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 _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.