Crystal Oscillator Load Capacitor Calculator
External capacitor selection for crystal accuracy
Required Parameters
Waiting for input data...
Quick Answer
CL_ext = 2 × (CL_crystal - CL_stray). For a crystal with CL=12pF and 3pF stray: CL_ext = 2×(12-3) = 18pF per pin.
Design Notes
The crystal's load capacitance (CL) is specified by the manufacturer (typically 8-20pF). The two external caps appear in series to the crystal. Stray capacitance includes pin capacitance (~2pF), trace capacitance (~1-3pF), and IC input capacitance. Wrong load caps cause frequency deviation — too much cap = lower frequency, too little = higher.
Common Mistakes
- 1
Using the crystal's CL value directly as the cap value — you need to solve for the series combination.
- 2
Forgetting stray capacitance — it can be 3-7pF, significantly affecting small CL crystals.
- 3
Placing long traces between crystal and IC — adds stray capacitance and may pick up noise.
Knowledge Base
How do I calculate crystal load capacitors?
C_ext = 2 × (CL - C_stray), where CL is the crystal's specified load capacitance (from datasheet) and C_stray is the total stray capacitance per pin (typically 2-5pF including IC pin capacitance and PCB traces). For a 16MHz crystal with CL=12pF and 3pF stray: C_ext = 2×(12-3) = 18pF per pin.
What happens if load capacitors are wrong?
Too much capacitance: frequency shifts lower than nominal, increased startup time, higher power consumption. Too little capacitance: frequency shifts higher, potential oscillation instability, may fail to start in some conditions. Typical sensitivity: ±5pF error causes ~10-50ppm frequency shift.
What is crystal load capacitance?
Load capacitance (CL) is specified by the crystal manufacturer (typically 8-20pF). It's the total external capacitance the crystal 'sees' for it to oscillate at its specified frequency. The two external capacitors appear in series to the crystal: CL_effective = (C1 × C2)/(C1 + C2) + C_stray. Usually C1 = C2.
How do I estimate stray capacitance?
Sources: IC oscillator pin capacitance (check datasheet, typically 2-5pF per pin), PCB trace capacitance (~0.5-1pF per cm), via capacitance (~0.5pF each), nearby copper pour coupling. Total stray per pin is typically 2-5pF for compact layouts. Measure with a VNA for precision applications.
What crystal frequency should I choose for my MCU?
Common choices: 8 MHz (STM32, AVR general purpose), 12 MHz (USB full-speed, requires exact frequency), 16 MHz (Arduino Uno), 25 MHz (Ethernet PHY), 32.768 kHz (RTC, divides to exactly 1 Hz). For UART: 11.0592 MHz eliminates baud rate error at standard rates.
Crystal vs MEMS oscillator?
Crystal: lowest cost, ±10-50ppm accuracy, requires external caps, 1-5ms startup. MEMS oscillator: higher cost, ±10-50ppm, no external components, faster startup (<1ms), vibration resistant. Crystal is better for: cost-sensitive, precision timing. MEMS for: vibration environments, PCB space constraints, guaranteed startup.
Related Engineering Tools
555 Timer Calculator
Monostable and astable timing
I²C Pull-Up Resistor Calculator
Optimal pull-up for bus speed and capacitance
UART Baud Rate Error Calculator
Clock divider and baud rate accuracy
ADC/DAC Resolution Calculator
LSB size, ENOB, and dynamic range