All electronic components transistors, analog and digital ICs require power supplies for proper biasing. The power supply rails (such as ±5V, ±12V or 3.3V etc) in most cases are produced from linear power supply or switch mode power supply techniques. Both have rectification and filtering or regulation stage prior to feeding to the electronic circuits. The linear power supplies have low frequency ripple-noise whereas the switch mode power supplies have high frequency ripple-noise.
In the electronic printed circuit boards (PCBs) a lot of capacitors are found. The quantity and value of capacitors becomes higher as the switching frequency of the IC rises. On the other hand value of the capacitors in the power supplies varies inversely so that their value decreases as the frequency of pulsating dc (ripples) increases.
The ICs degrade their output performance if there is any type of ripple noise in the power supply. This noise effect is more apparent in the analog ICs as compared to the digital ICs. All types of ICs require a certain level of power supply regulation which is a small fraction of required power supply mentioned in their data sheets. The power supply requirements and noise margins are becoming more and more stringent as the power supply requirements of ICs are becoming low.
A decoupling capacitor which is an ordinary capacitor placed very close to the supply pin of the component that suppresses ripple noise from dc power supply and also fulfils instantaneous current requirements of an IC. It removes very tiny ripples from power supplies which are already smoothed and regulated. They also act like an instantaneous power supply backup source for an IC when the voltage drops down due to the switching of IC. It is a common practice to place a network of decoupling capacitors i.e. a combination of different high value (hundreds of µF), medium value (a few tens of µF), and low value capacitors (a fraction of µF) as shown in the figure 1. This practice is effective for circuits where noise of multiple frequencies is apparent in the power supply. This occurs in the voltage levels either due to power supply bad regulation, length of power cables, PCB and wire parasitic, switching frequencies and EMI effects etc. The most commonly used values for small decoupling capacitors are 100nF, 220nF and 470nF. They are connected to the IC power pin (such as ±5V, 3.3V) , connecting one side to the power supply pin and the other side to the circuit current returns (ground). They are placed as close as possible to the pin but not compromising over the minimum components to component clearance as shown in the figure 2.
Figure 1: Decoupling capacitors network schematics
Figure 2: Decoupling capacitors placement