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What is the best practice for having digital and analog stuff on the same PCB?

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K3mHtH:
Hey everyone,
I'm designing a board that will have some analog stuff (dc voltage summing, opamps, audio rate signals, etc.) but also a digital microcontroller and associated parts (power supply, crystal, etc).

Is there some best-practices for keeping these things isolated, and stopping any digital noise from getting into my analog signals?

My PCB is 2-layer. Top layer has signals + ground pour, bottom layer has power, a few signals + ground pour.

I was thinking about doing a separate ground pour for the digital section so it's all on it's own digital-ground, that would then be connected to analog ground at a single point. But curious to hear what others have done in the past..

madires:
Please search YouTube for 'Rick Hartley' and you'll get plenty of answers.

coppercone2:
I recommend using two PCB's and to connect them with a cable

I think its a insane fad from how much difficulty it can cause, or pushed by cost sensitive bosses that know.. nothing of the sensitivities.

The design had a cable harness, it was easy to make revision 2 and no one died or worse went bankrupt because of it.

tooki:
Using a cable can make it worse.


I think the easiest measure is to make sure the digital area, with its potentially very high frequency content (due to fast rise times, regardless of clock rates), isn’t emitting/injecting noise: make sure that every signal has a clean, short, uninterrupted return path.

Feynman:
One of the keys is component placement. It's generally good advice to have an analog and a digital section on the board with components placed accordingly.
Another key is using a solid ground plane as return path. When using a two-layer board, the bottom side should be ground only with as little interruptions as possible. The key thing is, you neither want the signals nor the return paths of digital and analog signals share the same space.

If interference between digital and analog is actually a problem (try to quantify this by measurements) and you are having trouble implementing a solid ground plane on a two-layer board, it's probably worth considering a four-layer board. A stack-up that works well as a default is Sig/Pwr - GND - GND - Sig/Pwr. But this might depend on your application.

But using a digital and an analog ground on board level is a bad idea 90% percent of the time.

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