[SI-LIST] Re: Split Reference Plane Madness
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Chris Maryan <cmaryan@xxxxxxxxxx>
- Date: Thu, 07 May 2009 12:19:00 -0400
Chris,
yes, series termination will help. What you'll want to do is to series
terminate the lines prior to crossing the split, and as close as is
reasonable to the drivers. You'll want to over terminate, but be careful
to stay within the Vil, Vih margins of your receiver. Additionally, if
you have timing margin, place a small capacitor after the series
resistor to further filter the signals, and reduce the edge rate. At 30
MHz, you have a lot of latitude to play with.
regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Chris Maryan wrote:
> Hello All,
> First off, thanks for all the help pointing me to some SI tools earlier.
>
>
>
> I'm currently working with a mixed signal system that has a split ground
> plane around the quiet analog parts (D-A + filters and amplifiers).
> Ignore for a second the pros and cons of split planes and assume I am
> stuck with the current design of the planes and the signals that cross
> over them, position of the chips, etc.
>
>
>
> There is a set of (single ended) data lines and a clock line that go to
> the D-A that start over top of the digital ground, cross a split and
> then go over the analog ground and enter the DA. Needless to say, this
> causes noise in the analog system. (The grounds are joined in one place,
> unfortunately near (10mm), but not directly underneath the place where
> the crossing occurs). I've reduced the drive strength and edge rate on
> these lines as much as possible and this dropped the noise by 15dB.
>
>
>
> What about termination, is there anything I can do? Presently, there
> isn't any. (speeds are around 30MHz, trace lengths are about 20-30mm on
> the digital plane and less than 10mm over the analog plane). Is there an
> optimum place to put a series termination on these lines? What I've read
> says that location of series terminations doesn't matter, but that's in
> the case of a continuous transmission line. What about my split
> scenario? Also, would termination to ground be useful (my experiments so
> far say no, we had a termination to AGND on the clock line and this
> proved to be a terrible source of noise)?
>
>
>
> Trying to develop my intuition about this stuff...
>
>
>
> Thanks,
>
>
>
> Chris Maryan
>
>
>
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
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