For better EMC performance, decoupling capacitors should be placed as close as possible to each IC, because trace impedance will reduce the effectiveness of decoupling function.Ĭreamic capacitors are usually selected for decoupling choosing the value depends on the rise and fall times of the fastest signal. The value of decouupling capacitor is approximately 1/100 or 1/1000 of the bypass capacitor. Ideally, the bypass and decoupling should be placed as close as possible to the power supply inlet to help filter high frequency noises. The main function of the decoupling is to provide localised source of DC power for the active devices, thus reducing switching noise propagating across the board and decoupling the noise to ground. Larger values are required on the PCB with a large number of integrated circuits, fast switching circuits, and PSUs and having long leads to PCB.ĭuring active device switching, the high frequency switching noise created is distributed along the power supply lines. Usually, aluminium or tantalum capacitor is a good choice for bypass capacitors, its value depends on the transient current demand on the PCB, but it is usually in the range of 10 to 47 uF. The bypass capacitor is acting as a high frequency bypass source to reduce the transient circuit demand on the power supply unit. The main function of the bypass capacitor is to create an AC shunt to remove undesirable energy from entering susceptible areas.
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