What is Positive and Negative Mux input in ADC
This is a differential input ADC. It reads the voltage difference between the two input pins.

Typically, for a single input pin application, you would use the positive input pin as your input, and take the negative input pin to ground.

Read the data sheet carefully for the range of input voltages that each pin can be taken to. It may be that ground is not within the common mode range for that pin, in which case you would have to take it to some reference voltage that was in the required range.