The two different Grounds in ADC are
1) A GND - Analog Ground
2) D GND - Digital Ground
Ideally the Analog Ground and Digital ground in the Anlog Digital circuits are seperated for obtaioning noise immunity on analog lines
But while testing you can conncet the two grounds together
**broken link removed**
in the above datasheet there is basic ADC tester circuit available on Page 24
I guess that is the simples method to test your ADC
Digital and Analog grounds are separate grounds on the chip itself. In an application they should be connected together as close to the chip as possible. Idealy this should be the single ground point for the entire system, with the system analog and digtal grounds kept separate, and connected at the ADC chip.