OK, I'm using six ADC channels on an ExpanderPi to measure four voltage levels via resistor dividers and two currents using ACS712 sensors.
Each channel works individually, both with a known source voltage and the real input. I've checked all 8 channels on the ADC and all works fine.
However. For some reason, they don't work together in certain combinations that use adjacent pins. For example, inputs connected to:
pins 1, 3, 5, 7 or 2, 4, 6, 8 works
pins 1 & 2 doesn't work.
pins 2 & 3 works.
pins 2, 3 & 4 doesn't work.
When it fails, the symptoms are consistent. For example, with an actual voltage of about 2.4V measured at the pin, pin X reports 0V, pinX+1 reports ~0.6V, and pinX+2 reports the correct value. Disconnect X+2, or move it to X+3, and everything reads correctly.
What on earth is going on? Could this be an impedance issue?