I have always heard that it's best to tie data converters' grounds together under the part. You want the lowest impedance possible. The pins being designated analog and digital ground may make us wish they were a wall inside the part isolating the analog section from digital noise, but it's not like that. The bond wires between the die and the package have a non-zero impedance. So the part is designed so the digital return currents travel through a separate bond wire, and therefore a separate pin, from the analog circuit. Once you get to the board, this theory goes, you tied them together with the best connection possible. From there you split the grounds trying to keep digital return currents from PCB traces from travelling on the analog ground.
The ADS1254 datasheet has the opposite suggestion:
Each section should have its own ground plane with no overlap between them. AGND should be connected to the analog ground plane, as well as all other analog grounds. Do not join the analog and digital ground planes on the board, but instead connect the two with a moderate signal trace.
Wont this make any noise between the grounds try to flow through the ADC IC?
