Referencing this excellent question & answer Chassis ground question, how does one handle multiple isolated power domains?
- 3.3v for digital circuits
- 24v for serial connections
- 24v for analog outputs (4-20ma and 0-10v)
We isolate each of these from each other to prevent ground loops in industrial installations. Previous (old) designs were not isolated and would sometimes have noise issues (resetting uControlller, data corruption etc.) depending on the installed environment. Creating an isolated power domain (flyback with multiple outputs) for each signal type that leaves the metal chassis solved those issues. However sometimes, we've seen ESD will reset the cpu.
Here you can see that the three bottom mounting holes are connected with bottom side traces and then in three places they connect with ( the "Don't do that" ) parallel 1Meg and 0.1uF cap to the ground plane. The top 3 mounting holes have a 0.25" clearance to any other copper, including the internal planes.