cablesdataemcethernetshielding
With regard to termination of ethernet shielding I have been taught always to terminate only ONE end of the shielding to ground to prevent ground loops occurring.
Is this best practice?
and is there any other way of terminating the shield?
Finally.. and I'm sorry for the list of questions but the way I look at this schematic symbol is that the shield connects to pin QE. Is this what others think too?
Have a look at this tutorial made by Analog Devices. You can also go to Analog's and Kiethley's web site and search for shielding. You might find more white papers.
Your colleague was correct that the shield that's grounded at the host end can inject noise into the ground. Typically, the shield is grounded at the remote end. You could also connect the shield to the chassis ground.
Grounding scheme of the USB cable shield is not a good example, IMO. USB is designed to work with many different gadgets. Most of the USB gadgets aren't grounded by themselves. Perhaps, a trade between EMI immunity and versatility was made.
You may or may not be over thinking this, it depends on what your application is, and if you have to pass any regulatory inspections for a product. The general idea is to shunt the ESD to ground through the chassis and away from your electronics. This depends on if your enclosure is insulated or not. Another thing to keep in mind is you can also have RF running through the shield, and RF should be considered if you need to build a product to pass emissions testing (cables make great antennas, and can even help induct lightning RF onto your board). For now I'll talk about ESD. A good reference for all things ESD and RF is this book Electromagnetic Compatibility Engineering by Henry W. Ott. I'll quote it
So where should cable shields, transient voltage protectors, and I/O filters be connected when the product is in a plastic enclosure? There are three possibilities as follows:
In your case if none of the cables go on the outside of a vehicle I wouldn't worry too much about RF. If the cables aren't going to be in contact with people (buried in the dash) I wouldn't worry too much there either. If there going through the middle next to people I would worry. You can think of your vehicle ground like earth ground. The car may collect a charge but it also functions like a faraday cage so everything on the inside will be near 0 (except for something like a seat cover that has been charged up, any metal connected to the chassis will be near 0V (0v being the voltage with respect to the car and not "Earth" ground)).
I've also included an image to suppress ESD in a metal case from the same book:
Best Answer
Unfortunately junior engineers may have misunderstood advice about Single Point Grounds (SPG)
If a shielded cable is NOT grounded at both ends, H fields are attenuated by LPF and E fields are amplified by HPF effect.
The shield is not the issue; Engineers’ knowledge of ground potentials, isolation, and shielding is the issue.
The PHY of the ethernet has an effective high CMRR balun but may not be enough if RF noise couples to an open ended shield with 1/4 wave effects and high E field coupling.
The shield is supposed to be grounded at both ends.