[SI-LIST] Re: Split Ground Planes
- From: "Brent DeWitt" <bdewitt@xxxxxxxxxxxxx>
- To: <rakesh@xxxxxxxxxxxxx>
- Date: Thu, 28 Oct 2004 20:25:56 -0700
Rakesh,
In my dealings with ADCs in medical device use, I used the "moat and
drawbridge" scheme rather than a true split. The sensitive analog bits were
placed on an "island" to keep high speed digital currents from developing a
drop across the reference plane Z. The ADC was placed on the "drawbridge"
from the island that returned to the digital world. There was no reference
plane split _under_ the ADC and the digital pins were arranged to face out
of the island. No traces were allowed to cross the "moat" around the
island. Worked pretty well actually.
With respect,
Brent DeWitt
CKC Labs
Redmond, WA
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of rakesh
> Sent: Wednesday, October 27, 2004 10:36 PM
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Split Ground Planes
>
>
> Hi All,
>
> While appreciating what Guru Doug has mentioned,
>
> I have a doubt. Would the Split in the Analog and Digital ground below a
> ADC qualify as a split plane
>
> Best Regards
>
> Rakesh
>
> > > [Original Message]
> > > From: Doug Smith <doug@xxxxxxxxxx>
> > > To: <jose_moreira@xxxxxxxxxxx>
> > > Cc: <si-list@xxxxxxxxxxxxx>
> > > Date: 10/26/2004 3:04:16 PM
> > > Subject: [SI-LIST] Re: Split Ground Planes
> > >
> > > Hi Jose and the group,
> > >
> > > I normally reply personally to people to avoid the glut of
> > > "out-of-office" messages, but feel I needed to add to this discussion.
> > >
> > > Signal propagation is an important but small part of the trouble you
> > > can get into with split planes if not implemented properly. Even if
> > > you manage to get the signal propagation right, system reliability can
> > > be severely compromised as it is easy to turn every cable in the
> > > system into a great receiving array to pick up impulses you can easily
> > > miss in the lab, but your customers won't!
> > >
> > > I worry also about the common mode voltage that can result across the
> > > board and drive enclosures, cables, etc. This voltage will cause
> > > radiation that depends on a lot more that the board itself. Also
> > > common mode voltages can be developed from outside sources as
> > > mentioned above.
> > >
> > > There are probably thousands of modes to get in trouble with split
> > > planes of which the referenced paper covers a subset. You can cover
> > > many of the problems by not ever allowing a trace to cross the slit.
> > > Even DC signals like LEDs are generally too noisy to allow to cross a
> > > slit in many cases.
> > >
> > > Most of the examples of splitting up ground planes I have seen were
> > > not necessary and just caused problems (remember that at high
> > > frequencies everything is connected together whether you want them to
> > > be or not, lots of articles on my website about this effect). There
> > > are a few justified cases though. Video can be one example where the
> > > signal can be degraded by just a small amount of powerline current
> > > (hum bars in the video).
> > >
> > > In general, I would not recommend splitting planes unless you can
> > > state and MEASURE (to confirm) why a split plane is needed.
> > >
> > > Doug
> > >
> > > jose_moreira@xxxxxxxxxxx wrote:
> > >
> > > > Nikhil,
> > > >
> > > > you can find the article on the IEEE xplore site (if you have acess)
> or
> > =
> > > > in a library.
> > > >
> > > > Details:
> > > >
> > > > Signal propagation and radiation of single and differential
> microstrip
> =
> > > > traces over split image planes
> > > > Orhanovic, N.; Raghuram, R.; Matsui, N.;
> > > > Electromagnetic Compatibility, 2000. IEEE International Symposium on
> =
> > > > ,Volume: 1 , 21-25 Aug. 2000=20
> > > > Pages:339 - 343 vol.1
> > > >
> > > > Best Regards
> > > > Jose
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: Nikhil Patel [mailto:npatel@xxxxxxxxxxxxxxxxxxx]
> > > > Sent: Dienstag, 26. Oktober 2004 15:40
> > > > To: si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Split Ground Planes
> > > >
> > > > Hi all,
> > > > I have been doing a lot of reading on Split ground planes
> and am a bit
> > > > confused now. Do most people use just one solid ground in the
> schematic
> > > > and
> > > > layout for EMI issues or do you prefer split ground planes?
> Any input
> is
> > > > greatly appreciated.=20
> > > >
> > > > Thanks
> > > >
> > > > Nikhil=20
>
>
>
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