[SI-LIST] Re: Analog & Digital Grounds
- From: "Traa, Boris" <boris.traa@xxxxxxxxxxx>
- To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, Chris Adam <chris.adamsipi@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 12 Oct 2009 11:44:29 +0200
For what reasons should you separate analog and digital ground? If an IC is
provided with separate grounds on package and die and you measure the
impedance between these ground at the balls/pins you may come up with a low
impedance for the concerned frequencies due to the various capacitive coupling
paths. In my opinion it is better to have a defined short circuit than a
mysterious impedance between ground domains.
Kind regards
Boris Traa
System design engineer EMC
It's the currents that make circuits work or fail.
Philips Applied Technologies/EMC center
Room 2.020
High Tech Campus 26
5656AE Eindhoven, The Netherlands
Tel: ++ 31 40 27 43766
Fax: ++ 31 40 27 42224
E-mail: boris.traa@xxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lee Ritchey
Sent: 2009 Oct 12 2:16 AM
To: Chris Adam; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Analog & Digital Grounds
Chris,
Good question. I've never separated analog and digital grounds. That may
sound pretty strong, but it is true for 2000+ designs.
I think the recommendation to separate analog and digital ground stems from
the fact that mixed signal parts often are equipped with both type of
ground pins. This is not done so you have to design two separate grounds
in the PCB, but rather so the two internal circuits don't share the same
ground path out of the package. This is vital for A/D and D/A converters
and assorted video circuits. If not done, the "ground bounce" when the
digital circuits switch is coupled into the analog circuits which usually
are working with very small signals or small changes in signal value. A
ground bounce spike that would not disturb a digital circuit can totally
disable the analog function.
Sometimes the same issue applies to Vdd.
Lee Ritchey
> [Original Message]
> From: Chris Adam <chris.adamsipi@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/9/2009 3:53:48 PM
> Subject: [SI-LIST] Re: Analog & Digital Grounds
>
> Thanks Lee and all those who offered valuable suggestions offline.
> When do you really care to separate analog and digital grounds ? I see
some
> transceiver vendors recommend splitting the grounds while others don't.
>
> Thanks,
> - Chris.
>
>
>
>
> On Fri, Oct 9, 2009 at 8:29 AM, Lee Ritchey
<leeritchey@xxxxxxxxxxxxx>wrote:
>
> > First of all, there is no sacred rule that analog ground and digital
ground
> > must be separate. Who knows where that came from. Use only one ground
> > plane and manage the current flows so that digital ground currents don't
> > flow under the area where the analog circuits are situated. It's pretty
> > easy to do in most cases.
> >
> >
> >
> > > [Original Message]
> > > From: Chris Adam <chris.adamsipi@xxxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Date: 10/8/2009 5:47:35 PM
> > > Subject: [SI-LIST] Analog & Digital Grounds
> > >
> > > I am working on a PCB design where number of layers is limited to 4
to
> > > reduce cost. As opposed to the regular 'sacred rule', analog and
digital
> > > grounds are not separated though the respective VDDs are isolated. I
am
> > > particularly concerned that the merged ground is also a return path
for
> > > DDR2 signals. I would like to see how much noise does this ground
> > merging
> > > will introduce in sensitive power supply nodes such as for PLL and
CDR.
> > What
> > > would be the appropriate simulation set up for this ? I am thinking of
> > > multiport s-parameters, transfer impedance etc. I would appreciate if
> > any
> > > one shares his/her experience, thoughts and insights in this type of
> > > analysis.
> > >
> > > - Chris
> > >
> > >
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>
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