using the Arduino internal reference 1.1v
The Arduino Uno R3 has a default Reference Voltage of 5V.

You can change it to 1.1V using the analogReference() command as follows:

analogReference(INTERNAL)


Or you can apply an External reference to the ARef pin (0V to 5V):

analogReference(EXTERNAL)


Similar, but different, commands exist for the Mega.

From a circuit perspective, I assume you have an external voltage to apply to your resistor. If the external voltage is not that accurate connect it to an Analog pin for calibration purposes (&lt;1.1V). Or if it is accurate use that as an External Reference.