Selecting Decoupling Capacitors

Decoupling capacitors are one of those things where doing it right makes a real difference to circuit reliability. Here's the mental model that actually works.

Every IC power pin needs at least one decoupling cap, as close to the pin as possible. For most digital ICs (microcontrollers, logic gates), 100nF ceramic X7R at 0402 or 0603 is standard. This handles the high-frequency switching transients. For ICs with significant current draw (power amplifiers, RF transmitters, stepper drivers), add a bulk cap of 10-100uF in parallel — this supplies the low-frequency reservoir current that the PCB trace inductance would otherwise starve.

Placement is everything. A 100nF cap 5cm away from a GPIO expander provides nearly zero benefit because the parasitic inductance of the trace swamps the cap's impedance at the frequencies that matter. Route the decoupling cap before the IC, with the cap between the power trace and the IC pin, minimizing the loop area.