lms1487e National Semiconductor Corporation, lms1487e Datasheet - Page 11

• Manufacturer Part Number: lms1487e
• Description: Low Power Rs-485 / Rs-422 Differential Bus Transceiver
• Manufacturer: National Semiconductor Corporation

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
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FIGURE 11. Placement of by-pass Capacitors, C
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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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