In the Analog Gnd plane!



Both is possible. The way you did is sometimes the only way to do, when more than one ADC/DAC-chip is used on the same board. Then a connection of analog ground and digital ground at every chip isn't possible of course.
But your methode is a bit dangerous, as ground potential at analog ground pin and digital ground pin of your ADC can differ much more, than it is observed, when connecting grounds together directly at chip! Have a look especially at this narrow copper trace which connects your digital ground plane to common ground point of 'supply': If here digital currents are flowing, then an enormeous ground noise level can be seen between analog ground pin and digital ground pin of your ADC! This MUST be avoided!

Two things have to be done to prevent this:

1. This narrow copper trace shouldn't be that narrow. Make this connection be part of your digital ground plane.

2. A LC low pass filter consisting of 10µF Tantal, paralleled by 100nF X7R and 10µH soft ferrite choke should be located right at the point (and nowhere else!), where +5V supply enters digital ground plane. This filter will very effectively decrease high frequency ground return currents on the path from digital ground plane back to common ground point of 'supply'.

But maybe it's much simpler for you to connect analog ground plane and digital ground plane directly at your ADC-chip? Then both potentials are forced to be identical at this point.
Disadvantageous is, that digital ground return currents (from +5V supply) can flow now across analog ground plane. But even here the above discussed LC low pass filter will tremendeously help to hinder these digital ground return currents from degrading integrity of analog ground.

Kai