[volt-nuts] Help with 7081 AC buffer circuit

[David C. Partridge]

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: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] 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