Not able to get the exact sensor voltage

[bibhu223]

Hii all, I was trying to use the ADC with Rpi zero, I am expecting a single end voltage in 1-2 mV , But when I am connecting the sensor in ANI0 and COM, I am getting the voltage around 1.67 V which is too high. I check the AVSS and AVDD its around 5.08 which I took as REF.
i changed the Drate and also the Differnatial mode, But not getting the expected voltage, normal multimeter is showing the voltage of around 1-2 mV, I don't know how to resolve the issue.
Just simply I am using Python code in main.py