Electronic – 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.!

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…

Best 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:

enter image description here

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