Tuesday, 8 September 2015

capacitance - How do I choose the correct capacitors to use with my Crystal?


I am looking at using this crystal for a new circuit design. It will provide the main oscillator for my PIC application.


Here is the part on digikey.


The datasheet indicates that it has a load capacitance of 8pF. This seems very low compared to most. I have read that the load capacitance should be equal to:


$$ \frac{(C_1\times C_2)}{(C_1+C_2)} +C_{stray}$$


where Cstray is the capacitance of the PCB traces which are typically between 5 and 15 pF.


This equation is provided in a Microchip FAQ sheet. If this is correct my PCB trace capacitance is likely already equal to the load capacitance of the oscillator. So does this mean that I really don't need any capacitors connected between the crystal and ground?




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