[SI-LIST] Re: Question about split gnd planes

Ed,

Splitting more than one ground plane in the stackup requires a lot of 
consideration, and mostly it is not necessary.  Isolating a sensitive circuit 
(e.g., analog input, low-jitter oscillator) may be a good idea, but instead of 
cutting a large solid ground plane, you may want to try first to put the 
circuit to be isolated on a grounded patch on a 'non-ground' layer.  You can 
make ground surface patches under and around your circuit to be isolated, or 
you can put the patch on a signal layer.

Regards,

Istvan Novak
SUN Microsystems


From: Ed Troy <etroy@xxxxxxxxxxxxxxx>
Date: Thu May 25 15:38:33 CDT 2006
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Question about split gnd planes

If you have a circuit board that requires a split gnd  plane over a 
small section of the board, and you have several ground planes, 
should only one have the split (the one nearest the side containing 
the components that require analog ground)  while the rest of the 
ground planes are continuous, or should the split section be on all 
ground layers? I would think that you should only have it on one 
layer. Also, if it should only be on one layer, I would imagine it 
would be best to connect it to the digital ground with one, and only 
one, via. Is that generally correct? What are some good references 
for layer stackups, etc? I know I saw one, once, but can't remember where.

Ed


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