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. Long leads produce higher impedance

at higher frequencies due to stray inductance. Thus, this will

reduce the by-pass capacitor’s effectiveness. Surface

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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