When I apply 5 volts to one of the Nano Analog "A" pins with no power applied to the 5V or Vin pins, the Nano red LED light comes on and I read 3.16 volts on the 5V and Vin pins. I have tried this on several Nanos and see the same behavior. I would appreciate an explanation for what's happening within the processor to cause this to happen. Thanks,
Richard
Follow standard practices and then report problem, if any.
Apply power to the NANO using PC or Power jack or Vin and then apply signal to analog pin.
The 5V is conducting through the input pin substrate diode (sometimes referred to as "input protection diode". This powers the entire device, but not with the full voltage because there is the normal junction voltage drop across the diode.
It is usually referred to as, "phantom powering".
Thank you for the replies. I now understand what is happening.
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