Lee - I really must take issue with you on a statement you made in a chassis ground thread. In it you said: "You don't need to tie logic ground to the case many places to control ESD. ESD is a relatively low frequency event that can..<SNIP>" This is pure nonsense. In a series of ground breaking experiments W.M.King (http://www.systemsemc.com was measuring ESD impulses at sub nano second rise times [limited only by limitations of the measurement equipment] in the 1980s. More recently I was told by Doug Smith (http://www.emcemd.com) that he has personally measured ESD rise times at an astonishingly fast 80pS [again only limited by the measurement equipment]. ESD (and its effects on electronic equipment) is a significant design consideration for EMC Engineers. Please check your data before making these claims. BTW at multiGHz frequencies (such that an 80pS impulse would generate) Doug flatly maintains that even if you physically float logic from chassis, the frequencies are so high that the grounds are coupled anyway. Sorry Lee there goes your floating design.!! May I suggest that folks interested in mitigating ESD attend some free EMC meetings at their local Society. Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: charles.grasso@xxxxxxxxxxxx; Email Alternate: chasgrasso@xxxxxxxx -----Original Message----- From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] Sent: Wednesday, March 03, 2004 9:40 AM To: Chris Padilla; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: How to connect Chassis ground to DGND Chris, The last two sentences weren't truncated. The reason to avoid tying logic ground to the Faraday cage in more than one place is to avoid providing a secondary path for current that should flow in the ground planes of the system, namely the skin of the Faraday cage. A common symptom that this has happened is that EMI seems to leak out of the box at the cracks. I am aware that many companies, including Cisco, tie logic ground to card guides and face plates. They often have a difficult time making the box pass EMI as a result. You don't need to tie logic ground to the case many places to control ESD. ESD is a relatively low frequency event that can be controlled with a band of copper around the edge of the outer layers of a PCB that is tied back to the backplane only. Hope this helps. Lee > [Original Message] > From: Chris Padilla <cpad@xxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 3/1/2004 4:56:29 PM > Subject: [SI-LIST] Re: How to connect Chassis ground to DGND > > At 01:54 PM 2/29/2004 -0800, Lee Ritchey wrote: > >I've watched this one from a distance. I'm not sure what the objective of > >tying logic ground to "chassis" ground. There are at least tow possible > >reasons. > > > >1. To meet the UL safety specification. > > > >2. To help control EMI. > > > >In the first case, logic ground doesn't need to be attached to the chassis > >ground to do this. The chassis needs to have a good connection to the > >green wire that goes back to earth in the AC system, > > > >In the second case, using the word chassis when discussing EMI confuses > >things. In this case, the product is surrounded by a Faraday cage, which > >likely has parts of the chassis as sides. If this is the situation, it is > >important to make sure currents that belong in the logic ground system > >don't have an opportunity to flow in the Faraday cage. This can happen if > >logic ground is tied to the Faraday cage in more than one place. Symptoms > >of this happening are the complaint that EMI is "leaking" at the cracks. A > >common error is to tie the backplane logic ground to the Faraday cage > >around its periphery (Good Idea) to form one side of the Faraday cage. > >Then, the faceplates of the PCBs or their edges are also tied to the > >Faraday cage. (Not a good idea) > > Lee, > > I'm curious about the last two sentences above...I hope my truncation > of the message doesn't hurt the intended context.... > > You say a "common error" is to tie the BP periphery to the cage to form one > side of the Faraday cage but then say it is a good idea. I'm confused here. > > The last sentence states that it is a bad idea to tie the PCB board edges > and faceplate (presumably on a carrier tray) to the chassis or Faraday > cage. Why is that? We often do this to protect against ESD but I must > confess that I haven't tested any of my boards in a situation where the PCB > edges or carrier tray (faceplate) haven't been attached to the Faraday cage. > > Thanks, > > Chris Padilla > EMC Engineer > Cisco Systems > > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.org > > 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 technical documents are available at: http://www.si-list.org 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 technical documents are available at: http://www.si-list.org 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