[quote user="Eng351"]
What is the impedance of the sensor connection to your ADC, is it from a low impedance source like an opamp? The adc has a significant input impedance of a few hundred K which can cause significant drop in signal if your source impedance is too high. Also confirm the voltage of your AVDD.
[/quote]
Impedence mismatch was exactly the problem. My output was from an in-amp, so I tied it to ground using a 1k resistor and the ADC magically started registering an input from 0v upwards. Lesson learned.
However, my ADC value is still not correct. I have it configured as given in the initial post - 12 bits, with an AVDD reference voltage (3.045V coincidentally) and input taken on AIN2. However, I'm finding the ADC is maxing out at 0x07FF (2047). I had been expecting this to be 0x0FFF (4095). Based on the code above, would anyone have any idea why this is not the case?