I use 0603 ferrite bead from USB connector case to ground.
How big is what - EMC testing? I dunno, client keeps coming back so I guess it was OK.
There could be several reasons why you wouldn't connect the USB to solid ground plane. Imagine a machine making 16+ bit accurate measurements 24/7 having a USB port for debugging/servicing. Now, if USB connection breaks, that is not nice, but OK. If the shielding antenna picks up some noise, that is not acceptable on the other hand.
I use 0603 ferrite bead from USB connector case to ground.
I use 0603 ferrite bead from USB connector case to ground.
So do I. I have found that ground planes have ripple with fundamentals of the main system clock. For instance, one of my designs has a DDR2/FPGA running at 267 MHz. This draws a couple of amps so causes ripple on the ground planes. If the USB cable shield is connected directly to the GND plane then the USB cable will act as an antenna, with fundamentals showing up on the spectrum analyzer with various probes. The ferrite bead blocks this but does pass DC allowing DC level between systems to be aligned.
The differential signaling uses its own internal ground.
I suppose different shielding depending on environment and design?
Always ground shield directly to circuit ground plane. Anyone who says otherwise simply isn't smart. Yes.. you will find appnotes suggesting otherwise. They aren't smart.Quoteehh, i don't agree with that.
Proper USB ( i say PROPER usb ) has a difference between signal ground and shield. The ground wire is NOT zero-Z (0 ohms dc and 0 ohms ac : zero impedance or zero-z since z is impedance ) connected to the shield of the plug in the cable.
the Board (on both sides ) should be laid out in such a way that ESD energy shot into the shield (whether cable, connectors or enclosures) flies into the chassis ground on both ends and does not propagate into the system ground. that is why you create the r/c parallel network or the ferrite bead on both ends.
The resistor creates equipotential between system ground and chassis ground. the capacitor shunts Rf noise.
In case at least one end of the system will have a hard connection to 'earth'. ( computer with earthed power plug )
so in a properly designed system :
- ESD strikes shoot only into chassis ground and travel through the shield to an earth point without tilting the system ground. (human or machine discharge model using 'earth' as reference )
- system currents flow in a loop formed by the inner wires of the usb cable ( Vbus, Gnd, D+ and D- ) ESD energy never directly (it does couple ! more on that later) enters these wires as it is stopped by the shield , or the metal casing around the system boards.
In case of a lost earth connection, energy will dissipate slowly through the resistor / ferrite without upsetting the system.
The system works the other way around. Any power supply noise caused by the system is trapped inside the shield where it is shunted to earth.
Creating the system shield needs to be done properly. People put rings around the entire board. this can act as a tuned network and actually create a problem. Phat is the task of the small c : the shunt the energy so this does not happen.
In multiplayer boards the planes need to be shunted at the extremities using small value capacitors. Any plane system acts as a large dipole antenna. Take a square meter double sided board , in the middle put a clock source tuned to the resonance of the board and you have a massive dipole antenna radiating this energy. The solution is to place shunting capacitors at the edges and corners of the board. This shunts the rf energy.
The same goes with shooting arrays of via's. You need to know the wavelengths you are dealing with and shoot them closer together so they trap the energy.
These things are often done completely wrong and then the product fails EMC testing because it radiates.
There is another aspect as well (continuation form earlier) : When an ESD strike occurs on the shield, it will couple capacitively into the bundle of wires inside the cable. The Vbus, GND D+ and D- see this as a common-mode spike. sending these signals through a common mode choke prevents the coupled energy from causing any damage in your system. You force the energy to shoot into the shield and keep it out of the system (power , ground and signals)
To do it right you need a split system / shield , with the proper shunting for DC point and shortin RF energy getting out , and you need to stop the common mode being coupled in as well.
There is another aspect as well (continuation form earlier) : When an ESD strike occurs on the shield, it will couple capacitively into the bundle of wires inside the cable. The Vbus, GND D+ and D- see this as a common-mode spike. sending these signals through a common mode choke prevents the coupled energy from causing any damage in your system. You force the energy to shoot into the shield and keep it out of the system (power , ground and signals)
To do it right you need a split system / shield , with the proper shunting for DC point and shortin RF energy getting out , and you need to stop the common mode being coupled in as well.
Always ground shield directly to circuit ground plane. Anyone who says otherwise simply isn't smart. Yes.. you will find appnotes suggesting otherwise. They aren't smart.
If your circuit, for some reason, can't have an exactly grounded shield, you will need an isolation circuit. Probably, you'll have your USB chip on the ground side, then use simple serial communications to your MCU on the ungrounded side; or a USB+MCU on the ground side, with whatever interface to the ungrounded side. This is still more reliable than violating the ground shield.
The most dramatic case is this: suppose the USB cable is placed near some heavy switching equipment, say, a refrigerator switching on and off, or a lamp dimmer, or cheap Chinese unfiltered switching supplies, or anything industrial, etc. The spikes from these sources gets coupled onto the shield of the USB cable. The current from these spikes must find a path; if you don't shunt it around your circuit, it will go through it. Failure to ground the shield means some or all of the noise gets impressed upon the USB data signals, which can't tolerate more than a volt or two of common mode noise before logic levels are violated (USB has poor common mode rejection, even in true differential High Speed mode, and no, common-mode filtering won't help -- the USB signaling method is not designed to accommodate that, unfortunately). The end result is com failures, dropping packets, dropping the device entirely, or maybe even crashing the chip and needing a reset.
IEC 61000-4 Electrical Fast Transients (EFT) test normally involves very short spikes of 1-2kV amplitude: even if your shield is 99.9% effective, that's 1-2V left for your USB PHY to deal with, which is unacceptable! Very good shielding is needed, and this is why all proper USB devices integrate the connector, with grounded shell, into a shielded enclosure. Desktop computers, for instance, are solid metal, and for good reason (to keep internal noise inside, and to keep external noise outside!).
Tim
I remember at a previous job where we developed PLC's, HMI's etc for the industrial market we had problems with EFT. For USB to pass the test we inded had to connect the cable shield to the 0V of the board however the LAN connector right next to it needed its cable schielding connected to the board 0V with a 1-10Meg//1-10nF cap. This because our lead designers where certain that when u use a long (100m in case of Ethernet) cable u don't want a low ohmic connection of a 100m long between lets say your HMI and a PLC somewhere in the factory because it could pick up lost of shit and 'lift' the 0V at your board locally. Therefore we connected it with a bleeder resistor for DC and a cap for the AC crap induced on the shielding.
The best way to do cap-grounded shielding, in my opinion, would be to build a local ground plane around the shield (using typical THT or SMT shielded connectors with pads electrically and mechanically mounting the shield itself), and couple the edges of that with chip caps, one on each side, and preferably one where the signal pair crosses the split as well. The caps therefore have no trace length (or an absolute minimum, only pad thermals and maybe a via or two), and using multiple in parallel ensures low inductance.
This kind of solid grounding pushes you into the 80dB+ range for shielding. It's good medicine, and works for anything sensitive in a noisy environment.
Needless to say, if you're putting in RF-grounding caps, a ferrite bead is superfluous. You can use a resistor if you like; 0-33 ohms is probably best (this will tend to terminate the shield's common mode and differential (i.e., versus power/signal lines) impedance working against the bypass caps).
T3sl4co1l, you seem to have one of the sharpest grasps on this shielding issue and I thank you for taking the time to post about it. If I am understanding you correctly, something like the below images would be your recommendation? You would prefer no resistors/feritte beads, but just caps (or did I misunderstand that part)? What cap values? Something like 1nF, or a mix of different values? Are two caps enough on a small board (1.5in x 1.5in, for example)? If not, how many more?
Hi,
I've been pondering this myself recently with a design. Depending on who you talk to in the industry you get different results.
I found this link that deals with the different scenarios of USB shield and signal ground interface.
https://forum.allaboutcircuits.com/threads/usb-device-cable-shield-connection-grounding-it-or-not.58811/
cheers
Full speed devices use a shielded cable which requires that the connector shell be tied to the ground plane.
It is important to note that a ground plane does not behave like an equipotential surface at high
frequencies. The location of the connector shell’s termination to the Gnd plane is critical. The connection
needs to be made to the quietest area of the ground plane to prevent noise from the ground plane from
coupling to the shield. As shown in Figure 8, the quietest location on the ground plane is on the opposite
edge of the board, far away from the crystal and other high frequency signals
Always ground shield directly to circuit ground plane. Anyone who says otherwise simply isn't smart. Yes.. you will find appnotes suggesting otherwise. They aren't smart.
Always ground shield directly to circuit ground plane. Anyone who says otherwise simply isn't smart. Yes.. you will find appnotes suggesting otherwise. They aren't smart.
The cable shield should be grounded (i.e. directly connected to ground) on the host side, as per the USB specifications. On the device side, it should not be connected to GND, except via a 1M resistor with a 100nF capacitor in parallel. Other values are acceptable. There is a large leeway on the values you can choose.
Who says otherwise is wrong. The ground and shield, despite being at the same potential, serve different purposes. The ground wiring is used for power and signal return. The shield is for shielding. If you tie shielding to ground at both ends of the cable, you will defeat the shielding purpose and external noise will couple to ground.
Hi,
The cable shield should be grounded (i.e. directly connected to ground) on the host side, as per the USB specifications. On the device side, it should not be connected to GND, except via a 1M resistor with a 100nF capacitor in parallel. Other values are acceptable. There is a large leeway on the values you can choose.
Who says otherwise is wrong. The ground and shield, despite being at the same potential, serve different purposes. The ground wiring is used for power and signal return. The shield is for shielding. If you tie shielding to ground at both ends of the cable, you will defeat the shielding purpose and external noise will couple to ground.
6.8 USB Grounding
The shield must be terminated to the connector plug for completed assemblies. The shield and chassis are
bonded together. The user selected grounding scheme for USB devices, and cables must be consistent with
accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.
The cable shield should be grounded (i.e. directly connected to ground) on the host side, as per the USB specifications. On the device side, it should not be connected to GND, except via a 1M resistor with a 100nF capacitor in parallel. Other values are acceptable. There is a large leeway on the values you can choose.
Who says otherwise is wrong. The ground and shield, despite being at the same potential, serve different purposes. The ground wiring is used for power and signal return. The shield is for shielding. If you tie shielding to ground at both ends of the cable, you will defeat the shielding purpose and external noise will couple to ground.
I'll just assume this is what the USB spec says and leave it at that. But I want to second the "different purposes" comment. I have long been under the impression that in circuits where there is a ground line separate from a shield (e.g. not coaxial cable shield) that the shielding should be tied to ground only at one end so that it (1) functions as a shield, and (2) to avoid ground loop problems where two parallel but separate wires are carrying the "same" current.
I don't really understand how it's advantageous to have the 1MΩ || 100nF capacitor at the other end (but I'll trust whoever the EE was that convinced a committee that it was a good idea). I can see it as a "backup" coupling, or to dissipate frequencies high enough to wiggle through the shield, but I'm way out of my depth here to do anything but wildly speculate.
Hi,
The cable shield should be grounded (i.e. directly connected to ground) on the host side, as per the USB specifications. On the device side, it should not be connected to GND, except via a 1M resistor with a 100nF capacitor in parallel. Other values are acceptable. There is a large leeway on the values you can choose.
Who says otherwise is wrong. The ground and shield, despite being at the same potential, serve different purposes. The ground wiring is used for power and signal return. The shield is for shielding. If you tie shielding to ground at both ends of the cable, you will defeat the shielding purpose and external noise will couple to ground.
So the spec isn't smart. Quite common. Why would one expect from people expertising in digital transmission and bloated software protocols beeing smart in EMC? Most probably this part of the spec was created in a rather (EMC wise) clean environment - and yes it works, so the spec will be. And then there are many monkeys that just implement it that way because every monkey has done so before and no monkey knows why and why they are beating any new monkey that does it differently.
And I have to admit: The spec way works - with devices like mice and keyboards, these often have no shield at all.
Where in the spec?
I only see this,
usb_20.pdfQuote6.8 USB Grounding
The shield must be terminated to the connector plug for completed assemblies. The shield and chassis are
bonded together. The user selected grounding scheme for USB devices, and cables must be consistent with
accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.
Which defaults to what the experts in this thread have been saying, nothing else.
FYI, the only possible situation you would EVER lift the shield, is if the shield first connects to a solid metal enclosure, and if the interior is not quite at the same potential because of other considerations that are better solved this way (i.e., by floating the USB by less than a volt). Example, a high current power supply that has some unavoidable ground side voltage drop, and therefore internal connections must float slightly otherwise they could draw huge ground-loop currents.
If you need to float more than a volt (even a volt is pushing it to begin with), simply isolate the USB channel, using a digital isolator device. ADI makes all-in-one chips for this, or you can pair a USB-UART interface with a conventional isolator for a somewhat simpler (probably about same cost) solution. With the isolated circuitry floating, the shield can be tied directly to it without problems, maximizing signal quality.
Tim
Short answer: no. The shield tying to ground on the host side has a logical explanation, and I presented it. This is not just some cargo cult. But fell free to have your opinion on this matter. I take it as it is: just an opinion.
That is correct. Although, due to capacitive coupling in the switched-mode PSUs that computers normally use, that difference can go as high as 10's of volts AC, if unchecked (i. e., ground not tied to the chassis, at all).
Nothing particularly specific to USB, but there is IMHO nothing particularly specific to USB apart from the fact it is not properly balanced and was designed with scant attention to making it easy to filter.
Always ground shield directly to circuit ground plane. Anyone who says otherwise simply isn't smart. Yes.. you will find appnotes suggesting otherwise. They aren't smart. Not having the shield tied to ground on the far side, injects exactly the common mode voltage into the signal lines.
Short answer: no. The shield tying to ground on the host side has a logical explanation, and I presented it. This is not just some cargo cult. But fell free to have your opinion on this matter. I take it as it is: just an opinion.
Not having the shield tied to ground on the far side, injects exactly the common mode voltage into the signal lines.
With the bypass cap, that voltage is shunted at AC, but through the impedance of that lone capacitor, which is considerable in the frequency range where ESD and EFT live (10-300MHz).
...
...QuoteThat is correct. Although, due to capacitive coupling in the switched-mode PSUs that computers normally use, that difference can go as high as 10's of volts AC, if unchecked (i. e., ground not tied to the chassis, at all).
10s of volts AC, in a grounded circuit?? I don't know what part of my message you were replying to.
FYI, metal-enclosure appliances are internally grounded as well, where possible. For example, computer PSU outputs are grounded to their chassis. The motherboard isn't usually grounded to its mounting screws, but its connectors (and expansion cards) are.
Audio equipment often isn't, to avoid ground loop, but not completely so. A typical solution is a bleeder resistor or TVS, to shunt fault currents to ground, while allowing some compliance with input ground loop voltages.
Tim