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Below schematic shows a USB connection diagram.IN this you can see a common mode choke (DLW21HN900SQ2L)

The common mode choke impedance is given as 90 Ohms @ 100 MHz .The USB is working at 480Mbps.

I know that the characteristic impedance of USB is 90ohm.

My Question is we need to select a common mode choke with 90ohm impedance at 480Mbps. Please correct me if I am wrong.

enter image description here

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The differential mode impedance of these CMC is about 20 Ohm at 480 MHz. Ideally you want this to be lower because it will increase the connection differential impedance from 90 Ohm to 110 Ohm.

The common-mode impedance isn't related. It is coincidence. It could be even higher for more "EMI stopping prowess". But, generally, using CMC on USB 2.0 is a debated topic because the protocol is not purely differential. So too high common-mode impedance will impact signal integrity in USB 2.0.

If you want to use a CMC for existing EMI issues, I would make sure to use one that is rated for USB 2.0 by the manufacturer.

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  • \$\begingroup\$ My understanding about CM choke selection is that it's Differential mode attenuation will be 0dB or minimum at our frequency of interest.Common mode impedance will be maximum at it's operating frequency.Please correct me if I am wrong. \$\endgroup\$
    – Confused
    Commented Mar 4, 2023 at 8:08
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    \$\begingroup\$ @Hari ideally yes. But these CMCs fall rather short if your first criterion - in my opinion anyway. \$\endgroup\$
    – tobalt
    Commented Mar 4, 2023 at 8:09
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    \$\begingroup\$ @Hari and secondly, for USB2.0, you can't use very high common-mode impedance because the protocol is not purely differential. \$\endgroup\$
    – tobalt
    Commented Mar 4, 2023 at 8:15
  • \$\begingroup\$ May I know what you mean by 'protocol is not purely differential' and why \$\endgroup\$
    – Confused
    Commented Mar 4, 2023 at 8:55

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