# Data sheet inductance (R/A ?)

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.!

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...

## 1 Answer

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:

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

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).

• 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
• @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