7.2.4 Unused Inputs
Unused inputs can be disabled and left unconnected. Register 0x0949[3:0] defaults the input clocks to being enabled. Clearing the
unused input bits will disable them. Enabled inputs not actively being driven by a clock may benefit from pull up or pull down resistors to
avoid them responding to system noise.
7.2.5 Input Clock Rise Time Considerations
It is well known that slow rise time input clocks with low slew rates are a cause of increased jitter. If the slew rate is low enough, the
output jitter will increase. The following figure shows the effect of a low slew rate on RMS jitter for a differential clock input. It shows the
relative increase in the amount of RMS jitter due to slow rise time and is not intended to show absolute jitter values.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
100
200
300
400
500
600
Rela
teiv
e Jit
ter
Input Slew (V/us)
IN_X Slew Rate in Differential Mode
J
TYP
Figure 7.4. Effect of Low Slew Rate on RMS Jitter
Si5391 Reference Manual • Clock Inputs
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.5 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • January 11, 2022
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