Where are the AGND and PGND tied on the ADN8831 eval board?
It looks to me like the eval board has everything separated on the top layer and then going to one solid ground on Layer 2. If so, the analog and power ground symbols on the eval board schematic are the same net. It looks like AGND (pin 12) and PGND (pin 22) come out to vias going to a solid ground plane.
This sounds right to me: keep noise currents out of the chip by connecting AGND and PGND to a ground tied under chip.
That's right. The PGND pin is connected to a ground plane right at its location. The AGND and PGND are connected together.