All, For circuit packs and chassis, should the pack's logic ground planes be contiguous and connected to the metal faceplate with plated through-holes thus connecting logic ground to chassis ground? Currently, the packs have a chassis ground planes along the front edge of the pack which provide holes for mounting the bracket. Logic grounds are separated from this chassis ground with a gap. I've placed pads at each of the five through-holes for the bracket to allow logic and chassis to be connected or not based upon EMC testing. There are some unshielded I/O which are filtered using transformers and chokes. Connectors are metal and connected to the faceplate (chassis) by contact. I've had good luck using this scheme during EMC testing. The idea of having the option to isolate chassis from logic ground is for ESD, EFT, etc. immunity. Those test signals are referenced to chassis ground and keeping all offending currents on the outside of the shield (enclosure) would boost the system's immunity. Thanks in advance for any assistance. Grounding gets me every time. What to do with logic grounds and chassis for systems that operate into several hundred MHz and has unshielded I/O ports. Jack -----Original Message----- From: npischl <npischl@xxxxxxxxx> [mailto:npischl@xxxxxxxxx] Sent: Monday, February 24, 2003 2:05 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Shield ground isolation Annand, In addition to other good replies you received on the list: With shielded cabling, for low emission you need to minimize common- mode (CM) currents that flow on the exterior of the shield. In general, is not true that it is achieved by disconnecting the logic ground from shield ground. What is needed is high transfer impedance of the overall shield. Transfer impedance of a shielded cable for example is the (intentional signal) voltage between the center conductor and the shield divided by the CM current that flows on the exterior due to imperfections. If the cable shield is perfect, and the source and the load are perfectly terminated with 360-degree connection around the ends, the transfer impedance is very high (no "leakage") and the emission is low. You can easily expand the concept to the overall shield, including the "box", except that the definition of the driving voltage may become little more ambiguous. In most systems you will deal with, the two main causes of increased emission from unshielded cables will be imperfect connection of the shield at the connectors (pigtails), or compromised shielding-integrity of the chassis (e.g. a slot). They both let CM currents flow on the exterior of the shield and cause emission. Trying to "disconnect" the two metals (digital and shield) will not work well because, you want it or not, they are connected by their mutual impedance (C in the first place) anyways. This impedance lets current flow between them. This may also create serious functional problems when the cable shield is used as a reference (I have seen it unfortunately many times). It is important to keep continuous and low-impedance current path for both the intentional and CM signals, while minimizing external CM currents (maximize transfer impedance). This can be done by connecting the signal GND and the shield together at the connector, and maintaining low-impedance continuous connection for the intentional signals and 360 degree (or best approximation) of the cable-shield. The result is that the intentional signal current returns through low-impedance and that CM voltage at the connector (mainly the voltage between the PCB GND and the shield) is low, thus CM current on the cable is also minimized. What it does is that it lets CM current return to its source withini the box, instead of getting on the exterior of the shield. If doing so doesn't reduce emission from the shielded cable, you need to look at the shield-integrity of the box that is most likely compromised. Since we live in an imperfect world, sometimes you may see it happen, but it does not prove that disconnecting the signal from the shield is necessary for good EMC. Quite opposite is generally true. The only exception that may sometimes validate disconnecting the two may be if you drive the I/O through transformers or some other means of isolation. Even then, it depends on how much isolation (e.g. C between primary and secondary or across an optocoupler) you really have. Neven --- In si-list@xxxxxxxxxxxxxxx, "Kuriakose, Anand" <Anand.Kuriakose@xxxx> wrote: > > Hi All, > > On high speed boards with I/O connections running to outside world, the > shield ground of the connectors should be isolated from the digital ground > if the board has to meet the requirements of EMI/EMC compliance standards. > > Are there any side-effects of isolating the shield ground from the digital > ground? > > > I have seen on some boards, that the isolation is achieved by using ferrite > beads. On few others i have seen capacitors being used b/n shield and > digital ground. > > How are capacitors useful in acheiving isolation? Or are these caps used for > some other purpose, which i am not able to understand? > > What is the best method to achieve this isolation? > > Thanks in advance. > > Regards, > Anand. > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxx 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@xxxx 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 ------------------------------------------------------------------ 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