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_id=13448&family_id=607>. (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