[SI-LIST] Re: Why should we tie AGND and DGND
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Mon, 12 Sep 2005 10:16:52 -0700
I agree that the ground plane should not be cut for the reasons already
given. It is not necessary or good to have two separate ground networks
and the designers of these mixed signal devices did not intend this to be
done. What matters is that the analog loop be kept away from noise
suorces. This is best done by the way in which the two leads are routed to
the signal source. These two leads are the "input" to the analog section
and its return, which will be connected to the analog ground pin of the
part.
Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX- 707-568-3504
I just used the energy it took to be angry to write some blues.
Count Basie
> [Original Message]
> From: Mikhail Matusov <matusov@xxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 9/12/2005 8:56:14 AM
> Subject: [SI-LIST] Re: Why should we tie AGND and DGND
>
> Hi Nikita,
>
> The basic idea here is to keep digital noise currents out from entering
the
> more sensitive analog section. Your board seems to follow the paradigm of
> star grounding in an attempt to achieve this goal. By cutting the GND
plane
> one hopes that all the digital return currents will be contained in the
> digital part of the plane. However, we still need to have equal reference
> potential in both analog and digital sections to be able to pass signals
> between them. This means that the grounds have to be connected somewhere.
> According to the star grounding theory this is supposed to be done in a
> single point usually close to a power supply.
>
> In practice, cutting GND planes can make the situation worse as it might
> desrtoy natural return paths for some of the signals, usually the ones
> crossing between the sections. This can lead to EMI problems. In other
words
> if you have a track going from the analog section to the digital passing
> over the GND plane cut, this is not good...
>
>
> =======================
> Mikhail Matusov
> Hardware Design Engineer
> Square Peg Communications
> Tel.: +1 (613) 271-0044 ext.231
> Fax: +1 (613) 271-3007
> http://www.squarepeg.ca
>
>
>
>
>
>
>
> ----- Original Message -----
> From: "nikitanivan" <nikitanivan@xxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Monday, September 12, 2005 11:06 AM
> Subject: [SI-LIST] Why should we tie AGND and DGND
>
>
> > Hi Virtuosi,
> >
> > I did a design a couple of days back, which popped this question in my
> > mind,
> >
> > The design had both analog and digital circuits so I gave separate GND
> > references for both the circuits.I isolated analog circuits from DGND
> > by giving clearance for them in DGND.After everthing was over,I was
> > instructed to tie up AGND and DGND through an on board jumper.
> >
> > My question is,
> >
> > If the original intention is to tie both the GNDs together why was I
> > instructed to provide clearance for the Analog circuits in DGND area
> > and then tied together using a jumper,what difference does it make.
> >
> > What are the advantages and disadvantages of tying AGND and DGND
> > together?
> >
> > Thanks in advance.
> > Nikita.
> >
> >
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