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Issue #210 January 2008
Problem 1—What is the purpose of a “decoupling” capacitor, usually placed on a circuit board near a block of active circuitry (especially an integrated circuit)?
Answer 1: The capacitor’s purpose is to reduce the high-frequency impedance of the power supply as seen by the IC. This tends to be a problem because the apparent impedance is raised by the inductance of wiring harnesses and PCB traces. This causes the AC currents drawn by one chip to be seen by other nearby chips as changes in their supply voltages, potentially causing signal integrity problems.
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