Crystal oscillator load capacitance (CL) is a critical crystal parameter that determines frequency accuracy. The two external capacitors form a series combination seen by the crystal. Getting CL wrong causes frequency deviation — critical for RTC, USB, UART, and RF applications that rely on precise timing.
Crystal Parameters
From crystal datasheet. Common values: 6, 8, 10, 12, 18, 20pF
Typical: 1–3pF per pin (trace + pad + IC pin capacitance)
Enter crystal CL and stray capacitance to calculate load capacitors.
Common Oscillator Crystals
How to Use the Crystal Oscillator Load Capacitance Calculator
This crystal load capacitance calculator finds the correct discrete capacitor values for your crystal oscillator circuit. Enter CL from the crystal datasheet, estimated PCB stray capacitance, and get the capacitor values to place on each oscillator pin.
Load Capacitance Formula
CL = (Cg × Cd) / (Cg + Cd) + Cstray. For symmetrical layout (Cg = Cd = Cc): Cc = 2 × (CL − Cstray). For CL = 12pF, stray = 2pF per pin: Cc = 2 × (12 − 2×2) = 2 × 8 = 16pF. Place 16pF C0G capacitors on both XTAL1 and XTAL2 pins.
Stray Capacitance Guidelines
PCB stray capacitance comes from traces (0.5–1pF/cm), pads (0.5–1pF each), and IC input pin capacitance (1–3pF typical). Keep crystal traces short (under 5mm), avoid running parallel to other signals, and place crystals close to the MCU. For 32.768 kHz RTC crystals, stray capacitance is especially critical due to the small absolute CL value.
Capacitor Type Matters
Use C0G (NP0) ceramic capacitors only. X5R and X7R capacitors change value with temperature and voltage — they will cause frequency drift across operating temperature. For safety, select capacitors with at least 10% tolerance range around your calculated value, then verify frequency with an oscilloscope or frequency counter after assembly.
FAQ
Is this crystal oscillator calculator free?
Yes, completely free. Calculate load capacitor values without any account or payment.
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What is crystal load capacitance (CL)?
CL is specified by the crystal manufacturer and represents the external capacitance the crystal expects to see for accurate frequency operation. A crystal specified at 12pF must see 12pF of load (CL) from the external circuit, typically through two capacitors and PCB stray capacitance.
What is the load capacitor formula?
CL = (Cg × Cd) / (Cg + Cd) + Cstray. For balanced configuration (Cg = Cd = Cc): Cc = 2 × (CL - Cstray). Example: CL = 12pF, Cstray = 3pF. Cc = 2 × (12 - 3) = 18pF. Use 18pF capacitors on both oscillator pins.
What is PCB stray capacitance?
PCB stray (parasitic) capacitance comes from pad capacitance, trace capacitance, and IC pin capacitance. Typical values are 2–5pF total (1–3pF per side). It adds to your discrete capacitors. Failure to account for it causes the oscillator to run slightly above the nominal frequency.
What happens if my load capacitors are wrong?
Wrong load capacitance causes frequency deviation. Too much CL → crystal runs below nominal frequency. Too little CL → crystal runs above nominal. For a 10MHz crystal, 1pF error causes roughly 10ppm frequency shift — acceptable for most applications but critical for GPS, RTC, and communication protocols.
What capacitor type should I use for crystal loading?
Use high-quality C0G (NP0) ceramic capacitors with low ESR and stable temperature coefficient. Avoid X5R/X7R types — their capacitance varies significantly with voltage and temperature. C0G capacitors maintain accuracy across the operating temperature range and don't drift with applied voltage.