SI52131-A11A Datasheet, PDF (7/19 Pages) Silicon Laboratories – No termination resistors required

Si52131-A11A
2. Functional Description
2.1. Crystal Recommendations
The clock device requires a parallel resonance crystal.
Table 4. Crystal Recommendations
Frequency
(Fund)
25 MHz
Cut Loading Load Cap Shunt Motional Tolerance
Cap (max) (max)
(max)
AT Parallel 12–15 pF 5 pF
0.016 pF 35 ppm
Stability
(max)
30 ppm
Aging
(max)
5 ppm
2.1.1. Crystal Loading
Crystal loading is critical in achieving low ppm performance. To realize low ppm performance, use the total
capacitance the crystal sees to calculate the appropriate capacitive loading (C_L).
Figure 1 shows a typical crystal configuration using the two trim capacitors. It is important that the trim capacitors
are in series with the crystal.
Figure 1. Crystal Capacitive Clarification
2.1.2. Calculating Load Capacitors
In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate
the crystal loading correctly. Again, the capacitance on each side is in series with the crystal. The total capacitance
on both side is twice the specified crystal load capacitance (C_L). Trim capacitors are calculated to provide equal
capacitive loading on both sides.
Figure 2. Crystal Loading Example
Rev 1.0
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