I've gone back to my constant current project to fix an annoying fluctuation in the required current display. See schematics below.
The entire circuit consists of a CCS section with signal outs for both the requested current (represented by Vreq) and the actual current (represented by Vact developed across the sense R).
These signals are sent to an amplifier which scales them up x10 and the amplified signals are sent to a uP ADC with an external 2.5V ADC reference.
I'm using a star-ground approach and I keep digital (uP, display driver) and analogue (CCS, amplifier) grounds separate, with a separate ground point for the load return.
Despite this, the required current selected by the 100k POT moves up and down a mA or two on the display. In preparation for some low-pass filtering, I took some readings with my oscilloscope probe (set to x10) and to my astonishment, the display was rock solid. No fluctuation, perfectly smooth with the POT turns.
It didn't matter which "Probed" point I tried (see diagrams below), the effect was the same. Realizing that the probe contains some capacitance and a large resistance, I tried placing a 22pF and 10M in parallel at one of the points probed. So, far I haven't managed to replicate the "fix" brought about by the probe. I'll get a chance to do some more this evening, but in the meantime: can anyone tell me why the probe has had such a "beneficial" effect on my circuit?
UPDATE: I should have pointed out that the oscilloscope is mains connected and its ground lead is connected to earth ground. Also, the circuit itself is floating, that is it's connected to a 5V wall regulator that doesn't use earth ground. One other thing, in all cases when I probed the circuit ("probed" points), I connected the oscilloscope earth lead to the local ground at that point (not the star ground point) - not sure if that makes any difference.
While I'm updating, another intriguing thing is that if I attach a load (say a 100 ohm resistor) the readings seem to be quite stable. Not sure if this is related, but I thought it was worth mentioning.
Well, I had a chance to test things a bit further. Seems like it's the connection to earth ground that fixes the issue, so @WhatRoughBeast is on the right track. Unfortunately, the twisted pair trick didn't work.
I finally got the circuit to work reasonably well. I changed the 100k multi-turn pot for a 10k (and the 1M5 resistance to 150k) and the increase in current seemed to cure the flickering, except when the pot resistance was quite high. I also shortened the signal wires and implemented a simple digital low-pass. If I had this to do again, I'd use a proper PCB with a ground plane.