Analog to Digital Conversion
Contributors:
Nate
Nate 
Relating ADC Value to Voltage
The ADC reports a ratiometric value. This means that the ADC assumes 5V is 1023 and anything less than 5V will be a ratio between 5V and 1023.
Analog to digital conversions are dependant on the system voltage. Because we predominantly use the 10-bit ADC of the Arduino on a 5V system, we can simplify this equation slightly:
If your system is 3.3V, you simply change 5V out with 3.3V in the equation. If your system is 3.3V and your ADC is reporting 512, what is the voltage measured? It is approximately 1.65V.
If the analog voltage is 2.12V what will the ADC report as a value?
Rearrange things a bit and we get:
Ahah! The ADC should report 434.