Ed wrote:
The input bias current is paramount, of course, but depends on not just
the specs, but on how they actually work inside, how they are packaged,
and how they are applied. It's not easy to predict or discern real
performance to expect, without experimenting.
The input protection devices and structures of the CMOS opamps are key
to getting low bias current. The package and pinout determine the
external limits of performance. You can't do anything about these, but
you can choose the best trade-offs for an application. The more you can
figure out about the input behavior, the better you can use it.
* * *
Once you know what the input looks like, the main thing is to see how
the bias current responds to the power supply voltages, and with it, the
input common-mode voltage.
In particular, be very wary if the chosen op-amp is an "RRI" type
(rail-to-rail input). [Neither the LMC6001 nor the LMC6041 appears to
have a rail-to-rail input.]
RRI op-amps use different front end circuitry as the input common-mode
voltage moves from one power-supply rail to the other. This is
generally seen in astonishingly ugly (and discontinuous) graphs of input
leakage ("bias") current, offset voltage, and distortion vs. common-mode
input voltage.
This phenomenon is discussed (along with similar strangeness with
rail-to-rail output circuitry) [as Ed notes, the LMC6041 is an "RRO"
op-amp] in Horowitz & Hill's "Art of Electronics," 3rd ed., Section 5.9
(pp. 315 ff), and their "Art of Electronics -- The X Chapters," Section
4x.11 (pp. 336 ff). A few brave op-amp manufacturers also disclose
these behaviors in their literature.
Best regards,
Charles
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