USB device cable shield connection - grounding it or not?

Discussion in 'General Electronics Chat' started by colinb, Aug 30, 2011.

  1. colinb

    colinb Thread Starter Member

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    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).​

    (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).​

    - 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).​

    - “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).​

    (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.​

    (4) Do not connect cable shield to ground on the device at all.

    - As referenced in the AAC thread where to terminate usb cable shield?, Hardware Book says USB devices must
    not connect the shield to their own ground.
    Hardware Book. Universal Serial Bus: Shielding.

    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. I recently posted this question on si-list,
    and even there I got little in the way of answers.

    Colin
    endolith likes this.
  2. ian123

    ian123 Member

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    according to the specifications laid out by the usb-if
    A metal braid is required to enclose all the wires in the USB 3.0 cable. The braid is to be​
    terminated to the plug metal shells, as close to 360
    ° as possible, to contain EMI.

    apart from that there is no other requirement the same goes for usb 2.0

    it appears that this is a grey area but the sheild should be connected to both sides but not connected to ground in any way.

    will keep looking for you on the specs and see if i find something
  3. colinb

    colinb Thread Starter Member

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    Interesting. However, I think that the shield (cable braid and plug/socket shell) must be connected to ground on at least one end. If the shield were left floating, then other signals would be able to couple to it and use the shield as a nice long antenna to radiate EMI.
    endolith likes this.
  4. ian123

    ian123 Member

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    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.
    colinb likes this.
  5. colinb

    colinb Thread Starter Member

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    Thanks for posting that snippet. So the shield is connected the the chassis ground, but beyond that, handling of chassis ground is left up to the designer.

    That means most any of the suggestions in my prior cited sources are acceptable according to the USB specification; but how this shield/chassis connection is best handled as far as connecting to other grounds in the system is still a point of contention.
  6. ian123

    ian123 Member

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    according to the emi standards the connector requires a low impedance to ground
  7. ian123

    ian123 Member

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    The V​
    BUS and ground in a PC are subject to noise both from on-board sources and also from the switching
    power supply. Tests have shown that it is necessary to carefully decouple both V
    BUS and Gnd. This is done
    with ferrite beads. Separate ferrite beads may be used on each V
    BUS line to each downstream USB
    connector. Each ferrite bead on the V
    CC lines should be rated at 500 ma. Separate ferrites are useful, not
    only for EMI suppression, but also for their series DC resistance which limits the inrush current during a
    hot plug event. For a discussion on hot plugging and voltage droop see the
    USB Voltage Drop and Droop
    Measurements
    white paper. Ferrites should also be used on the Gnd lines to USB connectors.
    If bypassing V
    CC with ferrite beads still does not provide sufficient attenuation then small (1000 pF)
    capacitors can be connected on the connector side of V
    CC to the connector shell through very short traces.
    If this approach is used care must be taken to insure that the capacitors do not provide a path for ground
    plane noise to enter via the capacitors.

  8. colinb

    colinb Thread Starter Member

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    But what is ground when two pieces of equipment are involved? For mains-powered equipment, it must be earth ground, but what about portable battery-powered equipment? Obviously in USB there is the GND wire, which the USB Host supplies, so that could be the ground. What if the USB Device is self-powered and does not share ground with the USB Host? I think the answer is the USB requires a common ground from Host to Device and for the special situations where ground must be isolated between these pieces of equipment, special isolation circuitry must be used (optocouplers?).
  9. ian123

    ian123 Member

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    The shell should be provided with a low impedance connection to signal ground and, in cases where
    a metal chassis is present, a means of accepting a metal EMI gasket clip.

    thats all I have on the subject

    seems it need to be connected to ground to avoid emi​
  10. colinb

    colinb Thread Starter Member

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    ian123,
    Thanks for all the info you posted!
  11. ian123

    ian123 Member

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    ok there are two things we are talking about one is the sheild and one is the ground.

    the sheild needs to be jioned to the ground through a low impedance but the ground is supplied by the host.

    so therefor the way i see it is the sheild and the host ground will be joined through a low impedance on the host side but on the device side if they dont share a common ground you would need to isolate the grounds.
  12. ian123

    ian123 Member

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    my pleasure
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