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When a capacitor is connected in parallel to the power supply line of an IC, there is impedance in the power supply line, which is not shown in the circuit diagram, which can change the power supply voltage and cause a malfunction or interference between circuits. A capacitor is connected in parallel to control the voltage variations through charging and discharging. Additionally, since a capacitor passes alternating current, it removes or directs ripple noise to ground. This is called a “decoupling capacitor” (also called a “bypass capacitor”). For decoupling use, the ideal capacitor would have low impedance across a wide band of frequencies from low to high, but in reality the impedance-frequency characteristics of a capacitor follow a V-shaped curve. The frequency at the trough of the V-shape is called the “self-resonating frequency” (SRF), and it functions as a capacitor in the region below the SRF. For this reason, capacitors with different characteristics are typically connected in parallel to cover a wide frequency band in decoupling applications.

PTM Published on: 2015-09-10