He explained that the instrument needed to be calibrated 3 times before it was within tolerance and part of the reason was that the - 1 DCV was out on the 10 V range. The following resistances were roughly measured: 11.08 kΩ, 4.16 kΩ, 495 Ω (when connected the board, measured in both directions) Does anyone else get these readings from the board? If so, does your Advantest have trouble reading high-ohms? According to MickleT's schematics ("simplified diagram of the analogue board" and page 3/5 of "ADC"), the readings should be: 45 kΩ, 4.5 kΩ, and 500 Ω (when connected directly to the DMM) What makes me suspicious of R230 is that the -1 V range was reported to be a problem by the calibration technician. Out of curiosity, I cracked open the R6581T and measured the R230 network resistor because I have a hunch that this component could be the culprit. Then i started to think about modding the input MUX. I used the + and - voltages from zeners D202 and D203 inside the R6581T as test input and there were 1 V peaks on the 8K8 protection resistor when measuring - 5.6 V. The R6581T was open and the scope connected to the DCV MUX input. If you repeat that with Autozero On and Off, you will probably see strong leakage at negative input voltage and with Autozero On. I would not use a power supply for that as it may add leakage current, too. The interesting part is adding a battery to repeat the leakage current measurement at +6 and at -6 V (or whatever battery you have).
The R6581T input MUX emits RF, originating from the LM339 comparators that drive the FET switches. You may need to eliminate RF using a bypass capacitor parallel to the R6581T input. The "around zero" result with Autozero On appears strange. I think 5 pA is a lot, a good R6581T has below 1 pA. As far as i understand the studies above were near zero input voltage to the R6581T.
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