Calculator inputs
Example data table
| Example | Target CL (pF) | C1 (pF) | C2 (pF) | Total Stray (pF) | Calculated CL (pF) | Comment |
|---|---|---|---|---|---|---|
| Reference Clock A | 12.0 | 18.0 | 18.0 | 3.0 | 12.0 | Balanced selection for a 12 pF target. |
| Reference Clock B | 13.0 | 22.0 | 22.0 | 2.0 | 13.0 | Equal capacitors simplify tuning and sourcing. |
| Reference Clock C | 13.0 | 27.0 | 18.0 | 2.2 | 13.0 | Mismatched values can still meet the target. |
Formula used
CL = (C1 × C2) / (C1 + C2) + Cstray
Cstray = Cpin1 + Cpin2 + Ctrace1 + Ctrace2 + Cpackage + Cextra
If C1 = C2 = C, then C ≈ 2 × (CLtarget − Cstray)
Let Ceff = CLtarget − Cstray
C2 = (Ceff × C1) / (C1 − Ceff)
This calculator first computes the series equivalent of C1 and C2. It then adds all estimated parasitic capacitances. The result is the effective load seen by the crystal. Tolerance spread is approximated by shifting capacitor and stray values by their stated percentages.
How to use this calculator
- Enter the crystal datasheet target load capacitance.
- Enter your selected capacitor values for C1 and C2.
- Add pin, trace, package, and any extra stray capacitances.
- Set tolerance assumptions for both discrete capacitors and parasitics.
- Press Calculate load capacitance.
- Review the result section above the form.
- Inspect the graph to see how C2 shifts the final load.
- Download the result summary as CSV or PDF if needed.
Frequently asked questions
1) What is crystal load capacitance?
It is the capacitance the crystal sees across its terminals. If actual load differs from the datasheet value, frequency accuracy, startup behavior, and margin can shift.
2) Why are two capacitors usually used?
Most Pierce oscillators place one capacitor from each crystal pin to ground. Their series combination, plus stray capacitance, creates the effective load seen by the crystal.
3) What should be included in stray capacitance?
Include microcontroller pin capacitance, crystal pad capacitance, short PCB traces, socket or package parasitics, and any nearby layout effects that add measurable capacitance.
4) Can C1 and C2 be different values?
Yes. Unequal values can still hit the target load. However, balanced values are often easier to source, easier to tune, and may behave more predictably.
5) How do I pick equal capacitor values quickly?
Subtract estimated stray capacitance from the target load, then multiply the remainder by two. That provides a fast starting value when both capacitors are equal.
6) Do internal pin capacitances replace external capacitors?
Usually not completely. Internal capacitance contributes to stray load, but most designs still need external capacitors to meet the crystal’s specified operating condition.
7) Does more load capacitance always improve stability?
No. Excess load can pull frequency lower, reduce startup margin, or stress the oscillator network. Matching the crystal’s specified load is usually safer than simply increasing it.
8) Why does tolerance matter here?
Real capacitors and parasitic estimates vary. Tolerance analysis helps you see whether the design still stays close to the target load across normal part spread.
Design notes
This page uses a clean white single-column layout, while the input grid adapts to three columns on large screens, two on medium screens, and one on mobile. The result area intentionally appears above the form after submission, directly below the header.