[volt-nuts] Fluke 845 Modification LTC2057

[Randy Evans]

I guess I wan't clear in my previous e-mail.  The LTC2054 works down to 2.7VDC and since I need bipolar operation, I will use it with plus and minum 3 VDC supplies.  No problem with this IC. 

The ISO124 requires plus and minus 4.75 VDC minimum so it won't work with two lithium cells at plus and minum 3VDC.  Hence the problem.  

Didier, I am planning on using a specifically designed charger for lithium cells to avoid the problems you mentioned.

I was hoping someone knew of a bipolar isoation amplifier similar to the ISO124 that would work with plus and minus 3 VDC supplies.

Randy



On Saturday, December 28, 2013 7:46 PM, Randy Evans <randallgrayevans at yahoo.com> wrote:
 
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