Decoupling capacitor is also called uncoupling. The circuit always contained driver and load. If the load capacitance is large, drive circuit should charge and discharge the capacitor to accomplish the transition of signal, when the rising edge is steep, the current is high, so drive current will absorb much of the supply current. For inductance (especially the inductance on the chip base pins) and resistance will make a rebound in the circuit, such current is actually a noise to the normal conduction, which will affect the forestage work, this is so-called "coupling”.Therefore, it is also different from the high voltage ceramic capacitors and ceramic capacitors.
Decoupling capacitors just play the role of a battery to meet the electric current changes in drive circuit, and avoid mutual interference of coupling, then reduce high-frequency interfering impedance between power supply and reference. It will be easier to understand when combining bypass capacitor and decoupling capacitor capacitor. Bypass capacitor actually is also for decoupling capacitor, but it always refers to high-frequency bypass, it can improve high-frequency switching noise, and give low-impedance leakage prevention. High-frequency bypass capacitor usually is small, may be 0.1μF, 0.01μF and so on according to the resonant frequency, while coupling capacitor usually is higher, may be 10μF or more, which determined by the distribution of the circuit parameters and the changes of the drive current. Bypass is to filter the interference of input signal, while decoupling capacitor is to filter the interference of output signal and prevent the interference from returning to power supply. This may be the essential difference between them.
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