[SI-LIST] Split Reference Plane Madness
- From: "Chris Maryan" <cmaryan@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 7 May 2009 10:40:26 -0400
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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