[volt-nuts] Help with 7081 AC buffer circuit

[Mitch Van Ochten]

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" <david.partridge at perdrix.co.uk>
To: "'Discussion of precise voltage measurement'" <volt-nuts at febo.com>
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
>
>
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.