In message <ofaaf9b4a4.e676f095-on652576a3.0032e3a9-652576a3.00338...@scmmicro.co.in >, dated Wed, 6 Jan 2010, [email protected] writes:
> But many times I realised that connecting the shield conductor of >the USB cable to the digital ground of the circuitry in the PCB >improved the ESD / EFT immunity. Leaving the USB cable shield conductor >open (either at device end or at computer end) worsens the situation. I entirely agree; shields left open at one end are antennas. But shields should always be connected to enclosures, if conducting, and preferably to the outside surface. Think of the shield as a pipe connecting two water tanks. You need a full 360 degree seal at each end. No pigtails more that 10 mm long at most. If you connect the shield to the PC board, then all the hash on the shield is dumped into input circuits, where it can cause most trouble. > In the device introducing a ferrite bead between the digital ground of >the circuitry and the shield conductor of the USB cable also worsened >the ESD situation. Yes, because that open-circuits the shield at high frequencies. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK I should be disillusioned, but it's not worth the effort. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
