[SI-LIST] Re: Moat and drawbridge
- From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
- To: cchalmers@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 9 Mar 2004 12:05:56 -0600
Chris,
I've experimented with this sort of thing on several projects
(including GPS receiver where the analog signals were
70 dB below the digital signals and analog-to-digital
converters), and I've found the use of moats to be
"somewhat useful." For the GPS receiver, we got
0.5 dB improved performance with the moat (vs when
we tied the planes continously around). For ADC circuits,
we've seen more significant differences (likely because
the analog frequencies were much closer to the digital
signal frequency content).
When considering using moats (and other types of split planes),
I'll offer two items to consider:
* For ever voltage supply, pay close attention to
return path. Make sure the voltage path and associated
return path through ground is closely coupled at the "drawbridge"
and no where else. Otherwise, you'll lose most of the
potential benefit.
* Ditto for the signals. You mentioned some signals might
cross the moat; don't do this. If a signal needs to
cross the moat, do it only at the drawbridge. Otherwise,
you'll again lose the benefit of what you're trying to
accomplish. The only potential exception to this rule
is for differential signals.
Hope this helps,
Pat
*
> My question is, is this a good thing to do? Can I give an example.
> If you where supplying current to component BUF1 (which is
> just a buffer
> supplied by chip 1)would you not be increasing the loop area going
> from its supply Chip1 across the drawbridge and down to buf1
> rather than
> just going straight down to buf1 on a plane if the moat and
> drawbridge
> layout was not there. Is the moat and drawbridge layout only for a
> small analog area rather than for trying to contain all the switching
> noise of the power supply.
>
> Your comments would be much appreciated, Thanks
>
> Chris
>
>
>
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