Hi Alan, The I/O being isolated is standard PC I/O (ie. VGA connector, serial, parallel, audio). The thinking behind completely isolating the IO ground from main logic ground is threefold:- 1. To stop noise coming out from the logic ground onto the I/O signals. 2. To stop noise entering into the logic ground from the I/O cabling etc. 3. To provide isolation from ESD and external EMC events. I am aware that there would be an issue of traces crossing plane splits with respect to return currents. However, I have stitching capacitors fitted across the split at the point where traces cross the split (to provide a path for the return current). My board is a standard 4 layer motherboard with tracking on the top and bottom layers and power and ground in between. CHEERS ROSS -----Original Message----- From: Alan Hilton-Nickel [mailto:ahilton@xxxxxxxxxxxxx] Sent: 06 August 2002 22:55 To: Johnston, Ross Subject: Re: [SI-LIST] Ground plane split widths... Ross, Unless you have a desire for pain, I'd recommend *reducing* the split width...to zero. I don't know what I/O you are trying to isolate, but the return currents that would normally go through the ground planewill just find another route - probably through your chassis. This opens up a big inductive loop. Even if it doesn't result in bandwidth limitation due to signal integrity violations, it will cause your board to radiate EMI. Regards, Alan Hilton-Nickel Transmeta Corp Santa Clara, California "Johnston, Ross" wrote: > > Hi > > My design is basically a pc motherboard. I have isolated the I/O ground from > the logic ground. Currently the ground plane split is 15 thou - I am > contemplating increasing this width. Should the split be kept as small as > possible? If so - why? If the ground split was too small, wouldn't noise be > coupled across from each plane? What is the general consensus on this? > > CHEERS > > Ross Johnston > Electronics Design Engineer, > Core Electronics Group, > Lifecycle GST, > NCR FSG (Scotland) Ltd, > Discovery Centre 2nd Floor West, > 3 Fulton Road > Dundee, > Scotland DD2 4SW > Tel: +44 (0)1382 592920 Direct Dial > Fax: +44 (0)1382 591089 > email: ross.johnston@xxxxxxxxxxxxxxxx > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu