Crystal Load Capacitance

CL = (C1×C2)/(C1+C2) + Cstray

Calculator

Result
—

Formula

CL = (C1 × C2) / (C1 + C2) + Cstray

Description

A crystal oscillator requires external load capacitors (C1 and C2) to match the crystal's specified load capacitance for accurate frequency operation. The effective load seen by the crystal is the series combination of C1 and C2 (since they are in series from the crystal's perspective through ground) plus any stray capacitance from PCB traces and IC pins. If the load capacitance does not match the crystal specification, the oscillation frequency shifts: too little capacitance raises the frequency, too much lowers it. Typical load capacitance values range from 6 to 20 pF.

Variables

  • CL — Effective load capacitance seen by the crystal (F)
  • C1 — Load capacitor on one crystal pin (F)
  • C2 — Load capacitor on the other crystal pin (F)
  • Cstray — Stray capacitance from PCB traces and IC pins (F)

Practical Notes

When C1 = C2 (the typical case), CL = C1/2 + Cstray. For a crystal specifying CL = 12pF with Cstray = 5pF, you need C1 = C2 = 2 × (12-5) = 14pF. Round to standard 15pF values. Stray capacitance is typically 2-7pF depending on trace layout. Mismatched CL causes frequency errors of a few to tens of ppm.

Related Formulas

555 Timer Astable 555 Timer Monostable RC Oscillator Frequency Crystal Frequency Stability PLL Lock Time VCO Tuning Sensitivity

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App