LMC6464QML

SNOSAR8A – DECEMBER 2010 – REVISED MARCH 2013

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CAPACITIVE LOAD TOLERANCE

follower is the most sensitive configuration to capacitive load. Direct capacitive loading reduces the phase margin

of op-amps. The combination of the op-amp's output impedance and the capacitive load induces phase lag. This

results in either an underdamped pulse response or oscillation.

Capacitive load compensation can be accomplished using resistive isolation as shown in Figure 35. If there is a

resistive component of the load in parallel to the capacitive component, the isolation resistor and the resistive

load create a voltage divider at the output. This introduces a DC error at the output.

Figure 35. Resistive Isolation of a 300 pF Capacitive Load

Figure 36. Pulse Response of the LMC6464 Circuit Shown in Figure 35

Figure 36 displays the pulse response of the LMC6464 circuit in Figure 35.

Another circuit, shown in Figure 37, is also used to indirectly drive capacitive loads. This circuit is an

improvement to the circuit shown in Figure 35 because it provides DC accuracy as well as AC stability. R1 and

C1 serve to counteract the loss of phase margin by feeding the high frequency component of the output signal

back to the amplifiers inverting input, thereby preserving phase margin in the overall feedback loop. The values

of R1 and C1 should be experimentally determined by the system designer for the desired pulse response.

Increased capacitive drive is possible by increasing the value of the capacitor in the feedback loop.

Figure 37. LMC6464 Non-Inverting Amplifier,

Compensated to Handle a 300 pF Capacitive

and 100 K

Ω Resistive Load

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