In the case of the LabJack U12, a single-ended analog input has a voltage range of -10 volts to +10 volts (20 volt total span) and returns a 12-bit value. This gives a voltage resolution of 20/4096 or 0.00488 volts per bit (4.88 mV/bit).
What is analog reference voltage?
Description. Configures the reference voltage used for analog input (i.e. the value used as the top of the input range). The options are: Arduino AVR Boards (Uno, Mega, etc.) DEFAULT: the default analog reference of 5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards)