Hi Rae,
The ADS1248 is specified with the offset values given in the electrical characateristics after calibration. After power up and setting the desired PGA gain, did you issue the SELFOCAL command to remove device offset from the the measurement? You should always do this after power up and setting the PGA.
How did you acutally determine the RTD resistance was 108?
In see nothing in the schematic that would cause the offset other than the possiblility that the SELFOCAL was not issued. I do see you have an inductance (or ferrite) on the AVDD supply. We have found that this can be a really bad idea. If you want some filtering it is better to use a 10 ohm resistor instead of the inductance. The problem is with startup current demand where the necessary startup current is being choked by the inducatance.
System calibration may be required depending on the expected accuracy of your system.
Best regards,
Bob B