On the NANO I use Analog In 0 and 1 to read two pots.
I have connected each pot to a 5V supply and ground with the rotor to the A0 and A1 Pins.
The moment I connect the NANO’s ground pin to the suppy’s ground the board starts up. With NO Vin or 5v input how can this be ??
Many chips can be powered through an IO pin because of the ESD diodes, the ATmega 328 is such a chip. I suspect it is being powered through the pots.
Just to point out that this is not a good thing to do and could end up burning up your arduino.
Never attach any voltage signal to an unpowered chip.
Also, you seem to imply you are using the 3V3 version of the Nano. You shouldn't be applying 5V signals in this case.
Thanks for the info. What is the analog input voltage on the 3V3 ?
The maximum analogue input voltage is always equal to the supply voltage, any more == damage.