10

to

W

20

W

V+

V

IN

V

OUT

R

S

R

L

C

L

OPA322

70

60

50

40

30

20

10

0

0

3000

CapacitiveLoad(pF)

2500

500

1000

1500

2000

G=1,V =1.8V

S

G=1,V =5.5V

S

G=10,V =1.8V

S

G=10,V =5.5V

S

OPA322, OPA322S

OPA2322, OPA2322S

OPA4322, OPA4322S

www.ti.com

SBOS538A

– JANUARY 2011 – REVISED MAY 2011

CAPACITIVE LOAD AND STABILITY

The OPA322 is designed to be used in applications where driving a capacitive load is required. As with all op

amps, there may be specific instances where the OPA322 can become unstable. The particular op amp circuit

configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an

amplifier is stable in operation. An op amp in the unity-gain (+1 V/V) buffer configuration and driving a capacitive

load exhibits a greater tendency to become unstable than an amplifier operated at a higher noise gain. The

capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback loop that

degrades the phase margin. The degradation of the phase margin increases as the capacitive loading increases.

When operating in the unity-gain configuration, the OPA322 remains stable with a pure capacitive load up to

approximately 1 nF.

characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop

gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when

observing the overshoot response of the amplifier at higher voltage gains, as shown in Figure 29. One technique

for increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert a small

This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. A possible

problem with this technique is that a voltage divider is created with the added series resistor and any resistor

connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that

reduces the output swing. The error contributed by the voltage divider, however, may be insignificant. For

is decreased to 600

Ω, which the OPA322 is able to drive, the error increases to 7.5%.

Figure 28. Improving Capacitive Load Drive

Copyright

© 2011, Texas Instruments Incorporated

13