David,
Try enclosing the op-amp and nearby components with a little Styrofoam
"hat". The part dissipates a fair amount of power and the bias current,
although small, does show significant change with temperature. This change
multiplied by the 1 megohm input resistor may be responsible for some drift.
Regards,
mitch
----- Original Message -----
From: "David C. Partridge" <[email protected]>
To: "'Discussion of precise voltage measurement'" <[email protected]>
Sent: Friday, June 08, 2012 12:46 PM
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp has an
offset trim pot which I can use to adjust the output DC offset with the
input to the meter shorted - so far so good. The catch is that it's
really *very* noisy, wandering by as much as 20-100 uV in less than half a
second, and wandering around the zero set point by as much as +/- 800uV
over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at 120
celsius, as before I did that there was a huge monotonic drift as the PCB
warmed up with the result that after an hour or so the DC output of the
buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the service
manual which can be found here:
<http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solartron>
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit is
likely to causing this problem.
Thanks
David Partridge
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