Datasheet

LM6132

SNOS751D –APRIL 2000–REVISED FEBRUARY 2013

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DRIVING CAPACITIVE LOADS

Capacitive loads decrease the phase margin of all op amps. This is caused by the output resistance of the

amplifier and the load capacitance forming an R-C phase lag network. This can lead to overshoot, ringing and

oscillation. Slew rate limiting can also cause additional lag. Most op amps with a fixed maximum slew-rate will lag

further and further behind when driving capacitive loads even though the differential input voltage raises. With the

LM6132, the lag causes the slew rate to raise. The increased slew-rate keeps the output following the input

much better. This effectively reduces phase lag. After the output has caught up with the input, the differential

input voltage drops down and the amplifier settles rapidly.

Figure 32.

These features allow the LM6132 to drive capacitive loads as large as 500 pF at unity gain and not oscillate. The

scope photos (Figure 33 and Figure 34) above show the LM6132 driving a 500 pF load. In Figure 33 , the lower

trace is with no capacitive load and the upper trace is with a 500 pF load. Here we are operating on ±12V

supplies with a 20 V

pulse. Excellent response is obtained with a C

of 39 pF. In Figure 34, the supplies have

been reduced to ±2.5V, the pulse is 4 V

and C

is 39 pF. The best value for the compensation capacitor should

be established after the board layout is finished because the value is dependent on board stray capacity, the

value of the feedback resistor, the closed loop gain and, to some extent, the supply voltage.

Another effect that is common to all op amps is the phase shift caused by the feedback resistor and the input

capacitance. This phase shift also reduces phase margin. This effect is taken care of at the same time as the

effect of the capacitive load when the capacitor is placed across the feedback resistor.

The circuit shown in Figure 35 was used for these scope photos.

Figure 33.

Product Folder Links: LM6132