If I use ferrite with capacitor as pi network. What happen when I series or parallel the ferrite bead? Which one reject emi better? I known the ferrite bead is not inductance. It act like resistor at high frequency why dissipate energy as heat. But think about series and parallel really confuse me.
If you parallel two the same then the impedance (complex number) will be half, and if you put two the same in series then it will double. The DC current capacity should be higher with two in parallel (ideally it would be doubled, but that may not be a safe assumption in practice).
Ferrite beads are inductive at lower frequencies, and capacitive at very high frequencies. It’s possible that using a combination of two different types might be preferable in some particular situation. I can’t recall seeing that used in practice.
Anyway, in a pi circuit (2 caps to ground and one series ferrite bead), two beads in series will increase the attenuation.