Electronics & Software – Tips & Guide


Can I use ferrite beads as SMD resistors as they have a given impedance value?

Can I use ferrite beads as SMD resistors as they have a given impedance value?

not like resistors, no.

you can certainly thread them onto tinned copper wire and use them as inductors.

No, it will not work. impedance is not the same thing as resistance. impedance is for AC circuits and is not the same as resistance. the impedance in a DC circuit is nonsense.

In general, adding an impedance element to ferrite balls is a good thing, but the whole concept of protecting or reducing emissions has a lot of variables. note that your pearl is not really a pearl but rather a lump sum. In addition, the impedance varies over the frequency range of 30 MHz to 5 GHz, so do not expect 2,000 ohms everywhere. and also note that the DC current is limited to 50 mA. In addition, as you probably put it on a data path, you must be sure to understand what impedance will do on your data as well as all those unwanted signals.

All of this being said, you can make your additional component useless due to trace routing and placement errors of other components. The emi control is less of a black art than it was, but its experience is hard to gain before you can confidently turn new card models that have a decent chance to comply with their first verification test. The dark tip is to evaluate and observe every inch of your card for coupling effects, minimize signal strength as much as possible and keep the signal paths short. and finally, try to develop a pre-compliance test capability to check your designs at an early stage.

no, the impedance value given for a ferrite bead corresponds to a frequency tested, often 100 MHz. Ferrite beads are similar to inductances in this way (where the impedance is proportional to the frequency, in general). the ferrite bead can be specified at 2200 ohms, however this impedance will exist only at a specific frequency. you can see it if you consult the sheet of the ferrite bead. The datasheet generally provides a graph showing the impedance of the ferrite bead as a function of frequency. This is important for determining the proper ferrite bead to use to attenuate noise in a given frequency range.

On another note, I also want to emphasize that impedance and resistance are not exactly the same thing (even if they are both expressed in ohms). the impedance includes the actual resistance in the circuit / component as well as the reactance. in general, the resistors have a resistance and the capacitors and inductors have a reactance. I say in general, because with sufficiently high frequencies, for example, the self-inductance of a capacitor will become the dominant factor of its impedance and its impedance will increase with the increase of the frequency. similarly with ferrite balls … if you look at the frequency response of a ferrite bead, it will have a peak impedance at a certain frequency, then the impedance will drop as you go higher in frequency. Indeed, in a true ferrite bead, the own capacitance will become the dominant factor of its impedance as the frequency increases. ultimately, all actual components inherently have resistance and reactance and can be modeled in the real world as an arrangement of resistors, capacitors, and inductors.

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