A typical crystal oscillator circuit and an example layout design are shown in the figures. A crystal manufacturer normally specifies the load capacitance, C
L, needed for a crystal in its datasheet. This value is the load seen by the crystal which is in series with C
1 and C
2, including all parasites. Typically, overall stray capacitance, C
stray, for pin, board, and PCB trace is between 2 pF and 7 pF. The relation between them can be represented by the following formula.
C
L=(C
1 * C
2)/(C
1 + C
2) + C
stray
C
1 and C
2 are normally chosen to have equal value. Therefore,
C
1 = C
2=2 * (C
L - C
stray)
For example, assume that C
stray is 4 pF and crystal load capacitance C
L is 18 pF, C
1 = C
2= 28 pF. The normalized value of 30 pF can be used.
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