USB Filtering and Protection - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2015, 03:56:25 pm
content: With there being so much poor information out there, or misinformation even, I felt I should share a good article:https://product.tdk.com/en/products/emc/guidebook/eemc_practice_04.pdfRelevant things to notice:- Signal shield is shown going straight to ground. (This doesn't have to be circuit ground necessarily, but any AC voltage difference between this ground and circuit ground will be impressed on your signals. At the very minimum, it must be RF-bypassed to circuit ground. In almost all cases, tying the shield to the circuit ground plane is absolutely acceptable, and preferable!)- Common mode filtering on the circuit side (all 4 wires, but not the shield) is able to take up a little noise that the shield itself didn't catch (which might be because of poor wiring quality -- the shield should fully envelop the signals in all places, but if you have to tolerate cheap cables..).- But not too much, particularly for high speed operation. Two problems: 1. USB uses common mode signaling as part of the standard, and 2. there's no impedance to terminate the common mode filter. The best we can do is remove some potential high frequency interference, but heavily filtering it is actively destructive!- If you're using full speed, you can afford much more loading capacitance (e.g., TVS or schottky clamp diodes), and some really practical filtering (to the extent that your radiated immunity over 100MHz can actually be reasonable, also leaving some allowance for poor shielding). Heavy common mode filtering is still off the table (for the same reason: preserving common mode signal levels).For full speed filtering and protection, I'd recommend something like this,http://www.st.com/web/en/resource/technical/document/datasheet/CD00002023.pdfor building one out of discrete components if you like, but heck... they're so cheap and compact, you'll have a hard time arguing for anything else. (They're also available with pull-up/down resistors to identify host/client, low/full speed.)Tim

author: c4757p
author_rank: Super Contributor
timestamp: December 07, 2015, 04:25:01 pm
content: Quote from: T3sl4co1l on December 07, 2015, 03:56:25 pmGot any more info, or at least informed opinions, on this? They do indeed short the shield to ground, but they don't explain why, and I've seen alotof argument back and forth on this. I'd love to work it out myself, but I don't have the equipment to do empirical tests at this level.QuoteOooooooooh. That's going in my library. In particular I love the dual TVS with termination resistor between them - the extra series impedance should improve the effectiveness a bit over just one TVS - hence why I've never really trusted on-die ESD protection on USB chips.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2015, 03:20:12 am
content: Consider an unshielded length of cable. Say you have twinax (twisted pair in coax shield), and there's a gap where the shield is removed, replaced with a shorting wire instead.Consider a source of common mode energy. It might be coupled onto the cable with a transformer, or by this cable forming a loop through an RF field, or...The common mode energy tends to travel over the outside surface of the shield. Remember that the currents on the inside and outside of the shield are independent (it acts as a Faraday cage). And the inside shield carries the image current of the signal lines.So, in the unshielded length, there are three wires (two signal and one shield), which have equivalent inductance*. Normally these are well coupled (in effect, a wraparound shield ensures 100% coupling between the shield and what's inside), but the coupling coefficient is less than 1 for individual wires (twisted closely or otherwise). So we get a nonideal transformer, and different voltage drops for a given current flow.(*Again, inductance is an approximation, but that's sufficient for our purposes here.)So we have a voltage drop across the shield wire, which is not shared by the other wires. Looking at the signal voltages at the far end of the cable, it looks like a voltage source has been added in series with the shield, and therefore with the signals as well. Or to think about it a different way: the current that was supposed to stay 100% outside the shield, is now part of a current divider (i.e., the parallel equivalent of a voltage divider), mingling with the signal current(s) which were supposed to be separate!Or if there's voltage drop between shield and circuit ground, at the connector, same thing: the signals are pushed up by that amount.The fundamental point is: if you want to measure a voltage, you must do so at the point of origin. Whether it's an oscilloscope probe or digital logic input, you don't want to have stray wiring picking up outside signals (or, causing loss of the intended signal, since it works both ways).My most dramatic applied example of this was a client's application, which used USB signaling through ungrounded connectors. The equipment was failing EFT immunity even on the machine's lowest setting (300V). Ferrite beads did nothing. What I had to do was replace all the connectors with shielded metallic connectors (no small cost, sadly..) and string up all the shields (at least they started with shielded cable!) to get all the grounds together. After that, plus some grounding strips and a few well-placed ferrite beads, I had the EFT immunity beyond 2500V: at which point my crude setup was arcing at the (non-HV BNC) connectors. But that was 8dB beyond what we needed, so that's plenty!Consider that a 2500V EFT pulse has to be attenuated over 60dB (to 2.5V) before it will stop wrecking logic thresholds (USB's immunity range is about half the supply, or ~1.6V). That's a good shield! Just 1cm of missing shielding has ~10nH of  inductance, and if the average frequency content of an EFT pulse is 100MHz (probably not unrealistic, for being such a wideband signal), then the reactance is 6.3 ohms, which acts as a voltage divider with the ~150 ohm common mode source impedance**, putting a mere 100V on your logic signals!**Common mode impedance is rather arbitrary, but it's rarely less than 50 ohms, and rarely very far into the 100s. The impedance of free space is 377 ohms, and most transmission line geometries have an impedance some ratio to this value (usually a lower ratio, except at rather extreme distances). The common mode is measured between all conductors of a cable, and ground. Which is to say: only the shield matters, in a proper shielded type. But, unshielded multiconductor will be very similar, taking the dimension of the 'shield' as the outline of the bundle of wires. So you can still have well-defined common mode impedances, with, say, CAT-5 (UTP) cable running through a duct or raceway (= ground plane!). The impedance of a 6mm round cable about 25mm above ground plane is around 150 ohms, which is a very typical condition for immunity testing.This, by the way, is also why ferrite beads don't tend to do much, or at least much more than a few dB (say 3 or 6, but not much more than 10). They act as a common mode impedance divider with respect to the transmission line impedance. At frequencies of 100MHz or so, ferrite beads are most commonly 50-300 ohms (more for very large or multi-turn affairs, though multi-turn cores also peak at a lower frequency). The primary application of them is actually to dampen resonances: where you might've had a resonant cable before (where theworst caseamplitude might be +20dB, causing problems to your circuit), now it can be dampened, so that the wideband might be only a few dB better (or worse, as the case might be!), but the worst offending peaks (or dips!) will be shorter and wider (lower Q).To maximize the value of a ferrite bead, try to arrange them in a voltage divider against even lower impedances (e.g., grounding clips -- maybe you can't fully shield the connection point, but you can get a ground in there. Effectively, you're using the ferrite bead to increase the coupling coefficient of the unshielded wires -- you aren't reducing the leakage inductance (which is the difference that's causing problems), but if you can drop most of that voltage across the ferrite bead rather than the wiring (reducing the ground current), you can have it shared evenly, and thus improve signal quality.Tim

author: niconiconi
author_rank: Frequent Contributor
timestamp: December 17, 2021, 05:39:58 pm
content: Quote from: T3sl4co1l on December 07, 2015, 03:56:25 pmApologize for bumping this 6-year old thread. But I have a question to ask.In the TDK application note you shared (404, but web.archive.org has a copy), all the common-mode filtering examples it demonstrated were applied to USB data lines, D+/D- only. And I perfectly understand that heavy common-mode filtering is unfeasible for USB at the D+/D- lines because USB 2 High Speed is not truly differential, it actually uses single-ended signaling as part of the protocol (e.g. symbol SE0).However, what actually stops you to do heavy common-mode filtering on the USB cable as a whole? For example, why can't someone make a special 3 or 4-wire USB common-mode choke that winds the D+, D-, and GND (possible also VBUS) on the same magnetic core? The single-ended signals are common-mode from the perspective of D+ and D-, but it's differential-mode from the perspective of the cable as a whole. So the hypothetical 4-wire choke should work as a regular ferrite bead, but now exists as a SMD component integrated on the board, and not externally attached to the cable, it sounds like an attractive solution to at least give some high-frequency EMI suppression. But why nobody does it?I can imagine three reasons.1. The leakage inductance of the common-mode choke is still significant enough to degrade signal integrity and the result is not much better than a choke on the data lines.2. It's expensive or difficult to justify. For example, maintaining the characteristic impedance in such a choke can be difficult. And it's more difficult if you also want to filter power simultaneously.3. Ferrite chokes in general, as you said, are often just not very effective, and doing so is not worthwhile.What is the reason here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2021, 07:49:33 pm
content: Ah, looks like they moved it here:https://product.tdk.com/en/products/emc/technote/guidebook/index.htmlMulti-line works, I've done it before -- but it's a bit tedious to do. Also, keep in mind that "multiline" chokes may be poorly coupled (bead arrays and multi-hole wound types). Dual-line chokes can be used, using one line of each to carry just GND, and the remaining line for each signal. This is good for, for example, running I2C or SPI over a short distance (off board), while preserving signal quality and bandwidth, without having to push everything to RS-422 or something.You might still not do this for USB, because the impedance will be quite off: the differential impedance is twice the impedance of the chokes used. Basically, they act in series. And they're typically made with bifilar magnet wire, so the impedance is fairly high (~100Ω), so it's really far off compared to USB (200 vs. ~90). For Full Speed that's likely fine, but may be questionable for High Speed on up.Note that power counts as ground for RF purposes, so you can put any combination of VCC/GND into thse "GND" lines. Stick a bypass cap on both sides so they act together at RF, no problem. The leakage (the LF equivalent of the choke's TLT impedance and length) serves as filtering, so I wouldn't worry about adding ferrite beads or other supply filtering in that case.Tim