Application Information

POWER LINE NOISE FILTERING

A factor to consider in designing power and ground is noise

filtering. A noise filtering circuit is designed to prevent noise

generated by the integrated circuit (IC) as well as noise

entering the IC from other devices. A common filtering

method is to place by-pass capacitors (C

power and ground lines.

Placing a by-pass capacitor (C

the proper location solves many power supply noise prob-

lems. Choosing the correct capacitor value is based upon

the desired noise filtering range. Since capacitors are not

ideal, they may act more like inductors or resistors over a

specific frequency range. Thus, many times two by-pass

capacitors may be used to filter a wider bandwidth of noise.

It is highly recommended to place a larger capacitor, such as

10µF, between the power supply pin and ground to filter out

low frequencies and a 0.1µF to filter out high frequencies.

By-pass capacitors must be mounted as close as possible to

the IC to be effective. Longs leads produce higher imped-

ance at higher frequencies due to stray inductance. Thus,

this will reduce the by-pass capacitor’s effectiveness. Sur-

face mounted chip capacitors are the best solution because

they have lower inductance.

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FIGURE 11. Placement of by-pass Capacitors, C

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