Decoupling Capacitor Calculator

Decoupling capacitors (also called bypass capacitors) are small capacitors placed close to integrated circuits (ICs) on a printed circuit board (PCB). Their job is to filter out power supply noise — rapid voltage spikes and dips caused by sudden changes in current demand as the IC switches states.

Why Decoupling Capacitors Matter

Digital ICs switch millions or billions of times per second. Each switch causes a sudden current surge that creates a voltage glitch on the power supply line. Without decoupling capacitors, this noise can cause data errors, timing problems, or spurious resets. Decoupling caps act as a local energy reservoir — providing the burst of current needed by the IC without disturbing the main power rail.

The Formula

The key relationship is: C = ΔQ / ΔV = I × Δt / ΔV

Where:

• C = Required capacitance (Farads)
• I = Peak current demand of the IC (Amps)
• Δt = Rise/fall time of the current pulse (seconds) — approximately 1/frequency
• ΔV = Maximum acceptable voltage ripple (Volts)

For frequency-domain filtering, the capacitor’s impedance (Z) at the frequency to be filtered must be much lower than the source impedance. The capacitor impedance is:

Z = 1 / (2π × f × C)

Rearranging: C = 1 / (2π × f × Z_target)

Standard Decoupling Practice

For digital ICs, use two capacitors in parallel:

1. A large bulk capacitor (10–100 μF electrolytic or tantalum) to supply slow current transients
2. A small ceramic capacitor (0.1 μF or 100 nF) to handle high-frequency noise

Place them as close as possible to the IC’s power pins. The 100 nF ceramic is the most common decoupling capacitor in electronics.

Capacitor Types for Decoupling

Important: ESR

Real capacitors have equivalent series resistance (ESR). For high-frequency decoupling, choose capacitors with very low ESR (ceramic caps are excellent). High-ESR capacitors like older electrolytics are less effective at high frequencies.

Typical Values by Application