Re: gEDA-user: Mixed signal plane tie-in

[Eric Brombaugh <ebrombaugh@xxxxxxxxxxxxx>]

epswint wrote:
> First, I'd like to say thanks to all for their input and patience on 
> this list - especially towards the new users.  I've moved to PCB since 
> it is open and I'm tired of having to leave designs behind because I've 
> changed positions and locations and no longer have access to the layout 
> package I was working with last.  The user-list has saved me more than 
> once from my selective blindness when it comes to reading the 
> documentation! 
> 
> Now, the question:  I've observed two different techniques for tying 
> analog and digital ground planes on mixed-signal boards.  The first 
> technique placed the tie-in directly under the analog/digital converter 
> on the MCU, the second technique placed the tie-in at the far edge of 
> the board between the analog and digital connectors.  Both boards were 
> MCU development boards, laid out so that roughly one quadrant of the 
> board was analog, the rest digital.  I've got all of my components 
> placed and ready to go, just wondering what experience/thoughts the 
> user-list had on the subject.

That's a $64,000 question. Both ways will work if handled carefully, and 
which one you use depends on your previous experience and the design 
requirements.

Analog Devices has a pretty good tutorial on this:

http://www.analog.com/en/content/0,2886,761%255F795%255F97529%255F0,00.html

and some of their mixed-signal MCU datasheets also go into further 
detail on the subject. I've read a lot of different opinions on the 
topic and the one thing they all seem to have in common is that you want 
to keep the ground return current paths for the analog and digital 
circuitry separate. If you can do this by careful placement with a 
single ground plane then that will work. Just make sure 'not to cross 
the streams' to paraphrase "Ghostbusters".

Eric


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