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Defining Capacitor Functions on a Printed Circuit Board
Within electrical engineering, a descriptive word is used to describe a specific function or operation. The same word may be used to reference different applications thus making it confusing to understand what is being discussed. Three common words related to the use of capacitors are Bulk, Bypassing and Decoupling.
Bulk capacitors are used for maintaining a stable power distribution network (PDN) with components consuming large amounts of current over time. These are slow acting devices whose purpose is to minimize voltage sag due to IR loses (and other factors) caused by the removal of electrical charge by all components, especially resistive elements.
Bypassing refers to energy transference (shunting) RF current from one area to another. The advantages of using a capacitor for the purpose of bypassing is that the voltage rating of the capacitor is irrelevant, as its intended function is to shunt RF current, not provide charge. An example is removing undesired RF currents on the braid of a coax to chassis ground, especially when the braid is carrying the DC return current and cannot be directly bonded to a metal chassis. Also, when traces are routed over split planes, an RF return path must be present. A bypass capacitor performs this function and is commonly connected power-to-power or ground-to-ground. We are not interested in providing charge in a bypassing role. Many analog circuits mandate AC coupling in series within a signal trace to keep DC voltage from being impressed onto the input pins.
Decoupling is a means of overcoming physical and time constraints caused by digital circuitry switching logic states. A decoupling capacitor provides rapid recharging of the voltage and current of a power distribution network (planes for multi-layer or traces for single/double sided assemblies) to ensure voltage margins are within tolerance for optimal operations. If there is insufficient voltage and current present, and multiple components transition their signal pins simultaneously under maximum capacitive load, plane bounce may occur. Decoupling is accomplished by ensuring that a low-impedance, localized power source is present at all times. Because capacitors decrease in impedance up to the point of self-resonance, higher-frequency switching noise is effectively shunted from the power distribution system. Lower-frequency RF energy transference remains relatively unaffected.
A capacitor may also be used in other applications such as timing, wave shaping, integration, and filtering. When discussing capacitors among associates, proper description and use of a capacitor is required to ensure everyone understands what the other person is talking about.
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