Hi Ashwak,
Welcome to the forum! The ADS1248 takes a differential measurement from one analog input to another. It is not clear how you are connecting your inputs, so if you send me your schematic it would be helpful.
If you want to just measure your RTD and let's say you have it connected across AIN0 and AIN1. You assign the MUX to connect AIN0 to AINP and AIN1 to AINN (default settings). The voltage measured will be relative to the voltage drop across the RTD. If you use a current excitation for the RTD, Ohm's Law can be applied to find the resistance of the RTD.
Best regards,
Bob B