[SI-LIST] Re: Question about split gnd planes
- From: Joel Brown <joel@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Tue, 30 May 2006 09:26:25 -0700
This is similar to a question I posted about an Intel app note for an
Ethernet controller that recommended a plane split between the controller
andEthernet connector with built in magnetics. Intel also suggested that
capsof various values might be needed to bridge across the split which would
seem to negate the purpose of the split. Our EMI consultant said do not use
aplane split. Most responses I got back said the plane split was necessary
toprevent common mode noise on the ground plane from coupling across the
Ethernet magnetics which are designed to attenuate common mode noise. I also
had a thought to eliminate ground planes around the Ethernet connector
excepton the bottom layer of the board thinking there would be less coupling
from the bottom of the board to the Ethernet connector. My understanding of
plane splits is this: a plane split is a slot antenna, there is likely to be
an RF potential with respect to the ground planes on either side of the
splitwhich energizes the antenna and causes radiation. This can be mitigated
by bridging across the plane split with capacitors at intervals which are
small compared to the wavelength of the noise on the planes. This is more
commonly done with plane splits where the DC voltages of the planes are at
different potentials.
Joel
steve weir wrote: Manix, no. There are many misconceptions out there about
the myth of ground and its relation to noise isolation. Those myths get
propagated into misguided applications of moats and such. Think in fields
andthe misconceptions go away. Steve. At 09:50 PM 5/25/2006, Manickavelu M.
wrote: Istan, Is it not that the analog and digital grounds planes can not
beconnected together anywhere but only under the chip that sources the
analogsignals? Also that while coupling these two planes we should not use
direct Cu plane connection but couple them via inductors? Manix, MindTree.
-----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx[1]
[mailto:si-list-bounce@xxxxxxxxxxxxx[2]] On Behalf Of
istvan.novak@xxxxxxxxxxx[3] Sent: Friday, May 26, 2006 6:26 AM To: Ed Troy;
si-list@xxxxxxxxxxxxx[4] Subject: [SI-LIST] Re: Question about split gnd
planes Ed, Splitting more than one ground plane in the stackup requires a
lotof 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>[5] Date: Thu May 25
15:38:33 CDT 2006 To: si-list@xxxxxxxxxxxxx[6] 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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