A (perhaps THE) highlight of last October's Audio Engineering Society convention in San Francisco was a talk by Henry Ott. The topic was PCB layout for mixed-signal devices, obviously a subject of great interest. He started by saying, "Yes, every single converter data sheet you read tells you to split the analog and digital ground planes and connect them together at only one point, near or under the converter. So what do you do when you have more than one converter?"

He basically indicted all of the chip manufacturers as lazy.

(As an aside, Barrie Gilbert's columns on the Analog Devices web site and in the trade rags say the same thing and give the same advice.)



The key point Ott made (and it had many people scribbling in their notebooks because they never heard it before) is that return currents follow the path of least inductance, which for fast digital signals means that the return current is essentially constrained to the plane directly underneath the signal trace (which is why you need a ground plane in the first place). If your digital signals have to cross a split, then you're really in trouble.

The low-frequency signals' return currents spread out in the plane, so you do need to take care of that.

So Ott's recommendation is straightforward and obvious: while you can have a single solid ground plane, you really do need to partition your layout into digital areas and analog areas. You can't place a micro and its oscillator smack in the middle of your low-noise preamp area.

His final recommendation was: "You people need to start spending the money on multilayer boards."



I suspect that a solid ground plane is better in terms of radiated noise than a split plain.

-a