Hi, The other strangeness is the huge loss of OLG as they go to either rail.
________________________________ From: volt-nuts <[email protected]> on behalf of Charles Steinmetz <[email protected]> Sent: 27 July 2020 06:12 To: [email protected] <[email protected]> Subject: Re: [volt-nuts] Keithley 417 electrometer upgrade to solid-state 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 _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there.
