[volt-nuts] Solartron 7081 noise

[J. L. Trantham]

Mickle,

I am admiring your plots of data about your meter.  How did you do that?

I recently purchased a 7081 (SN 001523) and would like to do a similar assessment of my meter.  I have been able to use HyperTerminal to download the CAL constants and I just turned on the 'Output,RS232,ON' command and it is happily moving the readings into HyperTerminal.  Now, it would appear, I need to find a way to move the data into Excel then find a way to 'plot' it.  Perhaps it would be easier to do this via the GPIB?

Any thoughts from anyone?

Joe

-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Ìèõàèë
Sent: Sunday, September 11, 2011 7:48 AM
To: volt-nuts at febo.com
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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