Capacitor Self-Resonant Frequency

Formula

Description

Every real capacitor has parasitic equivalent series inductance (ESL) from its leads, internal connections, and electrode geometry. At the self-resonant frequency, the capacitive reactance (1/ωC) exactly cancels the inductive reactance (ωL), and the impedance reaches its minimum (equal to ESR). Below the SRF, the component behaves as a capacitor; above the SRF, it behaves as an inductor. This frequency determines the useful bandwidth of a capacitor for decoupling and filtering applications.

Variables

• f_SRF — Self-resonant frequency (Hz)
• ESL — Equivalent series inductance (H)
• C — Capacitance (F)

Practical Notes

Typical ESL values: through-hole ceramic 2-5 nH, SMD 0805 about 1 nH, SMD 0402 about 0.5 nH. A 100 nF 0805 ceramic has SRF ≈ 16 MHz with 1 nH ESL. For effective decoupling above the SRF, place a smaller capacitor (e.g., 100 pF) in parallel. Always check that your operating frequency is well below the SRF of your decoupling capacitors.