GoForSmoke:
The second one is what the OP's earlier schematic showed which is why I tried to explain why to connect the grounds, so the volts will be on the same scale, all relative to the same ground.
I'm not sure where the schematic is but I have just been trying to point out that for measuring very small voltage differences there is the option of using the differential voltage facility and the associated amplifier within a suitable Atmega MCU.
...R