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

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
4.75V.

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

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

volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

Looking at the LTC2054 datasheet, I'm pretty sure you should be able run
that fine off a 3V, 0, -3V as well.

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

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

volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

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@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