Can AVss and GND be same for ADC
Yes they can be the same, generally ADI and a few other manufacturers like to separate out the digital ground and analog grounding systems. This is not necessary if careful attention to grounding plane currents is observed.

The ground pin on any IC provides a current return path to ground. Ground is not 0V because it has a small amount of resistance and inductance, so any currents flowing through ground create a small amount of voltage. Currents take the path of smallest impedance back to the source. If this current return pathway runs across the pin of an amplifier or ADC it will create voltage on the grounding pin and change the voltage reference of the part relative to other parts. 

One way to control ground currents is by segregation and splitting planes. The other way is by proper placement of components. If you do a split plane, be sure that no traces are ran across the split. I prefer the solid plane method, which works well for low level voltage measurements. However there are cases in RF designs and digital designs where split planes work better for overall EMC control. 

A good explanation for this is found in chapter 17 of Electromagnetic Compatability Engineering, more particularly sections 17.3 and 17.5