Users should make sure that impedance of the ferrite bead is chosen to be about 1.8 kΩ between the 100 MHz and 1 GHz frequency range to reduce the emissions at the 180 MHz primary switching frequency and the 360 MHz secondary side rectifying frequency and harmonics on the ADuM5028 IC.
It is also important to note that the packaging of ferrite beads (specifically the physical dimensions of 0402 vs. 0603) can influence their impedance characteristics, particularly at higher frequencies. While the core material and its properties primarily determine the impedance, the physical size and shape (packaging) affect factors like parasitic capacitance and inductance, which in turn impact the overall impedance profile. Smaller packages like 0402 inherently have lower parasitic capacitance and inductance compared to larger packages like 0603. These parasitic elements can interact with the ferrite material's impedance, modifying the overall frequency response.
To select an appropriate ferrite bead, users must look at the impedance-frequency graphs of the selected ferrite bead's datasheet. Two such examples are given below: