Well that is a clue within itself. Use a can of circuit freeze very lightly on a few components, especially resistors and capacitors, and see if one in particular component quietens the noise. Sounds like you have a case of Johnson's noise aggravated with warm up.
Don ----- Original Message ----- From: David C. Partridge To: 'Discussion of precise voltage measurement' Sent: Saturday, June 09, 2012 8:43 PM Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit Argghhhh! Changing the pot didn't fix it - once the meter had warmed up for > 1hr it was backto its old tricks. Dave -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of David C. Partridge Sent: 09 June 2012 19:02 To: 'Discussion of precise voltage measurement' Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit Changing the potentiometer was indeed the solution - thanks to Ed and to John (whose post never made it here, though I found it on the site). The DC level at TP705 is now relatively stable and close to 0uV (jumping around a bit but typically well less than 100uV). Now that I get a stable reading near 0uV here, I see another problem :( If I then switch from the VAC to the VAC plus VDC setting, the DC output level at TP705 jumps positive by about 600uV, which results in a difference in reading of around 60uV which I don't believe should happen. Strangely, if I leave the input open circuit, I can switch between VAC and VAC+VDC modes without this shift occurring. I re-wired the circuitry around the relays using PTFE feed-through insulators and direct point to point wiring to eliminate leakage problems between the relay power lines (-10V, -15V) and the signals before I baked the PCB. Regards, David Partridge _______________________________________________ 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. _______________________________________________ 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.
