A ferrite bead between the signal GND and the enclosure would be quite dubious as it has the potential to set up a resonant network. Steve. On 8/29/2011 6:55 PM, Xu Shuai wrote: > 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.†>>> 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.†>>> 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.†>>> 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