Iraj,

Could you supply a schematic of the circuit?
If your Vref is 2.5V, the values read from the AD7730 are going to be very low. A typical load cell is in the range of 0 - 10 mV. I would connect the load cel as shown on p.25 or p.40, using 5V as you excitation voltage. In that way the AD7730 is connected to the load cel ratiometrically, which will make the readings more immune to supply voltage fluctuations.

Regards,

Jacob