[volt-nuts] Fluke 845 Modification LTC2057

David C. Partridge david.partridge at perdrix.co.uk
Sun Dec 29 04:20:44 EST 2013

It should work fine off TWO 3V lithiums then, as if you are running as
3V,0,-3V supplies, that gives you a total of 6V, which is plenty more than

David Partridge 
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of Randy Evans
Sent: 29 December 2013 03:46
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] Fluke 845 Modification LTC2057


I looked at the LTC2057 but it doesn't go down to 3V operation (4.75 V min)
and I want to power the Fluke 845 with 3V lithiums.  In addition, the
LTC2054 has a lower input bias current (1 pA at 25 C).  I wanted the lower
bias current since I want to use the circuit to upgrade an HP-419A also, and
the 419A measures current down to a FS range of 30 pA.  The LTC2057 bias
current is spec'd at 30 pA typ, which would make it useless for measuring
the lower current ranges of the 419A.

Regarding the isolation amp, I did find a good, relatively low cost amp (TI
ISO124) but it only works down to +/-4.5VDC, which means it is not
compatible with 3V lithium cells.  I would have to use 4 lithium cells
instead of 2.  I'm still looking for a good isolation amplifier that will
work on +/- 2 VDC but I'm getting pessimistic about finding one.  I might
have to just bite the bullet and use +/-6VDC power supplies.

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