120 ohms at 100 MHz — the noise floor it sets
The BLM15AG121SN1D delivers 120 ohms impedance at 100 MHz, which is the sweet spot for suppressing conducted EMI from digital clocks, switching regulators, and RF transceivers operating in or near the 100 MHz band. A single-line ferrite bead, it inserts one lossy element per channel — no shared attenuation between lines. Rated for 550 mA continuous current with a maximum dc resistance of 190 milliohms. At full rated current the voltage drop across the bead is about 105 mV; the self-heating from I²R loss stays within the 125 °C operating ceiling of the 0402 body.
0402 footprint — where it fits and what it limits
The 1.00 mm × 0.55 mm body (0402 / 1005 metric) is the smallest standard chip ferrite bead footprint. It mounts in the same pad pattern as a 0402 resistor or capacitor, so it drops into a common noise-filter land pattern without a dedicated footprint variant. The 0.55 mm seated height keeps it below low-profile components on the opposite side of the board.