Hello someguy:
Hope folks here are helping!
One good rule, (IMO), for criticism, is that NOW, one is obligated to see that person benefit.
So, took a second, before I start my own day, to illustrate the response (from Kleinstein).
Please see diagram, of basic low resistance in the supply line. SHUNTS, I think of more like less than one tenth (ohm)...but that's just me.
How about, we look at just TWO cases, as you've outlined.
Suppose the uP sleep mode is at 200 uAmps, and suppose running at ave. 85 mA...
That's, maybe your best quick guess, before starting anything. Grab your 0.58 ohm and do first glance, at the PC Board waveform, maybe some noise on that but the PC circuit generally has cap. and then regulator, so that input point shouldn't be wildly noisey (?)
So, a half-ohm, try expecting a drop of:
At 85 mA, that's an easy 85/2 = about 43 mV
Notice that has fairly good resolution...(Don't get all hung up on wanting an A to D converter show 'full scale' for best accuracy. I believe it's easy to get into false 'overconfidence', that lots of digits equates to 'lots of accuracy...
On the low end, that nominal 200 uAmps load current brings a (less accurate) voltage reading, of 100 uV.
My (Velleman) DMM appears to only show the first digit, on the '100 mV' scale. (The low 00.1 mA is what you see on meter display.)
But you can see, the point Kleinstein had, about not putting in too much supply drop, or the usual regulator does not get sufficient input voltage. Here, you get something like a tenth volt drop...less than what normal battery use involves.
(More response later)