LMC6482QML Datasheet, PDF (17/28 Pages) Texas Instruments – CMOS Dual Rail-To-Rail Input and Output Operational Amplifier

FIGURE 5. Pulse Response of
the LMC6482 Circuit in Figure 4
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Improved frequency response is achieved by indirectly driving
capacitive loads, as shown in Figure 6.
FIGURE 7. Pulse Response of
LMC6482 Circuit in Figure 6
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5.0 COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resistance
with amplifiers that have ultra-low input current, like the
LMC6482. Large feedback resistors can react with small val-
ues of input capacitance due to transducers, photodiodes,
and circuits board parasitics to reduce phase margins.
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FIGURE 6. LMC6482 Noninverting Amplifier,
Compensated to Handle a 330pF Capacitive Load
R1 and C1 serve to counteract the loss of phase margin by
feeding forward the high frequency component of the output
signal back to the amplifiers inverting input, thereby preserv-
ing phase margin in the overall feedback loop. The values of
R1 and C1 are experimentally determined for the desired
pulse response. The resulting pulse response can be seen in
Figure 7.
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FIGURE 8. Canceling the Effect of Input Capacitance
The effect of input capacitance can be compensated for by
adding a feedback capacitor. The feedback capacitor (as in
Figure 8), C_f, is first estimated by:
or
R₁ C_I ≤ R₂ C_f
which typically provides significant overcompensation.
Printed circuit board stray capacitance may be larger or small-
er than that of a bread-board, so the actual optimum value for
C_f may be different. The values of C_f should be checked on
the actual circuit. (Refer to the LMC660 quad CMOS amplifier
data sheet for a more detailed discussion.)
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