[volt-nuts] Fluke 887 mod with LTC1050

[Randy Evans]

Some time ago I presented a design where I replaced the non-working chopper
circuitry for a Fluke 887 differential voltmeter with an LTC1050 precision
chopper amplifier.  The design seemed to work fine, but I have discovered
an issue ( I haven’t actually used the meter very much).  The manual says
to zero the meter by shorting the input terminals and then adjusting the
zero adjustment for a zero reading on the meter.  This works fine with the
LTC1050 circuit, but I recently noticed the meter reads -0.08 when the
input is open on the 1V range and the null switch is on 0.0001 (it is fine
on all other ranges/null switch settings).  This equates to a -8uV
reading.  I believe this is due to the input bias current of the LTC1050
and the 1 megohm input resistance of the 887 on the most sensitive
range/null setting.  I tried three different LTC1050s and they all exhibit
the same problem.

However, this would imply an 8pA bias current, which is much higher than
expected.  My question is: does anyone else have a better explanation for
this issue and does anyone experience this issue with the original circuit?



Thanks,

Randy Evans AE6YG