nrf52840 HFXO PCB Stray Capacitance

RE: nrf52840 HFXO PCB Stray Capacitance

Bendik Heiskel — Thu, 22 May 2025 07:59:28 GMT

Yes, given that you keep the crystal close to the SoC 12pF should be OK for this crystal.

Best regards,

Bendik

RE: nrf52840 HFXO PCB Stray Capacitance

Samno — Wed, 21 May 2025 15:39:10 GMT

Thanks Bendik for your help. I saw somewhere that the Golledge MP06003 is used in a reference design, and the reference schematics have 2 x 12pF capacitors. Is this setup acceptable for the nrf52840?

https://www.digikey.ca/en/products/detail/golledge-electronics-ltd/MP06003/16965329

RE: nrf52840 HFXO PCB Stray Capacitance

Bendik Heiskel — Wed, 21 May 2025 15:22:48 GMT

Hi,

The PCB stray capacitance are the capacitance of the traces connecting XC1 and XC2 to the crystal. If the traces are kept short the capacitance can be assumed to be 0pF, as it will not have a significant impact on the total crystal load capacitance.

If you assume C_pcb1 = C_pcb2 = 0pF and C1 = C2 = C, the equation in the datasheet can be simplified to:

C = 2 * CL - C_pin

This gives a good starting point, which can be fine tuned by measuring the frequency offset of the radio. The radio frequency accuracy is directly dependent on the accuracy of the crystal oscillator, where the oscillating frequency of the crystal is dependent on the load capacitance.

Best regards,

Bendik