LMC6482 datasheet(24/50 Pages) TI2

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7.2.1.2.3 Compensating For Input Capacitance

The use of large values of feedback resistance with amplifiers that have ultra-low input current, like the

LMC648x, is quite common. Large feedback resistors can react with small values of input capacitance due

to transducers, photo diodes, and circuits board parasitics to reduce phase margins.

–

LMC648x

OUT

Figure 7-12. 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 7-12), Cf, is first estimated by:

12πR1CIN ≥ 12πR2Cf

(1)

R1 CIN ≤ R2 Cf

(2)

which typically provides significant overcompensation.

Printed-circuit-board stray capacitance can be larger or smaller than that of a bread-board, so the actual

optimum value for Cf can be different. Check the value of Cf on the actual circuit. (Refer to the LMC660 quad

CMOS amplifier data sheet for a more detailed discussion.)

7.2.1.2.4 Offset Voltage Adjustment

Offset voltage adjustment circuits are illustrated in Figure 7-13 and Figure 7-14. Large value resistances and

potentiometers are used to reduce power consumption while providing typically ±2.5mV of adjustment range,

referred to the input, for both configurations with VS = ±5V.

–

LMC648x

OUT

500k

499

V+

Figure 7-13. Inverting Configuration Offset Voltage

Adjustment

–

LMC648x

OUT

500k

V+

Figure 7-14. Noninverting Configuration Offset

Voltage Adjustment

LMC6482

LMC6484

SNOS674J – OCTOBER 1997 – REVISED SEPTEMBER 2024

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