[SI-LIST] 答复: [SI-LIST] Re: USB device cable shield connection
- From: Xu Shuai <xushuai@xxxxxxxxxxxxxxxxxx>
- To: 'steve weir' <weirsi@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 30 Aug 2011 09:55:29 +0800
I have not go through the references, but I guess the recommended series
ferrite bead is not between the shielding of the cable and the enclosure. It
should be put between the shielding of the cable and signal GND. The reason is
that if the connection between the signal GND and the enclosure is not so good
then maybe there is noise between them. If the cable is connected directly to
the signal GND the noise will be radiated and if there is one ferrite bead
between them the radiation will be reduced.
If there are enough grounding points between the signal GND and the enclosure
close to the interface the noise between the signal GND and enclosure will be
reduced and it is not necessary to put anything between them.
-----邮件原件-----
发件人: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] 代表
steve weir
发送时间: 2011年8月30日 9:26
收件人: si-list@xxxxxxxxxxxxx
主题: [SI-LIST] Re: USB device cable shield connection
The idea that the cable shield is an antenna for noise if it is
connected directly to a metallic enclosure, but is not if coupled
through a capacitor does not defies all reason. Faraday cages still
work. The absolute lowest EMI results from a solid 360 degree bond
between the shield and the metallic enclosure.
The series ferrite bead recommendation between case and shield is
perhaps the most absurd of all the recommendations. A ferrite bead
increases the impedance between the shield and case, increasing
radiation efficiency at frequencies where the ferrite bead impedance is
significant.
The only justification for a capacitor is to block low frequency and DC
loops. For the capacitor to be effective, its impedance must be low at
signal frequencies. 4.7nF is about 30 Ohms at 1MHz. The biggest problems
are going to be with even mode currents on the signal transitions at
much higher frequencies, so 4.7nF s/b more than enough for USB
applications. The capacitor adds series inductance that limits the
shield's effectiveness at high frequency. Low inductance components and
practices pay-off, even if you have to use more than one capacitor.
Steve.
On 8/29/2011 5:56 PM, earl albin wrote:
> Colin:
> I read the TI, Intel, and Cypress app notes. The Cypress app note is
> certainly insufficient as a guide. For one, the 4700pF cap as shown and as
> intended, RF mitigation, is incorrect in that it's really an LRC. Proposed
> network couples DC (1-MOhm) and depending on the cap, likely up to about
> 10-20 MHz AC between the shield and signal grounds. The 2-meter cable is not
> an efficient radiator at those frequencies. Again though this all depends on
> the cap and the layout. The ferrite bead accomplishes this effect sort of in
> the opposite direction
>
> I thought both TI and Intel were generally in agreement, perhaps not exactly
> on the values but on the approach. Intel did mention that this was what they
> found to work. Likely the first attempt didn't then they made
> adjustments/changes till they got it to pass.
>
> So at least you could start with either Intell or TI, better than guessing.
>
> On Mon, Aug 29, 2011 at 11:42 AM, Colin D Bennett<colin@xxxxxxxxxxx> wrote:
>
>> I have been trying to understand how to ideally handle the cable shield
>> on a USB device. (Full Speed USB, in this particular instance.)
>>
>> As seems to be the case with many signal integrity issues,
>> contradictory recommendations abound, each with its own unsupported
>> claims. Even authoritative-sounding sources such as Texas Instruments,
>> Intel, FTDI, and Cypress Semiconductor seem to disagree on the correct
>> way to handle the cable shield on USB devices.
>>
>> 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.
>>
>> Here are some of the options that have been recommended.
>> Note that (2)—series capacitor to pass high frequencies only—seems to
>> directly contradict (3)—series ferrite bead to block high frequencies
>> only.
>>
>> (1) Connect shield directly to signal ground.
>>
>> - “Full speed devices use a shielded cable which requires that the
>> connector shell be tied to the ground plane.â€
>> Intel. EMI Design Guidelines for USB Components. Sec 5.4 (p. 9).
>> <http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf>.
>>
>> (2) Connect shield to signal ground through a capacitor.
>> (Possibly with high-value parallel resistor approximately 1 Mohm.)
>>
>> - Connect shield to signal ground with 0.01 µF to 0.47 µF capacitor.
>> FTDI. Debugging FT232BM and FT245BM Designs. Section 3.2 (p. 11).
>> <http://www.ftdichip.com/Documents/AppNotes/AN232B-06_11.pdf>.
>>
>> - Cypress recommends a 1 Mohm resistor in parallel with a 4.7 nF
>> capacitor.
>> Steve Kolokowsky& Trevor Davis (Cypress Semiconductor).
>> Common USB Development Mistakes – You Don’t Have To Make Them All
>> Yourself! Figure 7 (p. 7).
>> <http://uk.farnell.com/images/en/ede/pdf/usb_dev_mistake.pdf>.
>>
>> - “Tying the shield directly to ground would create a direct path
>> from the ground plane to the shield, turning the USB cable into
>> an antenna. To limit the USB cable antenna effect, it is
>> recommended to connect the shield and ground through an RC
>> filter. Typically, R = 1MΩ and C = 4.7nF in Figure 3-5.â€
>> Atmel AVR1017: XMEGA - USB Hardware Design Recommendations.
>> Section 3.3.3 (p. 8).
>> <
>> http://atmel.com/dyn/resources/prod_documents/doc8388.pdf?doc_id448&family_id`7
>>> .
>> (3) Connect shield to signal ground through a ferrite bead.
>>
>> - “Place a ferrite in series with the cable shield pins near the
>> USB connector socket to keep EMI from getting onto the cable
>> shield.â€
>> Texas Instruments Application Report.
>> USB 2.0 Board Design and Layout Guidelines. Sec 2.2.4 (p. 3).
>> SPRAAR7 – December 2007.
>> <
>> http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=spraar7&fileType=pdf
>>> .
>>
>> Whether or not the device has a metal chassis, and the handling
>> of chassis ground and signal grounds, (as well as how the USB cable
>> ground is connected to either one) is certainly important as well, but
>> this isn't clearly discussed in most of the writings on USB cable shield
>> grounding.
>>
>> The device I'm developing is a bus-powered device which will likely be
>> in an unshielded plastic enclosure.
>>
>> Thanks in advance for any bits of wisdom on this topic full of
>> contradictory information.
>>
>> Regards,
>> Colin
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List technical documents are available at:
>> http://www.si-list.net
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
>>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
