I am designing a board with mixed signals. There is an SPI bus and also an ADC chip. Now, during layout phase is it recommended to to have a specific impedance for these traces ? The length is not much for both SPI and analogue signals. Have seen some engineers do layout of SPI with 50Ohms characteristic impedance and analogue of 75Ohms. Is this so critical ? My SPI is a max of 2MHz and analogue of 100KHz max. So, is impedance considerations coming in for such a design ?

  • \$\begingroup\$ Take care of the proper grounding and proper distance between analog and digital signals. \$\endgroup\$ – Manu3l0us Aug 25 '17 at 5:59

If the signal Trise and Tfall are 10:1 longer than the time-of-travel along the PCB path (and the wiring path), you do not need controlled-impedance design. [Note some people use 20:1. ]

What this means is: with 100 nanosecond Trise and Tfall, the electrical length of the trace/wire can be 10 nanoseconds (about 5 feet), if standard FR-4 (with Er of 4.5) is used. People will debate about using one-way or round-trip delays.

Also read this answer: Distance between SPI traces to prevent cross talk

Note I edited this (about 5 feet) is correct, since sqrt(4.5) for FE-4 is ratio of 2.2 slower than speed-of-light. Thus 5 feet of PCB trace, atop FR-4 where most of the energy is in the underlying FR-4 and NOT up in the air, becomes 2.2X longer or 11 nanoseconds. I use 1 nanosecond/foot as good working number.


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