V+

0.1µF

10µF

1000pF

10µF

0.1µF

50Ω

V–

Figure 39. Driving Large Capacitive Loads

UNITY-GAIN BUFFER APPLICATIONS

When Rf ≤ 100 Ω and the input is driven with a fast, large signal

pulse (>1 V), the output waveform looks like the one in Figure 40.

During the fast feedthrough-like portion of the output, the input

protection diodes effectively short the output to the input, and

a current, limited only by the output short-circuit protection, is

drawn by the signal generator. With Rf ≥ 500 Ω, the output is

capable of handling the current requirements (IL ≤ 20 mA at 10 V);

the amplifier stays in its active mode and a smooth transition occurs.

When Rf > 3 kΩ, a pole created by Rf and the input capacitance

(3 pF) of the amplifier creates additional phase shift and reduces

phase margin. A small capacitor (20 pF to 50 pF) in parallel with

Rf helps eliminate this problem.

OP270

2.4V/µs

Figure 40. Pulsed Operation

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