“If you’ve ever worked with the ADC of an Arduino you’ve probably discovered that the readings aren’t accurate at all. And it gets even worse about it’s rated temperature range. There are many reasons for this, but one of the main causes is within the way an ADC operates. It can not measure a voltage on its own, but only compare it with another voltage. The Arduino offers you to choose from an internal 1.1V, the AVCC system voltage as this reference voltage or an external voltage input. Unfortunately the first two ans simple solutions are far from perfect. The internal reference has an initial accuracy of 10% and an unspecified behavior over the temperature range. The AVCC voltage heavily depends on the used regulator, the capacitors used, the board layout, other load on the same rail and much more.”