6221/2182A Delta Mode - Current Range Commands Ignored by 6221

[ozgurrustem]

Hello,

We are trying to do a delta measurement with very low currents (on the 1 to 100 nanoamp range). We set up a Labview code (attached below) for this purpose and it should be working for any current between 10^-9 amp to 10^-6 amp, but it doesn't.

First of all, when we try to run the delta mode using 10^-6 amp current with 2*10^-6 amp range it works perfectly. However if we run it with 10^-7 amp (with 2*10^-7 amp range), 10^-8 amp (with 2*10^-8 amp range) and 10^-9 amp (with 2*10^-9 amp range) it doesn't work at all. We used auto range and it failed. We tried to write our own autorange code and it also failed (see the disabled part). Then we just manually entered the range, but 6221 seems to ignore our command to adjust the range whatever we do. Strangely enough, When we open Instrument Assistant I/O and run the CURR:RANG x manually, it works like magic for every current from 10^-9 amp to 10^-6 amp!

Can you explain why 6221 ignores our commands to adjust the range?

Thank you.