Dave, I am investigating some ideas for an isolation circuit for the 845A but the first priority is to modify the HP-419A first, which doesn't have the isolated recorder output. I want to use an isolation circuit that draws very little current and can run on a single lithium battery. Not a trivial design. The DC offset circuit of the HP-419A will use another lithium-ion battery with a linear regulator to work over a wide range of battery voltages is my current thinking.
Randy On Mon, Aug 4, 2014 at 10:05 AM, Dave M <[email protected]> wrote: > From: Randy Evans <[email protected]> >> >> >> I am looking at modifying my (defective) HP419A and Fluke 845A Null >> meters >> using the LTC2054 zero drift op amp. I am planning on using a single >> 4.2 V >> Lithium Ion battery to power them (no AC input required and may not >> include, TBD). The LTC2054 has a very low bias current of typically >> 1pA at >> room temp and the bias plots vs. temp show no appreciable increase >> until >> about 50C. Conceptually, it looks pretty straight forward and, based >> on my >> Fluke 887A modifications, likely will be very stable. >> >> Since this is a very astute group, does anyone have any comments on >> the >> feasibility of this modification before I get too involved? >> >> Regards, >> >> Randy Evans >> > > > Hi Randy. Good to hear from you again. > I have modified two 883A models and one 887A model DMMs using your > details, with great results. I have a model 845A null meter that is > working fine right now, but who knows how long that will last. I would > surely welcome some research on modifying it with a zero-drift opamp. One > concern that I would have with that mod would be that some mechanism would > need to be added to retain the isolation of the recorder output and guard > integrity that's afforded by the photo-modulator/demodulator circuit. > Perhaps some kind of optically-isolated interface might be in order > > Some Spice runs on the original circuit would be interesting to see. > > Dave M > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
