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Ferrite bead

A part that passes DC but blocks high-frequency noise, used with capacitors to give a sensitive chip (analog, radio) a quieter supply.

What it does#

A ferrite bead is rated in ohms at 100 MHz (120 Ω here). At DC it is a few tenths of an ohm, so supply current passes; at high frequencies it turns noise into heat. Followed by capacitors to ground it forms a low-pass filter between a noisy rail and a quiet one.

Example circuit: 3V3 → FB1 → 3V3A, with 10 µF + 100 nF after the bead for the quiet side (an ADC reference, a sensor, an audio chip).
3V3 → FB1 → 3V3A, with 10 µF + 100 nF after the bead for the quiet side (an ADC reference, a sensor, an audio chip). Open the ferrite_filter example from this page in PCB Canvas: “Insert into my schematic”.

Choosing#

  • Rated current above the load (BLM18PG121SN1D: 2 A).
  • DC resistance low enough that the drop does not matter.

Pitfalls#

  • Not a cure-all. Beads can resonate with the capacitors and make noise worse at some frequency. Add one where a datasheet asks for it, not everywhere.
  • No bead in a buck converter’s power path unless the datasheet says so.

Footprint#

0603 (C14709, basic part).