I need to check this out but one thing which need to correct over here is the proper selection of ref voltage. With the current selection, negative voltage of the device is -2.5V because of P2 +/- P2. You can connect a 1.25V signal on both the pins, this'll make the REFHI setting as 2.5V and REFLO as 0V.
I'll check this at my end and shall let you know.
Thank you for the info, this has solved my problem by using a 1.25Vref on p2 and p2.
First time I see this way of using an external reference.