Circuit Design: Know It All

Chapter 17

Dealing with High-Speed Logic

Walt Kester

Much has been written about terminating printed circuit board traces in their characteristic impedance to avoid reflections. A good rule of thumb to determine when this is necessary is: Terminate the line in its characteristic impedance when the one-way propagation delay of the PCB track is equal to or greater than one-half the applied signal rise/fall time (whichever edge is faster). A conservative approach is to use a 2-inch (PCB track length)/nanosecond (rise/fall time) criterion. For example, PCB tracks for high speed logic with rise/fall time of 1 ns should be terminated in their characteristic impedance if the track length is equal to or greater than 2 inches (including any meanders).