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HGC2262

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HGC2262 数据手册

HGC2261/2262/2264

Application Notes

Driving Capacitive Loads

Driving large capacitive loads can cause stability problems for voltage feedback op amps. As the load

capacitance increases, the feedback loop’s phase margin decreases, and the closed loop bandwidth is

reduced. This produces gain peaking in the frequency response, with overshoot and ringing in the step

response. A unity gain buffer (G = +1) is the most sensitive to capacitive loads, but all gains show the

same general behavior.

When driving large capacitive loads with these op amps (e.g., > 100 pF when G = +1), a small series

resistor at the output (RISO in Figure 1) improves the feedback loop’s phase margin (stability) by making

the output load resistive at higher frequencies. It does not, however, improve the bandwidth.

To select RISO, check the frequency response peaking (or step response overshoot) on the bench. If the

response is reasonable, you do not need RISO. Otherwise, start RISO at 1 kΩ and modify its value until

the response is reasonable.

Figure 1. Indirectly Driving Heavy Capacitive Load

An improvement circuit is shown in Figure 2. It provides DC accuracy as well as AC stability. RF provides

the DC accuracy by connecting the inverting signal with the output, CF and RISO serve 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.

Figure 2. Indirectly Driving Heavy Capacitive Load with DC Accuracy

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