I'm trying to get a 12bit (13 bit if you include the sign bit) ADC working accurately. Currently, if I connect the Vref, Vdd, and In+ pins I don't get a returned value of 4095. More often than not it returns about 3980. Shouldn't I be getting exactly 4095? The In- and Vss pins are also connected to the same ground. It doesn't matter what reference voltage I use; I've tried using the 3.3V and 5V rails from the arduino for Vref, Vdd, and In+ as well as an external voltage reference chip which provides 4.096V (part number ISL21009). I specifically bought a 12bit ADC so I could get 1mV accuracy with a reference voltage of 4.096.
Please help :-/