Hi Aamir,
You cannot exceed the compliace voltage with the IDACs, so the current source will not longer be able to maintain constant current which the compliance voltage is reached. You will not get the expected output once you reach the compliance voltage , but you will not damage the ADS1248.
The IDAC is a current source that is generated from the AVDD supply. This is why there is a compliance voltage restriction as there needs to be a source (AVDD), and a mechanism to keep the current constant. As there is some overhead to create the current source, you cannot create a voltage with the IDACs all the way to the supply. The outcome is simply Ohm's Law V=I*R up to the compliance voltage, which also can be described as I*R<= Compliance Voltage.
So V=I*R and V must be <= Compliance Voltage.
Best regards,
Bob B