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From: email@example.com (Tom Bruhns)
Subject: Re: Low leakage parts
Date: 6 Jan 2003 17:00:53 -0800
References: <firstname.lastname@example.org> <3E17D633.378A@sneakemail.com> <3E184B38.588B@sneakemail.com> <email@example.com> <3E186B3C.5BA0@sneakemail.com> <firstname.lastname@example.org> <3E18805E.4AA@sneakemail.com>
NNTP-Posting-Date: 7 Jan 2003 01:00:53 GMT
Mike Monett wrote in message news:<3E18805E.4AA@sneakemail.com>...
> Have you thought about the LMC662: 2fA typ, 1.3uV/C tempco? This would
> raise the minimum current that can be measured, but the drift with
> temperature is attractive for long-term meaurements.
An interesting experiment is to plot the input bias current versus
common mode voltage. You may find that it passes through zero at some
voltage. It did with the old CA3130 op amps. If that happens, and
it's stable enough, you can get to exceedingly low input bias current.
It may even be possible to bootstrap the power supplies to keep it
low over a moderate range of input voltages.
What was the best one could do with "electrometer" vacuum tubes? What
are the current best practices for low input bias current amplifiers?
I assume there are ways to get well below 1fA, but with what sort of
bandwidth? (How few electrons can we reliably sense electrically?)
1fA*1sec at 1pF is only 1mV, but even 0.01fA for a long enough time
(an hour, or a day) on a 1pF cap becomes easy to measure. If you
connected that LMC662C as a follower, leaving the NI input floating,
it would eventually drift to the zero bias current point (if such
point is within the input common mode range) but it would take a while
to get there, at probably less than 1mV/second slew (2fA and probably
a bit more than 2pF input capacitance). ... Maybe some of the
lurkers are doing work in the very-low-current area and can offer some
insights. For me, it's just an area of casual interest.
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