How to Choose the Load Capacitance for a Crystal Oscillator
2025-04-27
Selecting the correct load capacitance (CL) for a crystal oscillator is crucial for ensuring stable and accurate oscillation. Here's a detailed guide:
1. Understanding the Role of Load Capacitance
The load capacitance consists of external capacitors (typically C1 and C2) connected to the crystal, forming a resonant circuit with the crystal's internal capacitance. Proper load capacitance ensures frequency accuracy and stability.
2. Key Parameters
Crystal's Specified Load Capacitance (CL):
The datasheet specifies the required CL (e.g., 12pF, 18pF, 20pF). The circuit must match this value.
Stray Capacitance (Cs):
Includes parasitic capacitance from PCB traces, IC pins, etc. (Typically 3-5pF; must be estimated or measured.)
3. Calculation Method
The load capacitance is calculated as:
CL = (C1 × C2)/(C1 + C2) + Cs
Where:
C1 and C2 are external capacitors (usually equal)
Cs is stray capacitance
Simplified Selection:
If the crystal's CL = 18pF and Cs = 5pF, then:
(C1 × C2)/(C1 + C2) = 18pF - 5pF = 13pF
If C1 = C2, then C1 = C2 = 2 × 13pF = 26pF (common approximations: 22pF or 27pF).
4. Selection Steps
Check the crystal's CL value (from the datasheet)
Estimate stray capacitance (Cs) (typically 3-5pF; measure for high-frequency designs)
Calculate external capacitors:
If CL = 20pF and Cs = 4pF, then C1//C2 = 16pF
For symmetry, C1 = C2 = 32pF (standard value: 33pF)
Verify and adjust:
Test with an oscilloscope/frequency counter; fine-tune capacitor values (e.g., 30pF or 27pF) to match the nominal frequency
5. Important Considerations
Capacitor tolerance: Use high-precision (5%) ceramic capacitors (e.g., NP0/C0G)
Asymmetric designs: If C1 ≠ C2 (e.g., for startup optimization), recalculate equivalent capacitance
Passive vs. active crystals:
Passive crystals require external load capacitors
Active oscillators (built-in circuit) don't need external CL but may require input capacitance matching
High-frequency crystals: For >26MHz, minimize trace length and stray capacitance (Cs)
6. Common Issues
Frequency deviation: Likely due to incorrect CL or misestimated Cs
Failure to oscillate: Check if capacitors are too large (insufficient gain) or too small (frequency shift)
Temperature drift: Use stable capacitors and crystals for wide-temperature operation
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