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Crystal Oscillator Load Capacitor Calculator
Calculate crystal load capacitors from the crystal load capacitance, PCB stray capacitance, and selected C1/C2 values for Pierce oscillator designs.
Input Parameters
Results
Crystal frequency accuracy depends on load capacitance, ESR, drive level, PCB parasitics, and the oscillator circuit recommended by the MCU or oscillator vendor.
Equations Used
Crystal load capacitance: CL = (C1 × C2) / (C1 + C2) + Cstray
Equal capacitor estimate: C1 = C2 ≈ 2 × (CL - Cstray)
If Cstray is higher than CL, external load capacitors may need to be very small or omitted depending on the oscillator design.
Frequently Asked Questions (FAQ)
Q1: What does a crystal load capacitor calculator do?
It estimates the C1 and C2 capacitors required to match the crystal load capacitance specified in the datasheet.
Q2: Why is C1 and C2 not simply equal to the crystal CL?
The two capacitors appear in series from the crystal viewpoint, and PCB stray capacitance is added to the effective load.
Q3: What stray capacitance should I use?
A practical starting point is often 2–5 pF, but the real value depends on PCB layout, device pins, package, and measurement setup.
Q4: Can wrong load capacitors shift crystal frequency?
Yes. Too much or too little load capacitance can pull the oscillation frequency away from the nominal value.
Q5: Should C1 and C2 always be equal?
Equal values are common, but some oscillator layouts or drive circuits may use asymmetric values.
