Hi John, Some years ago, equipment in rack assemblies (sonar for example) used to have a "frame continuity" network. This network was isolated from the rest of the system except that it was joined at one point to the power supply ground. All the power returns of each electronic assembly where separately returned to a star-point ground terminal near the power supply unit. At that time, there was very little digital processing - it was slow - and high frequencies greater than 1MHz or so where not really of any concern. The idea was that a ground loop would be set up if the units were grounded at more than one point. Any such ground loop would be susceptible to inductive mains frequency coupling, and with lots of unscreened analog circuits in the system, this could easily be demonstrated if a unit was wrongly wired! I think maybe that kind of thinking has now changed and been replaced by a requirement for tight bonding between well-shielded units. Where necessary, isolated power supplies and differential signal interconnects or optical coupling are used for sensitive situations. Could it be that for lower frequencies, the Frame Continuity principle is still useful? However it seems to me that the decoupling caps you mention around the edges of the boards could possibly present a problem, coupling noise in at mid to low frequencies. Cheers Geoff > -----Original Message----- > From: John Coupland [mailto:coupswork@xxxxxxxxxxxxx] > Sent: 24 October 2003 17:59 > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Sacrificial ground: Is it useful? > > > Hi gurus, > We've traditionally used a "sacrificial ground" (a > trace around the edge of PCBs) which is connected to > chassis ground (when the board is installed). We also > distribute some caps (like 0.1uF) from this trace to > signal ground around the edge of the PCB. > 1. Is this trace used to help protect the active > circuitry when one picks up the board by the edges > (i.e. when ESD handling procedures are not being > followed) OR is it beneficial for some other purpose? > If beneficial: > 2. Are the caps beneficial or counter-productive? > 3. Should it be placed on all layers? > 4. How wide should it be? > 5. How near the edge it should be? > 6. How much spacing should there be from it to active > traces? To power/ground planes? > Thanks, > John > PS, I didn't see anything about it in the archives. > > ------------------------------------------------------------------ > 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 > > _________________________________________________________ Zetex Semiconductors - Solutions for an analog world EID Award Winners for 'Best Use of Technology' 2003 for the AcoustarTM ZXCW8100 End-to-End Digital Audio Amplifier Controller http://www.zetex.com _________________________________________________________ ###################################################################### E-MAILS are susceptible to interference. You should not assume that the contents originated from the sender or the Zetex Group or that they have been accurately reproduced from their original form. Zetex accepts no responsibility for information, errors or omissions in this e-mail nor for its use or misuse nor for any act committed or omitted in connection with this communication. If in doubt, please verify the authenticity with the sender. ###################################################################### ------------------------------------------------------------------ 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