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
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