Electronic – Are Common Mode Choke Coils needed on USB

chokeusb

I was looking over the schematic for the GumStix Palo 43 and noticed they used a common mode choke coil on the data lines coming in from USB.

I understand how this design can help remove noise coming in on the USB lines, but I wonder if it is actually something I should start doing on my designs. The datasheet for the FT232R has no mention of adding common mode choke coils, and I have used this chip before with out one.

So, would you recommend I change my USB design or keep it the way it is?

Best Answer

The USB signal is not entirely differential, so it's not a great idea. (The end-of-packet (EOP) signal is both pins pulled low, which, I believe, is why there's always noise at 1 kHz and harmonics in USB systems, since it's sending common-mode signals every 1 ms.)

  1. A common mode (CM) choke should be used to terminate the high speed USB bus if they are need to pass EMI testing. Place the CM choke as close as possible to the connector pins. See Section 5.1 for details.

Note: Common mode chokes degrade signal quality, thus they should only be used if EMI is a known problem.

Common mode chokes distort full speed and high-speed signal quality. The eye diagram above shows full speed signal quality distortion of the end of packet, but still within the specification. As the common mode impedance increases, this distortion will increase, so you should test the effects of the common mode choke on full speed and high-speed signal quality.

High Speed USB Platform Design Guidelines

Note: additional filtering may be achieved by winding the 4 wires through the ferrite bead an additional turn. As with the use of ferrite beads in signal paths, care should be taken to insure that the signaling meets rise and fall times, especially the EOP signaling. EOP signaling is single ended and may be strongly affected by a single bead, which acts as a common mode only filter.

Intel EMI Design Guidelines for USB Components

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