LM6482 National Semiconductor, LM6482 Datasheet - Page 13

• Manufacturer Part Number: LM6482
• Description: CMOS Dual Rail-To-Rail Input and Output Operational Amplifier
• Manufacturer: National Semiconductor

Application Information
nation of the op-amp’s output impedance and the capacitive
load induces phase lag. This results in either an under-
damped pulse response or oscillation.
Capacitive load compensation can be accomplished using
resistive isolation as shown in Figure 4 . This simple tech-
nique is useful for isolating the capacitive inputs of multiplex-
ers and A/D converters.
Improved frequency response is achieved by indirectly driv-
ing capacitive loads, as shown in Figure 6 .
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 .
Compensated to Handle a 330 pF Capacitive Load
FIGURE 6. LMC6482 Noninverting Amplifier,
the LMC6482 Circuit in Figure 4
FIGURE 5. Pulse Response of
FIGURE 4. Resistive Isolation
of a 330 pF Capacitive Load
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(Continued)
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13
5.0 Compensating for Input
Capacitance
It is quite common to use large values of feedback resis-
tance with amplifiers that have ultra-low input current, like
the LMC6482. Large feedback resistors can react with small
values of input capacitance due to transducers, photo-
diodes, and circuits board parasitics to reduce phase
margins.
The effect of input capacitance can be compensated for by
adding a feedback capacitor. The feedback capacitor (as in
Figure 8 ), C
which typically provides significant overcompensation.
Printed circuit board stray capacitance may be larger or
smaller than that of a bread-board, so the actual optimum
value for C
checked on the actual circuit. (Refer to the LMC660 quad
CMOS amplifier data sheet for a more detailed discussion.)
FIGURE 8. Canceling the Effect of Input Capacitance
f
f
, is first estimated by:
may be different. The values of C
FIGURE 7. Pulse Response of
LMC6482 Circuit in Figure 6
R
1
C
IN
or
R
2
C
f
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www.national.com
f
should be
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