The device has no means of providing a solid RF ground other than via the shield which seems to be chassis grounded back at the PC. I'm measuring 40 ohms from the shield to the earth on a power outlet. Do we really want to connect our 0V return path to the shield?
Yes! That's how ESD finds ground, for example.
I have had a quick look around the internet and there seems to be contradictory advice from IC manufacturers as was found by this chap here http://forum.allaboutcircuits.com/threads/usb-device-cable-shield-connection-grounding-it-or-not.58811/
The rampant misunderstanding is easy to see, for example:
Contrary to my initial supposition, the purpose of the USB cable shield
is not to protect the USB data lines from outside interference, but
rather to prevent the USB device from radiating EMI.
Both are necessary purposes, and both occur simultaneously! To deny this truth is to deny reciprocity. While there are circuits that do not exhibit reciprocity, a cable isn't anywhere near smart enough to do this!
All the (2) examples suggest RF grounding anyway, so this reason for example is preposterous,
To limit the USB cable antenna effect, it is
recommended to connect the shield and ground through an RC
filter.
It's freaking RF grounded anyway, what are you even trying to avoid? Probably yet another example of an appnote written by an intern... or an engineer with about as much EMC knowledge.
The TI appnote is even worse; the broken link is here,
http://www.ti.com/lit/an/spraar7c/spraar7c.pdf2.2.1: They bring out that old, bizarre practice of stacked capacitor values.
2.2.4: Ground is, well, grounded: says to ground to chassis. That's fine, so long as chassis ground is plane ground. If your board ground is noisy against that ground, you have other problems anyway, and you are bad and should feel bad.
The entire statement of this section is bizarrely incomplete and shows the author's lack of knowledge; while he knows signals must be conveyed over signal ground, he has no idea how that should be reconciled against chassis ground, except by playing the schoolyard "Not-Touching" game.
The second paragraph is the real doozy. This suggests a small ferrite bead, I expect cautiously small so as to avoid the inevitable problem. This statement:
"To prevent EMI from coupling onto the cable bus power wire (a very large antenna)"
Is all kinds of bizarre! How can bus power be an antenna
if it's wrapped in a freaking shield?! And how can that one wire, out of all 4-5 entering that shield, be the sole exclusive concern?
2.2.7, last list item: a bizarre suggestion, as with absolute minimum capacitive loading (as stated in other items), the common mode choke has absolutely nothing to work against. It's also a bad idea anyway, because the start/end packet signals
are not differential anyway, and attempting to filter the data lines as a pair only results in trouble.
The intermediate stuff is okay.
Finally, the ESD advice is strangely the best. 3.4 is general ESD advice, with the difference that the second item (GND isolation) is dangerous, and for non-isolated data signals, fatal. For example, Ethernet is designed this way: the isolation barrier is transformer coupled, with a 1nF cap to ground to provide a path for ESD (note that 8kV on 150pF discharges down to 1kV on 1nF, the rating of the Ethernet isolation barrier).
Perhaps I should attempt to scope the data lines with shield in both connected and disconnected states and then try some of the suggestions such as connection via 1M resistor and additionally with a paralleled small cap in an effort to see what, if any, are the effects
This is dangerous, as you won't observe much difference without an external source of EMI to introduce noise on your signals. Such measurements will lead to exactly the faulty conclusions above!
Tim
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