Re: How do I make a seperate analog and digital ground?
From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 01/11/05
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Date: Mon, 10 Jan 2005 16:05:55 -0800
Larry Brasfield wrote:
> "Henk Boonsma" <hboonsma@teranet.news> wrote in message news:1105388296.c51cc5aeb983a2938464a7c9a5217cb7@teranews...
>
>>I'm trying to design a circuit which has both analog and digital power
>>supplies but I'm not sure how to make seperate analog and digital grounds. I
>>assume just making seperate ground planes isn't enough because in the CAE
>>schematic that would merely look like two seperate ground symbols connected
>>to the same net. I think I've seen some schematics where an inductor is
>>placed between the analog and digital ground nets. How does this work?
>
>
>
> I have often told people, (and been paid for doing so), that
> split ground systems fall into 2 categories: those in which the
> split does nothing; and those where the split does harm.
>
> By the time you arrange that return currents do not have to
> flow across your split, you have done most of the work
> needed to avoid shared impedance coupling. The rest of
> the job is keeping field coupling below tolerable levels.
> Once that is done, the split will have no effect in ordinary
> circuits at ordinary frequencies. The exception is for DC
> where small potentials are significant.
>
> If you do not see why this is true, I suggest you procure
> and study "Noise Reduction Techniques in Electronic
> Systems" by Henry W. Ott. Or hire someone who has
> and can explain it better than I can here.
>
The description I like on how to use split grounds (on a PC board) is thus:
Split your ground planes, except for one "bridge" segment. RELIGIOUSLY
pay attention to ground return paths for ALL signals, keeping them over
the correct plane. ANY signals that gave to cross from one domain to
the other goes over the bridge.
Then when you're done make the ground plane one continuous pour.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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