[volt-nuts] FW: Fluke 845 Modification

m k m1k3k1 at hotmail.com
Tue Dec 31 07:53:49 EST 2013

> Date: Sun, 29 Dec 2013 21:44:03 +0100
> From: frank.stellmach at freenet.de
> To: volt-nuts at febo.com
> Subject: [volt-nuts] Fluke 845 Modification
> 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.

With the rise in leakage vs temperature, if 65C operation is important the 7650 is not for you. I think the 85C leakage will be similar to the 4238

> Frank
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