Decoupling capacitor

A decoupling (or bypass) capacitor is a small capacitor tucked right beside a chip's power pin, acting as a tiny local battery that delivers a quick gulp of charge the instant the chip suddenly demands current. Without it, the supply voltage at the chip dips and spikes every time the logic switches, and those wobbles couple into neighbouring circuits as noise — hence 'decoupling': it un-couples each chip's power demands from everyone else's.

The trick works because the wiring back to the regulator has inductance, so it cannot supply a sudden current fast enough — but a capacitor sitting nanometres away can. Designers sprinkle one (often 100 nF) at every IC, sometimes paired with a bigger bulk capacitor for slower demands and a smaller one for the highest frequencies. Forgetting them is one of the most common causes of a board that 'works on the bench but glitches in the field'.