Don't forget, an Arduino also radiates RF simply because it runs at 16MHz and does a lot of fast switching. It could play havoc if it is near a sensitive measurement. Depending on how sensitive the measurements you are trying to make are, it might be best to keep things analog inside the experiment's shield, and convert to digital outside?
I just now did the experiment. I plugged in an Arduino Uno right next to it and even powered it with a buck converter, hoping to create a worst case. I put it right next to the picoammeter, and it didn't perturb the measurements at all. Same thing with an Arduino Mega2560: no effect. It seems that high frequencies don't bother the picoammeter. Or, maybe it's their low power. Either way, all of that is really good news!
Apparently it's the opposite with lower frequencies though. That 60Hz AC frequencyis quite visible if I turn the oscilloscope up to 10mv/div. I need to do further isolation and removal of other SMPS's from the area though before I can be sure the 60hz noise is coming from the Rigol and not something else. I would think that a Rigol would be pretty well designed to avoid giving off that kind of noise, so I suspect it may not be to blame. With the bad SMPS's that I've already removed, they could affect the picoammeter measurements even at several feet away.
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In the scope shots I posted just above, it's clear that the swing of the output voltage on the picoammeter is getting reduced as frequency increases. This is not unexpected, given what Gyro said about the bandwidth of this design.
Question: Is the picoammeter still faithfully measuring and reporting the amount of picoamps passing through it on the way to ground while the bandwidth limitations are especially noticeable, or does the picoammeter lose accuracy under those conditions? What I mean is, there seem to be three possible cases: (1) are the picoamps being throttled but still accurately reported, or (2) are the picoamps actually the same (unchanged) and simply not accurately reported, or (3) are the picoamps both throttled and not accurately reported? i.e. which of the 3 cases is the reality? I can't answer the question experimentally without an accurate higher bandwidth picoammeter, which I don't have.