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Trial and error. Ferrite beads start to word at the frequencies above 1MHz. Find a family of beads that have the same package (like MMZ series tha comed in 0603 and 0805), add the pads to your board and see what works for you.
Beads are selected dependent on purpose in design, working or suppress frequency , needed inductance and max through current . Yet , geometric sizes does matter as well (sometimes).
i went trough this b4, so thrust me on this..get a spectrum analyzer and find the frequency range of ur noise profile..then only select the ferrites..and like artem said, selection are based on design, supression and max thru current..in some hi speed applications material selection is is crucial..
Look-up the impedance curve on datasheet and then use the bead with highest impedance value for the frequency band you want to suppress the noise.
Also check the DC resistance. only critical for high current line.
To chose the proper bead, you should consider the following:
1. What is the range of unwanted frequencies?
2. What is the source of the EMI?
3. How much attenuation is required?
4. What are the environmental and electrical conditions for the circuit (temperature DC voltage, DC bias currents, maximum operating currents, field strengths, etc...)?
5. What is the maximum allowable profile and board real estate for using this component?
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