51.6.4 Calculating the Correct Crystal Decoupling Capacitor

The model shown in the following figure can be used to calculate correct load capacitor for a given crystal. This model includes internal capacitors CLn, external parasitic capacitance CELn and external load capacitance CPn.

Figure 51-10. Crystal Circuit With Internal, External and Parasitic Capacitance

Using this model the total capacitive load for the crystal can be calculated as shown in the equation below:

∑ C tot = C L 1 + C P 1 + C EL 1 ) C L 2 + C P 2 + C EL 2 ) C L 1 + C P 1 + C EL 1 + C L 2 + C P 2 + C EL 2

where Ctot is the total load capacitance seen by the crystal. This value should be equal to the load capacitance value found in the crystal manufacturer datasheet.

The parasitic capacitance CELn can in most applications be disregarded as these are usually very small. If accounted for, these values are dependent on the PCB material and PCB layout.

For some crystal the internal capacitive load provided by the device itself can be enough. To calculate the total load capacitance in this case. CELn and CPn are both zero, CL1 = CL2 = CL, and the equation reduces to the following:

∑ C tot = C L 2

See the related links for equivalent internal pin capacitance values.