Hardware Hacking guys propose this quick tutorial on how to improve the accuracy of Arduino’s ADC readings, by measuring the actual voltage rail used to supply the onboard microcontroller. At a glance, this can be done by measuring the internal 1.1V reference voltage (it is available for ATMega 168 and ATMega 328 only) and, then, by normalizing the ADC readings.

More details can be found here.

[Via: Hardware Hacking]