Hi ufuk,
What is the common mode voltage on the IN-/IN+ pins? This differential ADC performs best when the common mode voltage is kept at Vref/2 which in your case would be 1.65V (3.3V reference). The IN+/IN- pins should swing equally around 1.65V (e.g. 1.5V and 1.8V). Is this the case? You should also have some capacitance of a few nF at IN+.
Best Rgds,
Alan
