Hi Randy,
to modify the 845 with a modern chopper OpAmp is a great idea, and will
for sure improve its performance, rather than degrade it.
Fluke never specified the leakage currents of its chopAmps, neither in
the 845, nor in the 332 / 335 or 334 MOSFET Chopamps..
The real replacement input resistance was never specified, only the
isolation resistance, but not described, against which potential, or in
which arrangement. Compare that to FET and Chopper OpAmp specifications!
And the optical chopper of the 845 goes into bipolar transistors, not
even FETs were used!
Therefore, the resulting input currents, caused by imperfections of the
optical chopper, may also be in the high pAs, or nA, nobody ever has
measured that, but will also give arithmetical 10E13 Ohm at voltage
differences below µV level...
The great thing about the 845 is its 1kV isolation of input to monitor
output, but this will not be changed, if you replace the bipolar /
optical chopper amplifier stuff..
I think, nobody dared to modify the 845, or questioned it, because this
instrument is a "myth", indicated by the price, you still encounter on
ebay...
For the Hp419, everything i described is the same here, the difference
of this circuitry is the lack of the isolation transformers only.
Therefore, go ahead, and maybe - please specify the bias currents before
/ after modification.
Frank
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