Hello!
I'm new here and I see a lot of skilled people, maybe someone could help me with a very strange problem that I have with Keithley 6517A.
First of all, I'm not an electronics engineer, so my knowledge in this area is limited.
I'm using this equipment to measure samples with very high electrical resistance (>100 GOhm). A few months ago I had to measure a sample with very dynamic resistance (ranging from hundreds of GOhm to several KOhm), so I was using autorange function.
I saw that when range changes from 200 pA to 2 nA, the displayed value is 10 times smaller (sometimes even negative) than it should be, so i decided to do a manual masurement. I sourced +10 V from the internal V-Source and measured the current through a 85 GOhm fixed resistor using Keithley low noise triax cable with Meter connect ON (it internally connects V-Source LOW to Ammeter LOW). The results were:
200 pA range -> +117 pA
2 nA range -> negative or 10 times lower current value (11.7 pA)
20 nA range -> +117pA
200 nA range -> +117 pA
Then, using the 85 GOhm resistor, I inserted one end of the resistor in V-Source High, and the other one (aprox. 10 cm long) in input High. Sourcing +10 V again, it measured:
200 pA range -> +117 pA
2 nA range -> +117 pA
20 nA range -> +117pA
200 nA range -> +117 pA
I extended the part that goes into the input High by 10 cm (20 cm total length) and measured again, using the same settings:
200 pA range -> +117 pA
2 nA range -> negative or 10 times lower current value (11.7 pA)
20 nA range -> +117pA
200 nA range -> +117 pA
It seems that if the input High connector is longer than 1x cm (no matter how good is it screened), the ammeter doesn't measure correctly only on 2 nA range, the other ranges work great. This problem doesn't always show up, sometimes the electrometer works fine, other times it doesn't.
I tried resetting the instrument using default bench settings, using digital filter, different integration periods (measurement speed), nothing seems to work. Any help would be much aprreciated.
I'm sorry for the bad english, I'm not a native english speaker, I hope I made myself understood.
Best regards,
Ovidiu