Form what I read:
"The digital ground pins (GND), the analog ground pins (AVSS), and the GND_CORE pins
on the LAN8720 QFN are all connected internally to the exposed die paddle ground. "
So use a separate analog ground for your ADC as you mentioned above. Maybe it will work if both share the same ground plane, but it's easier to omit a ferrite if you don't need it than add one in if you do.
Note that I've shown 0V (ground) and 3.3V power going straight to the noisy object (the digital block) and from where the connections hit this block I would tee-off to separate power and ground planes for each of the ethernet block and analogue block.
Reason: I don't like digital currents in a ground plane getting any further than they need to and this means minimizing digital ground currents (or 3V3 currents) from upsetting the ethernet and ADC sections.