The Fluke 845 is famous for its high resistance measurements, due to the
differential principle.
But its leakage or bias current is not specified at all!
Anyhow, one has to pay a premium price for this ancient technology,
perhaps having bad neon bulbs.
Well, all those new Super-Zero ChopAmps nowadays are specified for a
typical Ibias @ RT only, like the LTC2054, or the MAX4238/4239. There's
obviously no upper limit testing, and a specified 150pA max over 85°C
temperature range is much too much!
Therefore I would opt for the ICL7650, guaranteed / tested 10pA max @
25°C, with slightly higher noise.
This OpAmp is still available from MAXIM, and from Intersil, again.
Frank
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