CBM8631ASC7 datasheet(8/18 Pages) COREBAI

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CBM8631-CBM8632-CBM8634

OPERATION INSTRUCTION

Capacitive Load Tolerance

The CBM863x family is optimized for bandwidth and speed, not for driving capacitive loads.

Output capacitance will create a pole in the amplifier’s feedback path, leading to excessive

peaking and potential oscillation. If dealing with load capacitance is a requirement of the

application, the two strategies to consider are (1) using a small resistor in series with the

amplifier’s output and the load capacitance and (2) reducing the bandwidth of the amplifier’s

feedback loop by increasing the overall noise gain. Figure 2. shows a unity gain follower using

the series resistor strategy. The resistor isolates the output from the capacitance and, more

importantly, creates a zero in the feedback path that compensates for the pole created by the

output capacitance.

Figure 2. Indirectly Driving a Capacitive Load Using Isolation Resistor

The bigger the RISO resistor value, the more stable VOUT will be. However, if there is a resistive

load RL in parallel with the capacitive load, a voltage divider (proportional to RISO/RL) is formed,

this will result in a gain error.

The circuit in Figure 3 is an improvement to the one in Figure 2. RF provides the DC accuracy

by feed-forward the VIN to RL. 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 the phase margin in the overall feedback loop. Capacitive drive can be

increased by increasing the value of CF. This in turn will slow down the pulse response.

Figure 3. Indirectly Driving a Capacitive Load with DC Accuracy

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