I have been fairly successfully using an ADC Differential Pi on a model 3A+ to monitor a 12V battery via three voltage dividers built on a breadboard, each providing nominal 1.4V max input to three of the input channels of the ADC. The remaining channels are unused The same battery powers the Pi via a 12V to 5.1V converter, so the 0V of the Pi is about 20mV from the separate 0V of the voltage divider (due to the voltage drop from the current supplying the Pi). This was why I chose the Differential Pi for the job. I have been very impressed with this addition to the Pi, but ...
When I rebuilt the circuit on a bit of veroboard and reconnected, there was a puff of smoke and the ADC is now dead. I had very carefully checked that all the voltages were in the expected range so this was very surprising.
Having bought a new board and done some more checks, I find that all is still ok with my voltage dividers, but that some of the channels on the ADC are held about 0.53V above RPi ground. I'm almost certain that it is not the same channels all the time or when connected to a different RPi (model 4B) running the same python.
So my questions are:
a/ Is this because (as I just noticed) the RPi 3A+ is not listed as compatible? or
b/ Have I done something odd in the code or in the address selection which means that the ADC is not floating but for some reason holds some of the channels at a fixed voltage? I'm using the standard jumper settings 0x68,0x69, 18. or
c/ Now I've revisited the documentation and noticed that unused channels should be pinned to ground, should I do that? I have to admit that I'm afraid to do that because of this strange 0.53V offset. or
d/ Something else?
Thanks in anticipation for any insights.