Tips for mixed-signal circuits when doing PCB layout

1.Keep digital and analog grounds separate

As for the case of power circuits, you should keep the digital and analog grounds separate. The reason is the same: voltage and current spikes from digital circuits can generate interference (noise) in the analog circuits, affecting their performance. If you tie them together in the PCB, do it in a point near the end of the supply path (most notably near the PCB ground connection). Although there are other points of view to this problem, this is the most accepted.

2.Protect analog grounds from noise

Any interference in the analog ground has the same effect as it would be in the signal lines (all that matters is the differential voltage between any point and ground). You want a large ground plane to reduce its resistance, but that also makes it more susceptible to capacitive coupling to lines routed above or beneath it. To that end, analog ground should only have analog lines crossing it and the same for digital. That reduces the capacitive coupling between analog and digital circuits.