I have an electronics setup I'm using for research purposes. I'm having some strange behavior so I've tried to boil it down to the simplest working example. Most basically, a voltage is being applied to a device under test (DUT), which in this case is a 10kOhm resistor.
More specifically, I have two Source Measure Units (SMUs), both Keithley 236s. They're both in voltage source/current measure mode, but one of them (SMU1) actually sources a bias and the other (SMU2) just measures the current going through the circuit serially. I do this (rather than just recording the current SMU1 is supplying) because for my real-use case, the DC bias from SMU1 is going to be mixed with another signal by an op-amp, so the current SMU1 is supplying would not actually be the current going through the sample. SMU2 is set up as described on page 1-4 of the manual linked above, in "Measure (current) only" mode. It should basically function exactly as a simple ammeter you can stick serially into the circuit somewhere.
This is fine, but I'm finding a strange problem. If I test two configurations of this circuit at SMU1=0V:
I get slightly different behavior from the current read by SMU2. C1 gives a fluctuation of +-0.05 microamps, while C2 gives a steady current of ~0.03 uA, with smaller fluctuations around it, but definitely a steady 0.03 uA baseline current.
I'm not exactly worried that I'm not getting perfectly 0 current, because the manual says that in the voltage source range (11V) for the 236 I'm using, it has a 1mV resolution, so if it's a full 1mV across the 10kOhm resistor, that gives us fluctuations/an offset as large as 0.1 uA.
However, what I'm unsure of is, why are the two configurations giving different currents? (one a reasonable fluctuation, the other a strange systemic offset