FreeLists / si-list / [SI-LIST] Re: Analog and Digital grounds

[sdb@xxxxxxxxxx (Stuart Brorson)]

> 
> When I was coming up (longer ago than I care to admit to) I was taught th=
> e conventional wisdom that Analog and Digital signals should have seperat=
> e ground traces (later ground planes) which connect at a single point.  I=
>  have gathered that this is as old fashioned as using a single 0.1 uF cap=
> acitor for each power pin.
> =20
> Will some kind people point me to some papers or other resources where I =
> can learn what the current thinking is (and the theory behind it).  The l=
> atter is essential because trying to get some people to abandon the "trie=
> d and true" methods for something more up to date takes a lot of convinci=
> ng. =20

I first heard that multiple GND planes is a no-no at a National
Semiconductor analog seminar a couple of months ago.  The speakers
said that you are better off having one GND plane for analog and
digital, and just keep the analog and digital components as physically
separated as possible.  They said that the chances that you'd mess up
the AGND and DGND return currents -- with signal traces running over
ground cuts, and what not -- was larger than any noise improvement you
might get using separate planes.  This makes sense to me since any
analog/digital design I have been involved with has so many traces
running every which way that it's impossible to maintain separate AGND
and DGND planes underneath all nets without creating a gerrymandered
mess.  (And before I am attacked for using bad placement, please be
assured that I do try to place the components as logically as possible
before routing begins!)

Here's a pointer to some references I pulled off the National website
talking about this: 

http://www.national.com/kbase/category/Data_Conversion.html#57

You can search their site for more material.

HTH,

Stuart
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