I'm designing a board with 2 SFP connectors and I am not sure what I should connect the cage pins to. The manufacturer recommends connecting the case pins to chassis ground but my product is in a plastic housing.
I read this article but it assumes a metal housing. Should all the case pins be connected directly to ground? Or do I need a local plane under the cage and AC couple it to ground?
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The main reason that an SFP connector needs to connect to the chassis is to provide EMI blocking. That is, the cage around the SFP connector forms part of the overall shield that prevents the circuits behind it from radiating.
If you believe your design will not cause radiation even with a plastic housing, there is no particular need to connect the SFP cage to the chassis.
It's probably still a good idea to connect the cage to some circuit ground potential (possibly through a high-value resistor), just to prevent it from being floating metal, which could cause problems of its own, either in terms of radiation or susceptability.
The Photon
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Thanks. How about ESD? Should I have a cap in parallel with the high value resistor? – AMG Oct 28 '14 at 21:38
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Yes, it's probably a good idea. Also so that the impedance between the cage and ground is low for ac signals. – The Photon Oct 28 '14 at 22:06
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I know nothing about SFP, but if there's a cap and R, why not just connect it directly? – George Herold Oct 29 '14 at 01:27