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I have to implement a LC filter for a FT232H and its datas sheet (here) gives an example circuit (Fig. 6.1 VPLL- & VPHY-circuit). Strangely though a core-inductor is stated to be a 600R/0.5A.!

600R/0.5A Inductor Image

I have never come across such a way of writing inductance, or is that inductance at all? When I searched for the values, I found a ferrite-bead (here), but is that what I'm supposed to use as an inductor? It's just some ferrite... I'm confused...

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1 Answer

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It's an EMI suppression chip ferrite bead. Their inductance is often expressed as a resistance value at a certain (high) frequency. The 600\$\Omega\$ mentioned may be at 100MHz, for instance. This frequency should have been mentioned in the schematic. Let's say it's specified at 100MHz, then since

\$ |Z| = 2 \pi f L\$

L = 1\$\mu\$H at 100MHz. Why they don't simply give the 1\$\mu\$H value? That's because the inductance is frequency dependent, as illustrated in the following graph from Murata's BLM18 series chip ferrite bead datasheet:

enter image description here

edit
It doesn't look like a coil (more like a resistor) because the coil is actually inside the ferrite:

enter image description here

The picture below is an X-ray image of the coil inside the ferrite. This gives an idea of how thin the coil is, which explains the maximum current (in our case 0.5A).

enter image description here

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Ok, so does this mean it actually is a ferrite and not an inductor as I thought? – Max Z. Aug 26 '11 at 6:27
Thanks for that :) – Max Z. Aug 26 '11 at 6:43
@Max Z - An "EMI suppression chip ferrite bead" is an inductor. Steven is saying that such inductors are specified in terms of their impedance at a given frequency and not their inductance. Impedance is a useful indicator of how well they block a signal in this sort of role. An EMI suppressor has inductance plus some capacitance and the ferrite may be made purposefully lossy for RF to further damp the EMI signals. – Russell McMahon Aug 26 '11 at 10:39
I've deleted my answer as I agree that Steven is (probably :-) ) correct and so my answer was misleading. Note that digits 8 to 10 in the part numbers above are the impedance at 100 MHz eg ...601 = 600 ohms, ...102... = 1000 ohms etc. – Russell McMahon Aug 26 '11 at 11:18
As Russell states, the ferrite bead is made purposefully lossy, and it has a very low Q compared to normal inductor, which would normally be regarded as bad. They dissipate real power, so can maybe be thought of as a frequency dependent resistor. Useful for suppressing ringing on data lines, and other places where a pure inductance would not be as useful. – Oli Glaser Aug 26 '11 at 11:21

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