AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.4.2. Inductors and Ferrite Beads

A good ferrite bead has low DC resistance, high AC impedance, and high current handling capability. However, as the current handling capability increases, the AC impedance tends to drop, so there is a tradeoff involved. Figure 84 shows the impedance versus frequency plot of a typical ferrite bead.

Figure 84. Typical Impedence Profile of a Ferrite Bead

In Figure 84, the impedance at 2 GHz is more than 100 Ω. The ratio between that impedance and the power supply impedance, which is often lower than 1 Ω, is more than 100. As a result, most of the noise is blocked by the ferrite bead and is shunted to ground instead.

Some ferrite beads that meet performance are:
  • The Steward MI0805M221R-00 ferrite beads for transceiver power and ground planes. The DC resistance for this part is lower than 50 mΩ, and it can handle 2.5 A of current. The impedance is over 200 Ω at 1 GHz.
  • Two Steward ferrite beads connected in parallel can provide 5 A of current capability with 25 mΩ of DC resistance and over 100 Ω of AC impedance. This performance level is adequate for most applications.
  • The Murata BLM31PG500SN1 ferrite bead has 25 mΩ of DC resistance, 3 A of current, and 75 Ω of AC impedance at 1 GHz.

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