[volt-nuts] Solartron 7081 noise

[gbusg]

Excellent work, Mickle!

Nice touch adding the PTFE standoff insulators. How did you make connections 
on the trace-side to the PTFE stand-offs? (To reduce leakage and dielectric 
absorption, I assume you ran hard-wires instead of using existing PCB 
traces?)

Your PCB leakage problem reminds me of a similar problem Sony had with a 
high-impedance circuit back in the seventies. The problem began showing-up 
in older units of that model after years of use. Technicians were able to 
temporarily eliminate the electrical leakage currents by spraying the area 
with PCB cleaner solutions. But the problem would return in a few months. 
Finally Sony determined the problem was caused by bacterial growth in higher 
humidity environments due to a contamination problem at the factory when the 
products were made. Interestingly, Sony then released a Service Note, 
recommending that you spray the area with super-cold Freon to kill the 
bacteria by freezing it. ...I tried that approach and it worked. I've never 
forgotten that situation, because it was so counterintuitive and strange!

Apparently according to the 7081 manual, its integration time is determined 
by the Scale Length you select, hence number of digits displayed? So 
apparently your 7 1/2 digits resolution corresponds to 3.2 seconds 
integration time. Do I have that right?

It might be interesting if some other 7081 users will duplicate your tests 
for comparison.

Cheers,
Greg



----- Original Message ----- 
From: "Михаил" <timka2k at yandex.ru>
To: <volt-nuts at febo.com>
Sent: Sunday, September 11, 2011 6:47 AM
Subject: [volt-nuts] Solartron 7081 noise


Hi everyone,

I recently purchased a Solartron 7081 DMM. It▓s one of the first model, 
dated Apr. 1984.
I suspect that it is not as close to accurate as it could be due to aging 
and
storage in a humid environment. My test showed good accuracy, but the huge 
input
bias current and suspiciously large noise. After 8 hours of heating at 25
degrees Celsius input current was 330 pA causing a zero shift in the
short-circuited input √ 12 uV (internal resistance at the Hi input is 36000
Ohms). Standard warm-up procedure after prolonged low-temperature storage 
did
not help. One of the problem was found in the input amplifier and DC 
selector:
surface and volumetric leakage in the PCB from the relays power supply 
channel
(- 10 V), especially in places that are not protected by guard traces. After
checking each critical item in the input circuits (attenuator, DC ranging, 
input
amplifier with MDM channel) and installing the PTFE bushings and insulators
(http://img14.imageshack.us/img14/3432/pcb6top.jpg 
,http://img825.imageshack.us/img825/3896/pcb6bottom.jpg )
input bias has been reduced down to 1-2 pA!
After all, S7081 was warmed up again for more than 12 hours and has been 
tested
by drifts through the RS232 interface and a terminal on a PC:
1) Short inputs voltage drift while 6 hours preheating 
(http://img109.imageshack.us/img109/2384/7081zeropreheating.jpg);
2) Short inputs voltage drift in DC 10V range after preheating 
(http://img198.imageshack.us/img198/4846/708110vzerofront.jpg);
3) ADC zero test result, used by S7081 in the selftest procedure, periodical
(every 15 minutes) drift correction and while 6..8-1/2 resolution selection
(http://img155.imageshack.us/img155/2254/7081test0.jpg);
4) Internal reference test result, used by S7081 in the selftest procedure 
(http://img694.imageshack.us/img694/3379/7081test10.jpg).
After analyzing of the results it seems that the next big problem is the ADC
noise, since its amplitude does not decrease even with a short ADC inputs 
during
the test of zero (TEST0). Maybe I should change the integrators op-amp 
ICL7650S.

Mickle Timofeyev (Russia)







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