Decoupling Capacitor Impedance

Formula

Description

In power distribution network design, decoupling capacitors must present low impedance at the frequencies where the IC draws transient current. This is the same capacitive reactance formula but applied in the PDN context to determine whether a capacitor provides adequate bypassing at the target frequency. The PDN impedance target is typically derived from the allowable voltage ripple divided by the maximum transient current. Multiple capacitor values are used in parallel to cover different frequency ranges.

Variables

• Z — Capacitor impedance at target frequency (Ω)
• f — Target frequency (Hz)
• C — Capacitance (F)

Practical Notes

Real capacitors have ESR and ESL that limit their effectiveness at high frequencies. A 100nF ceramic capacitor is effective up to about 100 MHz, above which its parasitic inductance dominates. Use multiple capacitor values (10µF, 1µF, 100nF, 10nF) in parallel to maintain low impedance across a wide bandwidth. Place capacitors as close as possible to the IC power pins.