# Why are some common-mode chokes rated in ohms instead of henries?

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?

• Full data sheet here: datasheet.octopart.com/… – The Photon Jun 15 '15 at 18:07
• Shows the spec is for Z @ 100 MHz. – The Photon Jun 15 '15 at 18:08
• 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. – Stephen Collings Jun 15 '15 at 18:09

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.

• 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. – Matt Young Jun 15 '15 at 18:14
• @MattYoung Its still useful impedance below the crossover, just mostly inductive (and thus not damping ringing much). – Spehro Pefhany Jun 15 '15 at 18:28
• 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? – Stephen Collings Jun 15 '15 at 18:36
• 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. – Spehro Pefhany Jun 15 '15 at 18:54
• @SpehroPefhany Useful maybe, but more often than not adding the wrong ferrite will make things worse. – Matt Young Jun 16 '15 at 13:48

That particular part there is a ferrite bead, not an inductor, so it has no inductance.

• Well to be fair I guess everything has some parasitic inductance but the point being a ferrite is a different animal than an inductor – Some Hardware Guy Jun 15 '15 at 18:08
• 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. – efox29 Jun 15 '15 at 18:18
• 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. – Ignacio Vazquez-Abrams Jun 15 '15 at 18:22
• 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. – Spehro Pefhany Jun 15 '15 at 18:25