[volt-nuts] Fluke 845 Modification LTC2057

[Randy Evans]

John,

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.

Randy