Hi Vishal,
This is exactly the type of behavior you will see when saturating the PGA. The ADS1148 has a PGA that is similar to an instrumentation amplifier. With one input grounded, the output of the amplifier cannot get to the desired voltage level. As gain is applied the problem gets worse.
You need to make sure that your input is within the required common mode input range for the ADS1148. As you increase gain, the usable voltage range narrows so it is best to make sure that the common mode is near mid-supply of AVDD. The biasing resistor will set the sensor into the correct common mode range.
Best regards,
Bob B