I have an oscillator at 40 MHz which I would like to drive two ICs, 1.8V output. The ICs are in opposite directions so I have opted for star topology of the two clock traces. Trace length for the first IC is 13.75 mm while for the other is 6.25mm, so 20 mm in total, and the trace impedance is 50 ohm.
- I have used 50 Ohm impedance tracks as this is the recommendation by various sources I have read. Yet I have not seen any references for impedance in the datasheet. So is that the right track impedance?
- The documents I have read recommend a daisy chain approach in clock distribution. However in this situation is not possible (PCB layout, opposite directions, BGAs). Is a star topology a bad approach?
- In star topology, do I consider the traces as two different traces (13.75mm and 6.25 mm respectively, each terminated back at the clock pad) or as single 20 mm?
- The datasheet provides a typical rise time of 1.4ns for 15 pF load. Is this rise time considered for this specific load or up to this load?
- According to Max Conductor Length calculators, for rise time of 1.4ns max conductor length is around 50 mm before transmission line phenomena kick in. Is this applicable to this situation as well, since a) it is a star topology, b) trace lengths as individual and as a sum are below that value?
Any recommendations, sources and things to look for are always welcome!