Where are you getting this 0V from, does it share a common ground, are you measuring a voltage drop across a resistor (eg measuring a 4-20 mA ouput), how many readings are you taking? are you using a rolling average or taking 10 readings, ignoring the largest and smallist and taking the average of the rest?
Thanks a lot for the questions.
Actually, I'm trying to show the present voltage of a power source (0-30 volts) on Arduino serial port. I'm following the exact steps shown here:
When I want to see the output, I see that when the power source has, let's say, 10 volts, the serial monitor is showing nearly 13 volts.