I would guess that the OpAmps are set to such high amplification, that they pick up and amplify the voltage across the measuring resistor that is created by the alternating electric-magnetic fields in your room.
Well; that will be something easy enough to try; I can kill the power...
Did you get the scope to trigger on the fluctuations? The waveform is probably sinusoidal and the frequency around 50 or 60Hz (depending on the frequency of your power-net.
I didn't use a trigger, but attached is an image showing it. I have noticed with some sampling ranges that it's just off enough to give wrong readings -- but this was pretty consistent -- I'll replicate it later with more attention paid to getting a good reading.
My bigger concern was the failure to get a proper reading on the low range....
Also Dave talked about Bias-Currents in one of his videos, which he had to account for when calculating the resistors. Maybe your OpAmps have slightly different Bias-Currents that you have to calibrate out of the circuit with a Trim-Pot.
Hmm. I'll have to look for the video; thanks. I used the identical (part # wise) opamps. any suggestions on how/where to start with calibrating? I am a novice here; so I'm learning as I go w/o formal training...
Attached is an image that shows the waveform at nano amp range w/o anything connected. I will re-test with power off to the building...
The parts highlighted in yellow are part #'s I deviated from specified (basically same part as I can tell; -1 instead of -2); and (digikey) part #'s I used where none specified.
Thanks for your time and ideas.