Signal sampling at different points in a signals path without degrading the signal?
At low frequencies all the Op Amp gain is fedback to reduce the error thus 200ish internal R/1e6 or under 1 mOhm and Pico scope is probably &gt;=1M so any R value from 0 to &lt;10k is ok.  So near DC an Op Amp has almost ideal voltage source performance but with active current limiting.
hence my simulation boosts the current but feedback overcomes to null the Vbe drops.
You can also look at my swept f controlled current source that reads impedance or cell EIS directly from voltage . ( I doubt anyone knows how to do this in the EIS community, yet every time I think so, I am often proven wrong later)
I have no experience in your field but only as a test/design electronics expert for instrumentation.