Author Topic: USB Shield Ground Connection  (Read 31013 times)

0 Members and 1 Guest are viewing this topic.

Offline rd199

  • Newbie
  • Posts: 1
USB Shield Ground Connection
« on: January 29, 2015, 03:37:30 pm »
I've been evaluating options for connecting the USB ground pin and the shield of the connector and have read through various implementations, but haven't been able to come to a conclusion when dealing with ungrounded devices.

The best summary of implementations I've found is here:
http://forum.allaboutcircuits.com/threads/usb-device-cable-shield-connection-grounding-it-or-not.58811/

I'm considering the RC option (1Meg, 4.7nF) as it would provide some decoupling ground plane noise onto an unterminated USB cable (via inductance in the capacitor at higher frequencies), which could help with EMI and provides low frequency filtering, but have some reservations.  First, being an ungrounded device, when ESD is applied, the device could charge up to thousands of volts relative to earth.  The bleeder resistor would eventually equalize the potential between GND and Shield and when the device eventually was plugged into the PC, the shield would be tied to earth leaving thousands of volts across the capacitor until discharged through the resistor, which likely would blow the capacitor/resistor unless rated for the 4kV+ voltage that could be built up.  Am I missing something here?

Also, looking at connecting a TVS array to clamp, something like the Semtech - RClamp0503F, but with the RC implementation, where would you tie the 'GND' node of the array?  Ideally you would tie it to the shield, so an ESD event would discharge to the shield, but since the GND and the shield could have different potentials until the capacitor is fully bleed when connected to the PC, that wouldn't do well for the clamping diodes for D+ and D-, so perhaps you would connect to the internal ungrounded device GND net.  However, doing this, an ESD event through the device would now have to travel through the RC network to get back to earth, which isn't desired.

I did find another implementation on this forum using varistors here:
https://www.eevblog.com/forum/projects/tying-usb-shield-to-ground-%28vbus-0v%29-always-a-complete-fail/

This at least gives a fast discharge path of the capacitor when plugged into the PC, but doesn't really solve the problem of where to put the 'GND' not on a TVS array.

The only real drawback to a direct shield to GND connection is that you wouldn't get any filtering of ground plane noise on cables, which could lead to EMI issues or potential ground loop issues, but I would think because USB cables are so short, ground loops aren't really much of an issue.

Would appreciate feedback on my analysis of the RC implementation, varistor implementation, and direct short of shield to GND.  I would like to get to the bottom of this and create the best situation for ungrounded devices that can occasionally be connected to earth via connection to the PC in terms of ESD and EMI.

Thanks.
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #1 on: January 29, 2015, 04:40:20 pm »
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
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: ZL1CVD

Offline jhaislip

  • Newbie
  • Posts: 1
Re: USB Shield Ground Connection
« Reply #2 on: July 07, 2015, 12:07:47 am »
Thanks T3sl4co1!  This is one of the more straightforward answers I've seen thus.  I wanted to revive the thread because I'm currently working on a self-powered USB device which will be used in a very noisy environment (switching motors on/off nearby) and have many input wires coming from limit/position sensors.  It may be installed in both metal and plastic enclosures.

In my first design,  I made the mistake of not connecting the USB shield to anything. My plan for the next revision is to connect it directly to the signal ground plane.  However,  my PCB has no other direct connection to earth ground.  My understanding is that when the USB cable is connected,  the shield is effectively tying the signal ground to the earth ground via the computer chassis.  Is this correct?

Should I also ensure that my PCB signal ground is directly connected to earth ground via its own chassis and/or the mains grounding wire?

The PCB can also operate wirelessly without the USB cable connected.  In this configuration, is directly connecting the signal ground to earth ground even more important?

The first revision of the board was very susceptible to EMI and the USB port on the computer would frequently shutdown.  In the next revision,  I have added ESD suppression to the D+ and D- lines as well as a ferrite bead between the USB shield and the signal ground.  My hope is that these will help reduce the effect of EMI,  but I also want to make sure I handle the shield grounding correctly.

I've attached a screen shot of the current layout.  This includes a single chassis grounding pad for the mounting screw which is closest to the USB input.  I'll also need to determine if I should connect the chassis to ground via the other mounting screws.

Thanks!

Josh
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #3 on: July 07, 2015, 02:01:35 am »
Yes, unless you use an isolator, the USB is necessarily grounding you (shield or not).  If this is a problem because of other connections, you'll have to figure that out between the two.

The upside to a solidly grounded shield is, your USB will actually work, even when exposed to EMI (and won't radiate itself).  The downside is, all that EMI is conducted over your board's ground, and will find its way out any cables attached to it.  So, you need to follow similar process for those as well.  If they are low bandwidth, you can filter RFI and use TVSs to protect against ESD and EFT type signals, and avoid the need for a shield.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: USB Shield Ground Connection
« Reply #4 on: July 07, 2015, 02:18:05 am »
I use 0603 ferrite bead from USB connector case to ground.
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #5 on: July 07, 2015, 02:32:59 am »
I use 0603 ferrite bead from USB connector case to ground.

Has your product been subjected to EMC testing yet?

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: USB Shield Ground Connection
« Reply #6 on: July 07, 2015, 02:36:02 am »
Yes.
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #7 on: July 07, 2015, 02:37:38 am »
Hmm, how big is it?
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: USB Shield Ground Connection
« Reply #8 on: July 07, 2015, 02:42:28 am »
How big is what - EMC testing? I dunno, client keeps coming back so I guess it was OK.
 

Offline NANDBlog

  • Super Contributor
  • ***
  • Posts: 4275
  • Country: nl
Re: USB Shield Ground Connection
« Reply #9 on: July 07, 2015, 06:57:17 am »
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.
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #10 on: July 07, 2015, 08:27:34 am »
How big is what - EMC testing? I dunno, client keeps coming back so I guess it was OK.

The device, like is it a little dongle, or does it have a bunch of cables coming off of it? (And was it tested in that configuration?)

If it's a little box with no other connections, you can get away with a lot of sins, because the device is reasonably well isolated from its surroundings.  It gets very much more difficult to do things like this (broken shields) when you have more than one cable on a unit, or anything that gives a large capacitive load or other low impedance for RF to work against.

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.

Anywhere you've got cables attached, they want to do conducted tests.  Doesn't matter if it's USB or RS-485 or 16 bit analog channels.  You have to deal with that passing through your system, one way or another.  And it's going to be traveling down all cables and boxes, because you can't effectively filter common mode noise (the impedance is too high compared to ferrite beads).  You can only shunt it to ground, but that's usually a luxury.

If you break the shield, you still have a cable attached, so it's bringing in conducted energy.  Except now your USB connection gets shat on, because it's lost common mode range (only a volt or two!).  So you still have the noise problem, except you've got more emissions (from the USB, disturbing your own precision measurement) and dropped USB communications.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline John_ITIC

  • Frequent Contributor
  • **
  • Posts: 422
  • Country: us
  • Be the signal
Re: USB Shield Ground Connection
« Reply #11 on: July 07, 2015, 12:34:30 pm »
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, 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?
 

Offline John_ITIC

  • Frequent Contributor
  • **
  • Posts: 422
  • Country: us
  • Be the signal
Re: USB Shield Ground Connection
« Reply #12 on: July 07, 2015, 12:35:29 pm »
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?
 

Offline free_electron

  • Super Contributor
  • ***
  • Posts: 7103
  • Country: us
    • SiliconValleyGarage
Re: USB Shield Ground Connection
« Reply #13 on: July 07, 2015, 01:49:50 pm »
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. ;)
Quote
ehh, 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.
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 
The following users thanked this post: xani, kony

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #14 on: July 07, 2015, 03:23:25 pm »
(Bad formatting??)

A capacitor between shield and ground is a good step (I should be more specific and say: "the shield must be RF grounded"), but -- dubious in practice.  The reason is, any length of poor shielding introduces transients to the pair, and potentially disrupts communication.

A related example is using USB on a remote connector, on a cable going to a header: the cable, connector and harness might be fully shielded, but inevitably, there is that 3cm length where the shield conductor enters a pin in the header, goes through the header, and only then reaches the board.  (Substitute this length with a ground trace and chip component body, for the grounded-by-cap case.)  At the magnitudes of voltage, and rates of change, present in EFT and ESD, you simply can't have more than a few mm of unshielded length in your circuit.  Yes, that short link of wire or trace can easily drop 10, 20, 50V or more, during one of these transients!  That's not amazing, that's just physics and linear networks.

To put it another way, to have a valid USB signal traversing that cable while it's subject to 1kV EFT pulses, it needs at least 60dB of attenuation between shield and signal!  Some of that you might get for free -- you might save 3dB by geometry (the incident signal divides between shield, ground, and other cables), and another 3dB somewhere else (if EFT is being applied to a distant cable, it will be more rounded by the time that energy comes around).  A 3cm wire link for the shield might do 30, even 40dB.  A ground trace might even do 50dB.  Oh so close!  But still no cigar.  So it's a tough job, and demands best practices.

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).



The other thing about shields: they have impedance to the signals, whether you like it or not.  In a typical USB cable, the four wires are simply wires inside a shield (the D+/D- usually being twisted, and +5/GND being twisted or not).  Therefore, there is common mode impedance between the D-pair and power/ground, and to shield.  In general, these will be in the 50-100 ohm range, hardly insignificant!

This is why:

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.

And so, if you're having to deal with solid kilovolts of spikes, your only recourse is to shunt that noise around your precious signals.  You must keep a low impedance shield, to ground, so that the energy is bypassed around your circuit, along its ground.

Note when I say "shunted around the circuit": it can be done explicitly with an enclosure shield.  This looks like a desktop computer, for example.  The case is solid metal, and all the shielded connectors are bonded to it (with EMI fingers).  All noise is shunted around this path, all noise currents due to self-capacitance, due to current riding along to other cables, whatever.  Everything inside is oblivious to all this happening, because it's a Faraday cage.  The continuous, low impedance shield is an absolute requirement to achieve this.

If you don't have a metallic enclosure, you're a bit more pressed for options.  A circuit board with solid ground plane is as good a substitute as you can get.  The shield must be quickly bonded to this ground.  Once the noise is "on" the ground, it's also on all the signals -- this is good -- it acts as a Faraday cage as well, so that the entire circuit's potential floats on this level.  All the circuit knows is the differences within, which because they are referenced to this (noisy, only in an absolute sense) ground, means nothing to it!

When you combine these ideas, you can meet the question: does it matter if you ground the shield, when it's already inside the box?  Probably not -- but -- you can never be too safe, and doing so will only improve performance*.

(*There are no guarantees, when it comes to EMC.  Obvious exceptions include badly made connectors that aren't, in fact, fully shielded, so endogenous currents inside the system cause ground loops within the cable -- not much, only a few volts, but at higher current than otherwise, maybe just enough to trash some logic thresholds.  Maybe there's a poorly grounded power supply, injecting common mode switching noise into the motherboard and peripherals.  Lots of possibilities -- but these, at least, are more of a secondary issue, and much more likely to be solvable with ferrite beads (to reduce RF ground loop currents), because the voltage is already small.)



Also, there's always the caveat: if you can tolerate high BER, you can do some truly nasty things.  Like leave off filtering, shielding and maybe ESD altogether.

The last example I tested was a fairly standard USB host on a Linux board.  I don't think the driver did any auto-retry whatsoever, it just dropped as soon as it found a malformed packet.  Once disrupted, it would sometimes recover the connection automatically, always after a soft reset.  I have no idea if that's something under user or kernel control, for such a platform.

If you're doing something as good as TCP/IP, with self-resetting or retrying interfaces, or simply don't care and it's more like UDP, you might not care for all but the most intense RF susceptibility tests.

Susceptibility consists of bombarding the cables (conducted) or system (radiated) with AM modulated CW, over a frequency sweep, inducing a known V on the cables / V/m in the air.  So, if noise on the order of a few volts is invading inputs, it would tend to just completely stomp out continuous data flow, during the AM peaks or continuously.

Basic FCC Part 15 or IEC 61000-4-3 tests are around 3V/m, so if you figure a < 1m cable is acting as an antenna and receiving about 3V, it's not too painful to have that divide down accidentally by circuit layout or filtering, and still get a stable threshold.

This is another reason why you can sometimes get away with nasty things.  But apply the same rules to an automotive, aero or military application (with 30V/m or more), and you'll find yourself stumped, that your sage advice falls flat on its face.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: ZL1CVD

Offline DutchGert

  • Frequent Contributor
  • **
  • Posts: 256
  • Country: nl
Re: USB Shield Ground Connection
« Reply #15 on: July 07, 2015, 05:11:08 pm »
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.
Until this day I am still not sure what the right approach was and if there thinking was flawed.

My personal opinion is to use a frame ground/shield on your pcb that connects all shields of all cables coming in and out of the board together and then connect this frame ground/shield at one point to your board level 0V/GND. If you do this with either a 1-10Meg//1-10nF or a Bead or a 0R resistor depends on your application entirely.
 
« Last Edit: July 07, 2015, 05:24:37 pm by DutchGert »
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #16 on: July 08, 2015, 11:25:38 am »
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.

Yeah -- the Ethernet standard requires transformer coupling, usually with some common mode filtering to take the edge off, and a 1nF capacitor, from center tap to ground, to handle ESD.  Note that an 8kV, 150pF ESD source drops down to about 1.5kV into 1nF, exactly the isolation those transformers are rated for.

Ethernet still needs a shielded housing for the connector, but this is more for providing a strike path for ESD.  Most cables (unshielded twisted pair = UTP) don't even connect to it!  Proper STP should be grounded to it, however.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline acker9

  • Newbie
  • Posts: 1
  • Country: us
Re: USB Shield Ground Connection
« Reply #17 on: August 08, 2015, 10:15:09 am »
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?

(I understand the 4 usb pins could use a TVS diode array as well, but this question is about the shield-to-ground circuit, not the TVS diode array protection, so I left it out to focus specifically on the shield circuit.)

In the below board image, the top pour is the pour for the USB connector shield, and the bottom pour is the pour for GND (USB A pin 4).  For this example, assume a bus-powered USB device connected to a host.   The device board has a plastic enclosure.



« Last Edit: August 08, 2015, 10:22:27 am by acker9 »
 

Offline wraper

  • Supporter
  • ****
  • Posts: 9742
  • Country: lv
Re: USB Shield Ground Connection
« Reply #18 on: August 08, 2015, 10:20:01 am »
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?
No, he said to connect directly. No any parts in the middle. He mentioned cap as the option if the device have metal case which is grounded, and USB connector is grounded to the case.
« Last Edit: August 08, 2015, 11:04:17 am by wraper »
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #19 on: August 08, 2015, 01:14:33 pm »
Yeah, there's not much point in separating grounds, since they're tied internally (at one end or the other) anyway. :)

Still, if you wanted to keep them (galvanically) isolated, you want to keep all four wires (VCC D+/-, and GND) referenced to the GND wire, and you want that bypassed (at RF) to the shield.  So in that case, you would have to do something like what you've shown.  To that extent, your reading is correct. :)

Beware about small capacitor values: you must always be mindful of the RLC circuit formed, but especially for small values where the resonance will be of a comparable impedance and frequency to the signals you're trying to protect.  The effect being, if you have a strong resonance there, your signals corrupt themselves due to the shield-ground voltage developed.

So you're better off using as much capacitance as is convenient (like 0.1uF, or direct shorts), and managing that resonance by placing lossy capacitors in parallel with the main bypass capacitors (>= 2.5 times the capacitance, and resistance/ESR equal to sqrt(Lstray/C)).

Worked example:

Typically, the inductance of the ground wire to shield, inside a 2m USB cable, will be on the order of 2uH.  That resonates with 0.2uF (2 x 0.1 in parallel) at 250kHz (which means "humps" on the order of tens of USB symbols/bits, at Full Speed), and has a resonant impedance of 3.2 ohms.

That the impedance is less than the characteristic impedance of the cable (50-100 ohms, depending on how many wires together you're talking about), means the 3V signals coming through the cable (give or take pulse skew, because it is balanced, and the remainder of that balance is what's actually the problem) won't be able to drive a full 3V across it, but more like 0.3V or less.  Which means less error for your receivers to worry about.

The damping should be 0.5uF or more, in series with about 3.2 ohms.  Only one damping capacitor is needed, because it doesn't do much at RF (it's got a minimum impedance, at high frequencies, of only 3 ohms, after all!).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline technix

  • Super Contributor
  • ***
  • Posts: 3088
  • Country: cn
  • From Shanghai With Love
    • My Untitled Blog
Re: USB Shield Ground Connection
« Reply #20 on: August 08, 2015, 04:37:20 pm »
Both M180 and M180v3 had USB shield shorted to the ground right at the connector.
 

Offline snoopy

  • Frequent Contributor
  • **
  • Posts: 612
  • Country: au
    • Analog Precision
Re: USB Shield Ground Connection
« Reply #21 on: September 24, 2018, 09:02:29 am »
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

 

Offline wraper

  • Supporter
  • ****
  • Posts: 9742
  • Country: lv
Re: USB Shield Ground Connection
« Reply #22 on: September 24, 2018, 09:52:20 am »
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
If you are talking about top post, author fails to understand at least some documents he listed. On a quick glace one from TI. And further posts are yet again various opinions without any conclusion. But the reality is, when you don't connect shield directly to GND of the PCB, in that case IT MUST BE DIRECTLY CONNECTED TO GROUNDED METAL ENCLOSURE. If you don't have metal enclosure, there is no question that shield must be directly connected to GND on the PCB.
« Last Edit: September 24, 2018, 09:59:13 am by wraper »
 

Offline snoopy

  • Frequent Contributor
  • **
  • Posts: 612
  • Country: au
    • Analog Precision
Re: USB Shield Ground Connection
« Reply #23 on: September 24, 2018, 10:29:39 am »
Hi wraper, in my situation I have a USB and HDMI connector sharing the same board. Previously I just had the USB connector only and connected the USB shield directly to the chassis with the USB GND connected to the shield via 1Meg in parallel with 10 nF cap.

Now because I am sharing the two connectors on the same board what's the best way to deal with the shields in both cases ? I would assume the HDMI shield and HDMI GND gets connected together with a direct path to the case ? So that leaves me with a direct connection between USB shield and USB GND ??

According to the TI document (figure 8) this is the accepted practice.

Quote
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

cheers
david
« Last Edit: September 24, 2018, 10:33:55 am by snoopy »
 

Online T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13005
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: USB Shield Ground Connection
« Reply #24 on: September 24, 2018, 04:48:25 pm »
A single capacitor isn't enough.

If you must have galvanic isolation on the shield, use multiple caps, at least two, distributed around the connector (preferably four, especially for SMT connectors where you can make it work better).

Every single nanohenry of stray inductance counts against you.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf