End of the line for this iteration. While the change of grounding to a star configuration is a good plan, in my reality those new jumpers all acted as antennae.
I was able to choose the error seen at the output by moving my arms, tilting the probe, bumping a wire, etc. Putting the circuit board into a metal project-box helped, but wasn't enough. I was never able to get a consistent, repeatable set of readings.
Coming in version four: a double sided PCB, for better grounding and a more compact layout; trimmer pots, to save a bit of resistor wrangling; protection diodes, because it sucks to kill an ADC.
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