SI52146 Datasheet, PDF (7/20 Pages) Silicon Laboratories – PCI-EXPRESS GEN 1, GEN 2, & GEN 3 SIX OUTPUT CLOCK GENERATOR

Si52146
2. Functional Description
2.1. Crystal Recommendations
The clock device requires a parallel resonance crystal. Substituting a series resonance crystal causes the clock
device to operate at the wrong frequency and violates the ppm specification. For most applications there is a
300 ppm frequency shift between series and parallel crystals due to incorrect loading.
Table 4. Crystal Recommendations
Frequency
(Fund)
25 MHz
Cut Loading Load Cap Shunt Motional Tolerance Stability Aging
Cap (max) (max)
(max)
(max)
(max)
AT Parallel 12–15 pF 5 pF
0.016 pF 35 ppm
30 ppm 5 ppm
2.1.1. Crystal Loading
Crystal loading plays a critical role in achieving low ppm performance. To realize low ppm performance, use the
total capacitance the crystal sees to calculate the appropriate capacitive loading (CL).
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. It is not true that load capacitors are in parallel with the crystal and are approximately
equal to the load capacitance of 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 (CL). Trim capacitors are calculated to provide equal
capacitive loading on both sides.
Figure 2. Crystal Loading Example
Preliminary Rev. 0.1
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