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Magnetic beads knowledge
The English name of the magnetic bead is Bead. Among them, the ferrite magnetic bead is an anti-interference device with rapid development. It is cheap, easy to use, and filters out high-frequency EMI noise. It is equivalent to a series connection of resistors and inductors, but both resistance and inductance values vary with frequency. It has better high-frequency filtering characteristics than ordinary inductors and resistive at high frequencies, so it can maintain high impedance over a wide frequency range, thus improving the high-frequency filtering effect.

(1) Main characteristic parameters of magnetic beads

DC resistance DCResistance(mohm):

The resistance value exhibited by this magnetic bead when a DC current passes through the bead.

Rated current RatedCurrent(mA):

Indicates the maximum allowable current when the bead is operating normally.

Impedance [Z]@100MHz(ohm):

This refers to the impedance corresponding to the magnetic beads at a frequency of 100 MHz.

Resistance-frequency characteristics:

Describe the curve of resistance value as a function of frequency

Inductive reactance frequency characteristics:

Describe the curve of the inductive reactance as a function of frequency.

(2) Magnetic bead selection

The magnetic beads have almost no impedance at the low frequency end, and only exhibit high impedance at high frequencies. Therefore, magnetic beads are generally selected when suppressing high frequency interference.

In addition to paying attention to the three parameters of 100 mega impedance, DC impedance and rated current, the selection of magnetic beads should also pay attention to the use category of magnetic beads. For example: high-frequency high-speed magnetic beads, power beads (high current), ordinary signal beads.

(3) Typical application of magnetic beads

Ferrite beads are widely used on printed circuit board power lines and signal data lines. If ferrite beads are added to the input end of the power line of the printed board, high frequency interference can be filtered out.

Ferrite beads are designed to suppress high frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference.

Magnetic beads are typically used in signal interfaces, mainly to suppress interference from ports and external interference, such as 232 interface, VGA interface, LCD interface, etc.

Ferrite beads are also used in digital power circuits to filter out high frequency noise, such as crystal power supplies, PLL power supplies, etc.

In digital circuits, since the pulsed digital signal contains high-order harmonics with high frequency, it is also the main source of high-frequency radiation of the circuit, so it can be used in such applications to play the role of magnetic beads, such as clock signal lines, data address buses, etc. .

(4) Precautions for magnetic bead selection

What is the frequency range of the unwanted signal;

Who is the source of noise;

How much noise attenuation is required;

What are the environmental conditions (temperature, DC voltage, structural strength);

What is the circuit and load impedance;

Is there room for placing magnetic beads on the pcb board; the first three can be judged by observing the impedance frequency curve provided by the manufacturer. All three curves in the impedance curve are very important, namely resistance, inductive reactance and total impedance.

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