My end goal is to measure supply current of a microcontroller with best-as-possible dynamic range. In sleep, it draws ~10uA and peak, it's about 300mA, so of course, it'll be tricky to get that all in one go. But I figure catching that 300mA with 1mA resolution without browning-out should be doable with a microcurrent, and I'll set up a different measurement to quantify sleep mode. But in trying to get this on a scope without a savage amount of noise, I'd like to use a 1x probe rather than 10x. [As it happens, I think this doesn't much help my cause, but it's how I arrived at the problem below.]
I actually have 2 microcurrents, one modded to run from 3x 1.5V AAAs and a newer one just running off a normal coin cell. Call them old and new.
I've got a test circuit set up to figure out a repeatable DC measurement before I go adding the actual DUT. The test circuit is a 1V power supply, through a decade resistance box to dial in the current, through a microcurrent to ground. A multimeter (10Mohm input) is reading that microcurrent. I'm also trying to probe that microcurrent with a scope. I discovered that connecting the scope probe ground in parallel actually caused a noticeable measurement error due to the additional ground path, so I un-grounded the negative of the 1V bench supply to eliminate that. Now, when I plug in a 10x scope probe in parallel (10Mohm input impedance), the multimeter reading doesn't change appreciably (as you'd expect). But when I switch the probe to 1X (1Mohm impedance), then things get interesting. On the old (modded) microcurrent, nothing much happens. The output stays appreciably the same on the multimeter (and the scope measures what I'd expect, plus a whole bunch of mains noise which is annoying AF). On the new microcurrent, however, dropping the impedance on the output port to 1MOhm drastically changes the measurement - 20% or so at 50mV (reading of 50mA actual drops from 50mV to 40mV), and much more at lower currents (reading of 10mA actual drops from 10mV to 1mV).
Both are uCurrent Gold Rev 5, and on visual inspection, have identical populations.