In my specific case, I have an arduino and a servo motor.
- Arduino is powered from PC via USB (5V).
- Servo is powered from a separate AC-DC adapter because it needs more current.
In order to control the servo from Arduino, they must have common 0V, so grounds have to be connected. That part is clear.
My question is should I worry about current going through this ground-to-ground connection? In other words, can I consider those two voltage sources to be completely decoupled or is there a situation in which a voltage will be maintained between them by some external source?
Would it be a good idea or an extremely bad idea to place a resistor between the two grounds?