(Not) Impossible to separate AGND from DGND
For what it's worth, at my last two jobs I've had the pleasure of working with a super-talented electrical engineer (different person at each company), and in both cases the engineer's preferred design approach was to use one single ground plane for everything. Yes this runs counter to the conventional wisdom of how to lay out circuit boards, and they freely acknowledged that. But these were smart, methodical, informed, well-educated individuals who consistently produced great-quality designs, using just that one plane for everything both analog and digital.

Now unfortunately I don't have deep knowledge of how they worked their magic, but I can pass along the bits that I do know:


A good ground plane might be all you need.



  The use of double-sided or multilayer PCBs with at least one continuous ground plane is undoubtedly one of the most successful design approaches for high-performance mixed-signal circuitry. Often the impedance of such a ground plane is sufficiently low to permit the use of a single ground plane for both analog and digital parts of the system.


Source


Understand all your return current paths. Your signal, plus the return current, forms a loop. The signals on your board will couple to one another through these loops -- if you keep these loops small and well-separated, then there will be very little coupling. Source Remember that the loop at low frequencies may look different than the loop at high frequencies, since at low frequencies the current will follow the path of least resistance, and at high frequencies the current will follow the path of least inductance. Source
Minimize the chance for your circuitry to introduce noise in the first place. This means having good decoupling located right at your ICs (and make sure high frequencies are sufficiently covered -- what's the self-resonant frequency of that capacitor?) and, possibly more importantly, put a little series resistance such as 10Ω right at the output of your drivers to soften the edge rates on your voltages (and also help to mitigate the current spikes on your power supply).
Be meticulous, detail-oriented, and fussy. Worry about the smallest details of getting the signals and routing just right. Follow the IC vendor's recommended layout rules exactly.


It's not easy -- if it was easy, everyone would be doing it -- but I've seen that it can work, and work quite nicely.

EDIT

My Google-fu did not work out for me earlier, so I had to make do with my second-hand perspective as stated. Now that I see this excellent answer from @peter-smith I can just say "yes. this."