Never connect the two ground planes AGND and DGND at more than one place it will surely result in ground loop.
- The most ideal place for joining AGND and DGND, for a system with single ADC is under the ADC.
- You shouldnt join the two Gnd at any other place not even at power supply.
- The place where you join AGND and DGND becomes the star ground point and the at same point the chasis ground should be connected.
- If you use same power supply for feeding both analog and digital section than the power connector must be towards digital section and the analog section must be placed on the other side. This practice avoids any noisy digital current flowing towards analog section since the connector is close to digital section the digital return currents happily go to the connector without touching analog section.
- When using separate supplies you have two power connectors one in analog section and the other in digital section. The grounds of the two power supplies should be connected at no other place than under the ADC/DAC. [ Star ground point ]. Not even at the supply modules or the power connectors.


Regards,
Prahlad Purohit