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Ferrite beads, Impedence at 100MHz.

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Dec 6, 2013
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I am trying to design a linear regulator stage following a switching step-down regulator, in a application note from Linear Technology the "secret sauce" is ferrite beads before and after the linear regulator each with its own capacitor.

But when I go to digikey to select a bead I find something that makes me quite unsure, I can get a bead that can handle the current in my circuit that presents a impedance of 100Ω at 100MHz, actually there are several but if I am willing to pay a little more(or 2* more) I can get a bead that presents 800Ω at 100MHz.

When the signal of interest is low frequency, is the higher the impedance at higher frequency's the better?

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If you look at these two beads, the price differ much but so does the impedance at 100MHz.
Is it worth paying more in a case such as this?

Microwave ferrite has much lower permeability and more conductive oxide filler.

<30MHz ferrite has the highest mu with cobalt doped iron oxide with much less conductive metal particles as beads around conducting wires. But high permeability and lower resistance SMD in a ferrrite power may use silver oxide with iron oxide and is more expensive.

But as you may have found there are thousands of variations in ferrite. with a dozen basic types.

The goal for load regulation is to have a high ratio of load to source impedance ratio, while the goal of ripple or spike rejection is just the oppositve with high series to shunt ratio.

You must balance cost with these variables for choices of impedance of ferrite , low ESR and high ripple current rating of Caps and low RdsOn FETs of switches to achieve low drop in LDO's and switches and reduce or eliminate heatsink costs, while achieving the desired noise levels.

You have the right approach using SMPS to LDO for noise. Magnetic costs in high density can be expensive compared to a large air coil and a few small ferrite beads or a torroidal coil with heavy gauge, but come down significantly in volume. More exotic materials are only worth the cost if the size is that important.

Also look at Toko, Murata and TDK. The best magnetics often come from Japan TW & Korean sources.
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