Does that matter on the AC buffer in this case? Maybe that's the
normal 1/f noise of the circuit down near DC.
If it is problematic, then you may want to try reconditioning the
low level signal relay contacts. You can rig up a circuit with a 9V
battery and a 1 k or so ohm resistor, temporarily hooked up across
each contact, then run the relay through a few dozen cycles. If you
do that with each contact in the signal path, it may clean it up.
You may have to disconnect some of them to avoid damage to the
circuits - especially at the OP-16 input - it depends on the
overload rating. You may be able to first see if it's worth doing
this laborious process by very gently tapping each relay body to see
if there's any response.
Also, if you are working on the circuit while it's out in the open,
you could have light generating currents in some of the devices, so
make sure it's protected.
Lastly, the zero adjust pot itself could be the culprit - try running
it to each extreme a few dozen times to clean the wiper and element.
Ed
----- 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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