OPA2991IDSGR datasheet(25/78 Pages) TI

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Cap Load (pF)

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

C008

RISO = 0 , Positive Overshoot

RISO = 0 , Negative Overshoot

RISO = 50 , Positive Overshoot

RISO = 50 , Negative Overshoot

Figure 7-5. Small-Signal Overshoot vs Capacitive

Load (10-mV Output Step, G = 1)

Cap Load (pF)

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

C007

RISO = 0 , Positive Overshoot

RISO = 0 , Negative Overshoot

RISO = 50 , Positive Overshoot

RISO = 50 , Negative Overshoot

Figure 7-6. Small-Signal Overshoot vs Capacitive

Load (10-mV Output Step, G = –1)

For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small

resistor, RISO, in series with the output, as shown in Figure 7-7. This resistor significantly reduces ringing

and maintains DC performance for purely capacitive loads. However, if a resistive load is in parallel with the

capacitive load, then a voltage divider is created, thus introducing a gain error at the output and slightly reducing

the output swing. The error introduced is proportional to the ratio RISO / RL, and is generally negligible at low

output levels. A high capacitive load drive makes the OPAx991 well suited for applications such as reference

buffers, MOSFET gate drives, and cable-shield drives. The circuit shown in Figure 7-7 uses an isolation resistor,

RISO, to stabilize the output of an op amp. RISO modifies the open-loop gain of the system for increased phase

margin.

load

out

iso

-V

Figure 7-7. Extending Capacitive Load Drive With the OPA991

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OPA991, OPA2991, OPA4991

SBOS969F – OCTOBER 2019 – REVISED JANUARY 2022

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OPA2991IDSGR Datasheet(PDF) 25 Page - Texas Instruments