Shaheen,
If the pin isn’t connected to anything, I don’t think I’d expect the value to remain perfectly consistent. Have you tried connecting 3V or ground to the pin and seeing if your ADC reading is correct?
The following equation can be used to convert the 16 bit ADC reading to a voltage:
voltage = (read_u16() /65535) * (float)VREF
where VREF is the supply voltage to the mDot.
Cheers,
Mike