Ferrite beads sit somewhere between an inductor and a resistor, and understanding which one they're pretending to be at a given frequency is the difference between killing EMI and creating a resonant disaster. A ferrite bead is a lossy magnetic core wrapped around a conductor. At DC and low frequencies, it's essentially a small inductor with milliohms of DCR. As frequency rises, the core material's complex permeability shifts from reactive (inductive, X_L) to resistive (R). Above the crossover frequency, the bead behaves like a real resistor that dissipates RF energy as heat instead of reflecting it back into the circuit.

The datasheet spec you care about is impedance at 100 MHz. A "600 Ω @ 100 MHz" bead has ~600 Ω of impedance at that frequency — but only a few ohms at 1 MHz and essentially zero at DC. Also critical: the DC current rating (typically 100 mA to 6 A). Exceed it and the ferrite saturates, collapsing the impedance to just the copper resistance.

The classic mistake: Using a ferrite bead on a power rail with a large bulk capacitor downstream. The bead's inductance (typically 0.1–1 µH at low frequency) plus the decoupling capacitance (say 10 µF) forms an LC resonant tank. At resonance — often 100 kHz to a few MHz — the impedance peaks and can amplify noise by 10–20 dB instead of attenuating it. The bead only becomes lossy above its crossover; below that, it's a Q-multiplier.

Rule of thumb: Resonant frequency f_r = 1/(2π√(LC)). For a 1 µH bead and 10 µF cap: f_r ≈ 50 kHz. If your switcher runs at 500 kHz, you're actually fine — noise is above resonance. But if you have a 100 kHz noise component, it gets amplified. Fix: add a small series resistor (0.1–1 Ω) or use a lossy cap (higher ESR) to damp the Q.

Real example: Isolating an ADC's analog VDD from digital VDD. Use a 600 Ω @ 100 MHz bead rated for 500 mA between the digital 3.3 V rail and the AVDD pin, with a 10 µF + 100 nF cap on the analog side. Check the datasheet impedance curve — you want the bead resistive (not inductive) at your dominant switching harmonics. Simulate the LC resonance and add a 1 Ω damping resistor if the peak lands in a sensitive band.

Never use a bead to filter high-di/dt paths like MOSFET gate returns — the voltage spike (V = L·di/dt) will exceed the bead's saturation and possibly your silicon.