adding a ground plane to a digital effect: which rules applies?

[add4]

Hello everyone

I'm usually always putting a ground plane to my pcbs. but since it's usually pretty simple circuits without LFOs or digital parts, i'm not worried about ticking and other digital noise sources.
Now i've heard that if you go digital you have to separate ground planes for the analog and digital part. How can i physically do that?
- the ground is, in the end, common for all my circuit and comes from my power jack, at one single point of the PCB. Do i need to do it another way?
- The way i imagine this is to make a ground plane on top for the digital part of the circuit (where does it stop? coupling cap beyween the digital chip and the first analog 'brick' of my circuit the signal with hit?) and the ground plan on the bottom for the analog part. Will it be enough?

What are the general rules to do this right?

Thanks in advance
 

[EBK]

I'm afraid this won't be much help, but I have a pedal with digital stuff on separate boards from the analog stuff.  I threw some extra shielding around the digital stuff for good measure. I actually made the additional shielding out of copper tape and gaffers tape, with a wire by to my DC jack ground -- there are no doubt more professional ways to do that, but the pedal is beautifully free of digital noise.

[Ralfg]

This is a great question. I’ve been wondering the same thing.

[sonnyboy27]

From what I've read, it seems like you should keep them separate then join them at the point where you make contact with the DC jack's ground reference. Haven't done any digital boards myself though.

[gtr2]

Mixed signal design is complicated and is largely dependent on the circuit and layout requirements.

It's all about the return current...analog and digital behave differently.  You can find some good info on the web from Henry Ott.

A solid ground pour on a 4-layer PCB with proper analog and digital placement is the ideal IMO.  NO split plane(s).  It's always better to have a single reference plane.

If you are just doing a two layer board it's near impossible to have a solid ground pour and you need to be aware of placement and currents.

For something like an FV-1 or a PIC you can have a successful design on a two-layer PCB with careful design.

Josh

[raulduke]

This is a nice article on the subject:

http://www.effectrode.com/wp-content/uploads/partitioning_and_layout_of_a_mixed_signal_pcb.pdf