[volt-nuts] Help with 7081 AC buffer circuit

[Don @ True-Cal]

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: 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


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