Crystal Load Capacitor Calculator (Oscillator Capacitors C1, C2)
The two capacitors to fit on a crystal oscillator for the crystal's specified load capacitance, the load you actually get with the parts you have, and the frequency error that difference causes.
Results
The dashed line marks the value you entered.
What this tool does
A crystal oscillates at its specified frequency only when the circuit presents the load capacitance C_L it was calibrated for. The two capacitors on the pins act in series, so each must be about twice the load capacitance minus the stray capacitance of the board and the chip pins, which is why a 12 pF crystal typically needs about 15–20 pF capacitors, not 12 pF. Stray capacitance is typically a few picofarads, so treat the result as a starting point. Each picofarad of error moves the frequency by a few ppm to a few tens of ppm, depending on the crystal.
Formula
C₁ = C₂ = 2 (C_L − C_stray) · C_L = C₁C₂ ÷ (C₁ + C₂) + C_stray · f_L = f_s √(1 + C_m ÷ (C₀ + C_L))
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cl | Load capacitance the crystal is specified for | pF |
cs | Stray capacitance of the board and pins | pF |
c1 | Capacitor you fitted on one pin | pF |
c2 | Capacitor you fitted on the other pin | pF |
c0 | Shunt capacitance of the crystal (C0) | pF |
cm | Motional capacitance of the crystal (C1 in the datasheet) | fF |
CN | Value of each capacitor needed (equal pair) | pF |
CA | Load capacitance you really have | pF |
CD | Difference from the specified load | pF |
PP | Frequency error caused by that difference | ppm |
Worked example
- Load capacitance the crystal is specified for12 pF
- Stray capacitance of the board and pins3 pF
- Capacitor you fitted on one pin18 pF
- Capacitor you fitted on the other pin18 pF
- Shunt capacitance of the crystal (C0)3 pF
- Motional capacitance of the crystal (C1 in the datasheet)10 fF
- Value of each capacitor needed (equal pair)18.0 pF
- Load capacitance you really have12.00 pF
- Difference from the specified load0.00 pF
- Frequency error caused by that difference0.0 ppm
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.