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About CAN Transceiver stub length

Gary An
Intellectual 440 points

Hi all, 

http://www.ti.com/lit/an/slla270/slla270.pdf?&ts=1590043577481

What I understand on the apps notes is that there are signal reflections on the tub-lines, because stub-lnes are unterminated.

This note shows if stub-length is 1/3 shorter than line's critical length(bus line length?), the reflection is minimized.

I do not understand how the stub-length is related with the reflection, and why it is 1/3 of the line.

Could you please help me to understand this?

Thank you

  • 0
    TI__Guru 62920 points

    Hi Gary,

    There are a couple of factors that determine the effect of a reflection on a signal: its amplitude and its phase.

    The amplitude of a reflection depends on the degree of mismatch in impedance seen by the signal (incident wave).  The bigger the mismatch, the bigger the reflection.  That's why termination is used at cable ends - when the signal reaches the end of the cable the impedance transitions from the characteristic impedance of the transmission line to the termination resistance value.  If these are matched, no reflection is generated.  Stubs are not terminated, though, and so when the signal reaches the end it will have to transition from the characteristic impedance of the line to something much larger (i.e., the receiver input impedance, which is often >20 kOhm).  So, the amplitude of reflections from stub nodes will always be fairly large.

    However, if the reflection bounces back and combines with the intended signal (i.e., the portion of the transmitted signal propagating down the main line) without much phase shift, then it will not have an observable effect.  The phase shift associated with a stub depends on its length - the longer the stub, the higher the phase shift.  That's where the 1/3 critical-length rule of thumb comes from.  It ensures that reflections from the stub interact with the signal occur only near the transition period and limits how observable their impact would be.

    Regards,
    Max