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Renesas Electronics America - Knowledgebase

What is Bypass capacitor?

Latest Updated:11/01/2006


What is Bypass capacitor?


Bypass capacitors are used to provide an AC path around a circuit element. They come in two types.
The first type is used for analog amplifier circuits. The signal in this type of bypass capacitor is connected in parallel to a current limiting resistor, so that it is not attenuated by that resistor.

The other type of bypass capacitor is placed in the required location on the board between the power supply and the ground, where it absorbs power supply noise on the board, performing the function of a smoothing capacitor in areas with a low current density. This type of bypass capacitor is particularly important in the vicinity of devices in which a large current flows or high-speed-operation devices.

The necessary capacitor capacitance differs depending on the system (such as the pattern, power capacity, and noise environment of the circuit board). A rule of thumb is as follows.
Place one ceramic capacitor (a few tenth of a uF) near each power pin of an LSI or one ceramic capacitor per several SSIs and MSIs such as gate ICs. On the entire circuit board, several electrolytic capacitors (several tens of uF) should be connected, though the number varies depending on the scale of the board.
Use ceramic capacitors with frequency characteristics that are as good as possible. Avoid higher capacitance laminated ceramic capacitors because some of these capacitors have poor frequency characteristics.
Place a ceramic capacitor near an IC that handles high-frequency signals, such as a clock, even if the IC is not an LSI.
Place an electrolytic capacitor near a part that handles a high current.

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