Stray capacitance on PCB (32kHz Crystal Cx calculation)

[amazed]

Hello,

There are several equations for external capacitor calculation, this explanation is the most convincing to me, which contains:

The crystal datasheet specifies 12,5pF as load capacitance. The crystal itself will be just a backup option, so on the PCB there are currently two unpopulated 0805 footprints which can be soldered (into a short) if needed.

What would be a good stray capacitance guesstimation for this layout?
(bottom layer is GND pour, standard 1.6mm FR4 material, 1oz copper weight)

Thanks in advance.

[herbschwarz]

The 12.5 pf load capacitance is the value that the manufacturer used to tune the crystal to the specified frequency. Using a different value can 'pull' the crystal to oscillate at a slightly different frequency.

[Wawa]

18pF would be a good start, which gives you 9pF without the board.
Leo..

[amazed]

@herbschwarz

The 12.5 pf load capacitance is the value that the manufacturer used to tune the crystal to the specified frequency. Using a different value can 'pull' the crystal to oscillate at a slightly different frequency.

Yes, this is why I tried to choose the needed value with caution.

@Wawa

18pF would be a good start, which gives you 9pF without the board.

Thanks! Stray capacitance is usually assumed to be 2-5 pF with short traces, which results in 20pF-15pF load capacitors based on the formula in #1. Also considering that the basic (cheap) C0G capacitors at the PCB assembler company have +-5% tolerance, I will go with this middle value.