[volt-nuts] Fluke 887 mod with LTC1050

[David]

Could it be current noise on the LTC1050 inputs?  Charge pumping
produces a current noise and bias current spikes which are deceptively
large given the input bias current specification.

On Wed, 26 Apr 2017 19:17:08 -0700, you wrote:

>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