I have a question about HFXO load capacitors and LFXO load capacitors.
Each load capacitors of nRF52840-DK are like below.
And i checked BOM file to find out load capacitance of each xtals.
XTAL SMD 2016, 32MHz, Cl=8pF, Tol: ±10ppm, Stab: ±15ppm
XTAL SMD 2012, 32.768kHz, 9pF, ±20ppm
As you know, load capacitor equation is like below.
C1 = C2 = C_L * 2 - (Input capacitance XC1 and XC2 in case of HFXO)
C17 = C18 = C_L * 2 - (Input capacitance on XL1 and XL2 pads in case of LFXO)
So, if i calculate these values based on document,
Input capacitor of HFXO C1=C2=8*2-3pF=13pF
input capacitor of LFXO C17=C18=9*2-4pF=14pF
Input capacitors of nRF52840-DK and values of above are different.
Could anybody answer why this is different?
You forgot the C_pcb which is ~1pF. This gives 12 pF for HFXO and 13 pF for LFXO load capacitors. 12 pF was probably chosen for both as it's a standard value for a capacitor and is easily available. In addition, the requirement for the accuracy on the LFXO is not as high as for the HFXO.
Thank you very much.
Your answer is clear for me.