[SI-LIST] Re: Why should we tie AGND and DGND
- From: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 12 Sep 2005 11:56:59 -0400
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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