So, here we go again with the typical "what's the best ground layout?" question. Seems this question has different answers depending on the application, manufacturer, and decade.
From what I've read, the trend nowadays is to use a single ground plane for everything. Avoid star grounds, ground plane is where it's at. Dedicate a complete layer to ground and add plenty of vias, especially for decoupling capacitors. Do not split the ground plane
This article, however, recommends making a slit to split the ground plane around the precision analog sections.
Regarding Analog VDD, the same article seems to suggest the DVDD in certain ICs is not really a DVDD in the same sense a AVDD to DVDD in a MCU is. An ADC, for example, doesn't need to be high speed, its digital pins don't need to drive large capacitive loads, and don't drive high currents. They can also be slew rate limited by source termination resistors. All of this makes the DVDD line on an ADC much quieter than a MCU driving 50mA on each pin, all of them switching at very high speed.
So I'm assuming, for the particular case of an ADC, a ground plane containing both digital and analog grounds may be "good enough". Then again, the part I'm using, a HI7190, has an evaluation kit with a rather complex layout, consisting of 4 layers with split ground and power planes. But this layout dates from 1994, back from when split planes were all the rage.
This particular ADC uses about 1 to 3mA for each power rail (AVdd, DVdd, AVss), so I don't see a reason why it should need different power planes, as there is no high current draw, especially if using source termination on the SPI lines.
It will be, however, connected to a rather noisy ESP8266 microcontroller. The ESP, as most other RF chips, is known to draw a lot of current (peaks in excess of 200mA) from the supplies. In this case, should the ground plane be split? Or, since the ESP8266 micros come usually in a PCB module, it will be connected to the main board with just one GND point, which will effectively "split" the ground plane.