Unwanted constant offset in ADC reading
AD7323 (from data sheet): -


The offset error could be up to +9 LSB single ended or +10 LSB differential.
The gain error could be +/- 8 LSB single ended and +/-14 LSB differential
Integral and differential non-linearites could add another 2 LSB
The internal reference accuracy is +/- 5 mV in 2.5 volt