lms485 National Semiconductor Corporation, lms485 Datasheet - Page 11

• Manufacturer Part Number: lms485
• Description: 5v Low Power Rs-485 / Rs-422 Differential Bus
• 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. 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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