Keithley 2400 Error +505

[JohnPi]

I am trying to calibrate my Keithley 2400. It isn't (wasn't) very far off calibration, but now I get error 505 ('Source offset data invalid') when calibrating the 1 uA current range (calibrating voltage seems to go OK).

Basically, I measure about 1.004 uA when sourcing 1.0 uA on the 1 uA range. There are other calibration steps with -1 uA (reads 0.997 uA) and 0 uA (reads about 3 nA) -- but then when I apply the final calculated calibration code, it fails with the 505 error.

I think the way the calibration works is that it expects 4 numbers which are close to 1 uA, -1 uA and 0 uA. It then calculates the internal cal constants from these -- but if those are too far off, it rejects the calibration.

Does anyone understand how the cal constants are calculated ?

Calibration actually ran OK for me during a couple of trials (with shorter settling time etc.) -- but when I ran the slow (more accurate) one, now I get that error -- how do I recover ?

p.s. I tried to calibrate manually on the 1 uA range -- I get the same 505 error. My cal is only ~ 0.3 % off (manual says limits for cal are ~ 10 %), so it should be OK. Even when I say it is 'perfect' -- e.g. tell it it reads precisely 1.000 uA when supplying 1.0 uA, I still get the 505 error.