USB device cable shield connection – grounding it or not?

Colinb, an user from allaboutcircuits.com forum post this here:

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.

Source: http://forum.allaboutcircuits.com

Leave a Reply