What is a Decoupling Capacitor?

If you are having issues with system noise, or any digital noise interrupting your power and signal lines, you can put a stop to that problem with a decoupling capacitor. This is a type of capacitor that can help “de-couple” or isolate local circuits from noise emitting from other devices on shared power. Decoupling capacitors are usually placed around power sources so as to supply a localized current source. Once applied, they can also isolate the local circuit from power noise coming from other areas.

When discussing the decoupling capacitor, it’s important to mention bypass and coupling capacitors.

Bypass capacitors can be used to supply a low impedance for high frequency noise on high impedance paths, and because they can help minimize higher frequency noise prior to it spreading to other areas of a circuit, they are referred to as the main decoupling capacitor. They help in mitigating risks of circuit malfunction or issues with containing EMI.

On the other side of this discussion is the coupling capacitors, which serve to provide DC isolation while producing a path for audio, video, RF, and high speed digital data. Coupling capacitors are often found on high speed interfaces to ensure that DC potential differences do not develop as ground currents between devices.

So what is the main function of a decoupling capacitor?

Their main function is to counter interferences from different electrical sources. Data busses and synchronized switched logic can create large instantaneous current flows that can produce significant charge from a local power delivery system (PDS). When there is an overload of charge, the PDS can prevent the power supply from instantly delivering added current. In turn, this causes the voltage to dip. Along with countering interferences, the decoupling capacitors aid in providing an instant charge source that prevents a bypass path and further dips in voltage. This instant charge source can help dampen noise on the PDS and thus allow the local circuits to be untouched by the potentially perturbing power plane ripple. This effect also extends to noise from other portions of the design when they experience instantaneous current draws.

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