Because the PC board (with signal or power return planes) is in close proximity to the metal enclosure or chassis, parasitic capacitance almost guarantees a high-frequency connection to the chassis. Therefore, you may as well connect it down good in multiple locations.
I may have this wrong, but I believe the only time signal return is connected via a capacitor to chassis is for low-frequency isolation and high-frequency connection. I may have to research this more.
Remember, the chassis is not a "sink hole" for electrons. Currents flow in loops and the major goal to endure EMC is to make these current loops as small as possible. Therefore, there is really no such thing as a "good" ground.
Thanks a lot again! Yes my question was related to analog/digital ground planes (or rather signal return planes as you have correctly reminded) primarily and I have got a satisfactory answer from the Q&A. Another question I had was related to the connection between the chassis ground and signal return planes. I have seen systems which had direct connection (through plated through holes etc.) between the signal return plane and chassis ground, again for certain systems the connection is through a capacitor & 1M resistor parallel combination. If the chassis ground is a good ground, is it always better to tie the signal return planes to the chassis ground directly at some point, if there are no safety isolation related requirements?
Sanjib, I presume you're referring to separate analog versus digital ground return planes. I would suggest, besides Dr. Van Doren's slide set, checking out the references on Dr. Todd Hubing's CVEL web site, I provided earlier. He addresses that topic and is more of a proponent (as am I) of using a single return plane. You really need to be aware of the paths of the critical return currents, however.
Thank you for sharing Dr. Tom Van Doren's slides on grounding. I will read through this taking some more time, but I just scanned through. I have a question that occurs to me repeatedly: single solid ground or separated ground planes with proper decoupling. Both has some pros and cons. Hope I will get a convincing answer from this article. Again, thanks a lot!!
Yes, there were quite a few papers presented on grounding and shielding - especially in the Monday and Friday workshops. Henry Ott's Fundamentals of EMC workshop was a good case in point. In addition, these topics were demonstrated during several of the EMC Experiments sessions (no papers there, unfortunately).
If you're a member of the IEEE and the EMC Society, you should be receiving a CD with all the published papers and workshops. If not, I imagine they'll all be uploaded into the IEEE "Explore" database - for a fee.
EMC is a topic that always haunts when the prototypes are built for testing and the ocean of learning always remains unexplored. Are there any of the papers presented in the symposium related to grounding, shielding etc published for public view?
What was there in the EMC pocket guide? Sounds attractive. I always miss one such small reference with consolidated information needed during troubled times. Is it still available for sale?
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