差模共模噪声Differential and Common mode

差模和共模噪声的产生原理

4.1. Differential and Common Mode Noise

Noise is classified into two types according to the conductionmode.

The first type is differential mode noise which is conducted onthe signal (VCC) line and GND line in the opposite direction toeach othe. This type of noise is suppressed by installing a filter onthe hot (VCC) side on the signal line or power supply line, asmentioned in the preceding chapter.

The second type is common mode noise which is conducted onall lines in the same direction. With an AC power supply line, forexample, noise is conducted on both lines in the same direction.With a signal cable, noise is conducted on all the lines in the cablein the same direction.

Therefore, to suppress this type of noise, EMI suppression filters

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methods are applied.

1.Noise is suppressed by installing an inductor to the signal line

and GND line, respectively.

2.A metallic casing is connected to the signal line using a

capacitor. Thus, noise is returned to the noise source in thefollowing order; signal/GND lines ¨ capacitor ¨ metalliccasing ¨ stray capacitance ¨ noise source.

差模和共模噪声的产生原理

Common mode choke coils are used to suppress common modenoise. This type of coil is produced by winding the signal or supplywires one ferrite core.

Since magnetic flux flows inside the ferrite core, common modechoke coils work as an inductor against common mode current.Accordingly, using a common mode choke coil provides largerimpedance against common mode current and is more effectivefor common mode noise suppression than using several normalinductors.

[Notes]

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差模和共模噪声的产生原理

Since magnetic flux cancels out inside the ferrite core, impedanceis not produced for differential mode current. The magneticsaturation problem is small. Common mode choke coils are suitedfor common mode noise suppression on lines with large currentflow, such as AC/DC power supply lines. Since they do not affectsignal waveform, they are also suited for common mode noisesuppression on lines where signal waveform distortion causes aproblem, such as video signal lines.

The above graph shows examples of impedance characteristicsof DC common mode choke coils. Actual characteristics alsocontain differential mode impedance, and this must be consideredwhen using common mode choke coils in circuits where the signalwaveform is significant.

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[Notes]