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Some common-mode chokes, including this one, are rated in ohms instead of in henries. Why? Isn't the impedance injected by the common-mode choke entirely frequency dependent? Is there an assumed frequency at which the specified ohm-age is calculated?

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  • \$\begingroup\$ Full data sheet here: datasheet.octopart.com/… \$\endgroup\$ – The Photon Jun 15 '15 at 18:07
  • \$\begingroup\$ Shows the spec is for Z @ 100 MHz. \$\endgroup\$ – The Photon Jun 15 '15 at 18:08
  • \$\begingroup\$ Ah, so the answer is that I'm not looking at the freaking data sheet, I'm looking at the part number guide. Bah. Thanks. \$\endgroup\$ – Stephen Collings Jun 15 '15 at 18:09
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It is measured in ohms because it doesn't behave like an inductor. It's inductive at low frequencies, lossy/resistive in between, and capacitive at very high frequencies.

The resistance is usually measured at 100MHz, but check the datasheet to be sure.

enter image description here

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  • \$\begingroup\$ If you're going to use a ferrite CMC, look where the \$X_L\$ and R plots cross. That is the crossover point, and needs to be lower than the frequencies you're trying to suppress to be of any use. \$\endgroup\$ – Matt Young Jun 15 '15 at 18:14
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    \$\begingroup\$ @MattYoung Its still useful impedance below the crossover, just mostly inductive (and thus not damping ringing much). \$\endgroup\$ – Spehro Pefhany Jun 15 '15 at 18:28
  • \$\begingroup\$ I think perhaps I could use some clarification. I was looking at this specifically for its property of adding inductance to differential currents in the <5 MHz range. It sounds like it will still do that. It's just not what perhaps most people would use it for. Am I understanding correctly? \$\endgroup\$ – Stephen Collings Jun 15 '15 at 18:36
  • \$\begingroup\$ You get < 50 ohms at <5MHz if that's okay for you and the rated current is okay, then it shouldn't be a problem. \$\endgroup\$ – Spehro Pefhany Jun 15 '15 at 18:54
  • \$\begingroup\$ @SpehroPefhany Useful maybe, but more often than not adding the wrong ferrite will make things worse. \$\endgroup\$ – Matt Young Jun 16 '15 at 13:48
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That particular part there is a ferrite bead, not an inductor, so it has no inductance.

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  • \$\begingroup\$ Well to be fair I guess everything has some parasitic inductance but the point being a ferrite is a different animal than an inductor \$\endgroup\$ – Some Hardware Guy Jun 15 '15 at 18:08
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    \$\begingroup\$ I'm not sure if thats entirely corrrect. I think its just a different kind of inductor that has slightly different properties at certain frequencies. \$\endgroup\$ – efox29 Jun 15 '15 at 18:18
  • \$\begingroup\$ Even a single, uncoiled wire has inductance. We don't usually use it for such, but it certainly can have an effect on circuits sensitive to inductance. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 15 '15 at 18:22
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    \$\begingroup\$ You can get into real trouble assuming ferrite beads have no inductance (don't ask how I know). The one he's using has tens of uH inductance at frequencies below the XL-R crossover. That inductance can be quite high Q. \$\endgroup\$ – Spehro Pefhany Jun 15 '15 at 18:25

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