Hi, just thought I'd say thanks for the design, I think I have got it to work, but have a few questions.
I copied the schematic in the first post, although I used a fubarino SD board (I had one lying around). This needed a couple of tweaks to remove avr-specific code (see https://github.com/JamesGlanville/esrmeter). I also used trimpots as I had no 1% resistors.
Are trimpots ok? I seem to remember they have a higher inductance than normal resistors, and I don't know whether this matters here. I have a fairly good LCR meter I used to trim the pots, so I think they are accurate to within 1%.
The fubarino SD board does not have an internal voltage reference, so I'm using the 3.3V supply (and 3.3V for everything apart from LCD power). I think this should be fine, although with less ADC precision. However, with vRef=3.31V (measured, and very stable), I need to set current to 0.02468 to get "good" results. By calculation, it should be 0.0331, a pretty big difference. When I say "good" results, I am getting correct readings to about 2% of 1ohm, 2 ohm, and 2.5ohm resistors. I have no way of testing ESR otherwise, but my results seem reasonable (my supply of caps is mostly very good, a few ones are terrible). Is my weird current value ok/is there a better way to calibrate the meter?
Thanks again :)