Hi Sivanantham,
The reference voltage has a minimum limit of 0.5V ( = VREFP - VREFN). (The Requirements are in the Electrical Characteristics Table on page 3).
Therefore, you can set VREFN to 2V if you are using a single-ended 5V supply.
You can also connect VREFP to 0.5V and VREFN to AGND. WIth a bipolar +/- 2.5V supply, you may have to do something like this because there is a 2V headroom requirement when also using the buffer.
Best Regards,
Chris