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I am designing a circuit with USB 3.0, and looking at the reference design from Cypress in the link below.

SuperSpeed Explorer Kit User Guide (CYUSB3KIT-003)

I see on page 47., that the USB connector shield is connected to system ground using a LC circuit, consisting of a 100nF capacitor and a 220ohm Ferritbead.

EMIFILr (Inductor type) Chip Ferrite Bead BLM21P Series (0805 Size)

At DC the ferritbead allows current to flow, and at 100MHz it has a resistance of 220ohm. At only 1MHz the capacitor has an impedance of around 1.6ohm, which is almost a short.

I cannot understand the purpose of the ferritbead ?

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  • \$\begingroup\$ That first download is a zip file and nobody sensible is going to download it. \$\endgroup\$
    – Andy aka
    Nov 19, 2015 at 8:26
  • \$\begingroup\$ You are probably right, but that's how Cypress decided to save there schematic. I found a pdf file with the circuit. \$\endgroup\$
    – JakobJ
    Nov 19, 2015 at 8:35

1 Answer 1

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A 100nF capacitor might have this type of frequency response: -

enter image description here

As you mentioned, at 1 MHz the impedance is 1.6 ohms but if frequency rises you will reach a point where the capacitor self resonates and becomes inductive and, maybe the ferrite bead is intended to always restrain the net impedance to 200 ohm resistive no matter what frequency of signal might exist between shield and chip?

At much lower frequencies than 1 MHz the ferrite will behave like an inductor with fairly low losses and this may also be a feature desired by the designer.

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