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 havent 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 _______________________________________________ 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.
