Hi John and the group, Yes, I did mean it! If you connect it to a different "ground" then the difference between the two grounds will show up as a common mode signal to the USB chip. A shielded cable is nearly a perfect transformer meaning that the voltage impressed across the inductance of the shield is induced into the center conductor (like a common mode choke). The result (for an ideal cable) is that the signal that arrives at the end of a shielded cable is just what entered it, in this case at the mouse end (shield inductive drop cancels the induced voltage in the center conductor). But, if the shield goes somewhere else than the signal ground the difference between signal ground and that point will be added to the output of the shielded cable.
I have a nice (intuitive, non-mathematical) description of how shielded cables work in my seminars which I have not posted on my website yet. May put it on CircuitAdvisor.com when it is up later this month. You can have the shield connect to the chassis on the way in to dump ESD currents there, but it generally has to end at signal ground as well. A well designed board will not be much affected by the ESD currents. If you are trying to do 20 kV contact (no correlation to reality) then everything gets much more difficult. Might need ferrite on the shield before entry and between the chassis ground point and signal ground on the cable. There are many articles on my website with data on this type of thing. A few that apply are: http://emcesd.com/tt110199.htm http://emcesd.com/tt2005/tt030105.htm http://emcesd.com/tt2007/tt020107.htm (not ESD but cable shields work similarly (better) than coupled bonding conductors) http://emcesd.com/tt2007/tt030407.htm http://emcesd.com/tt2009/tt020309.htm I have confirmed this with countless measurements over the last 20 years and fixed countless ESD problems this way as well. Doug On 1/6/10 12:27 AM, John Woodgate wrote: > In message <[email protected]>, dated Tue, 5 Jan 2010, Doug > Smith <[email protected]> writes: > >> the cable shield has to be closely referenced to the signal ground of >> the receiver to prevent common mode overload problems. > > That could be incorrectly interpreted as a recommendation to connect > the shield to the PC board. I'm sure you didn't mean that. -- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: [email protected] \ _ / ] \ _ / Website: http://www.dsmith.org - 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]>
