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MARCH 2020 I DESIGN007 MAGAZINE 41 ringing. Generally, when the trace length exceeds one-sixth of the electrical length of the rising edge rate, then termination is required. Regardless, it is always a good pol- icy to keep critical signals and re- turn paths as short as possible. Key Points • The source impedance of a digital IC driver is typically lower than the impedance of the transmission line. This results in reflections and electromagnetic radiation if not addressed • Whenever a signal meets an impedance variation, along a transmission line, there will be a reflection which can seriously impact signal integrity • Impedance discontinuities can also occur due to input gate capacitance, branches, stubs, or test pads; variations in dielectric materials; a neck- down in a via field; skew in a differential pair; or a gap in the return signal path • Series termination is excellent for point-to-point routes, one load per net. It works well for traces that are electrically short and is used to fanout multiple loads radially from a common source • Impedance back-matching slows down the rise and fall times and reduces the ringing (over/under- shoot) of clock drivers • Series termination is only used when there is only one receiv- er/load, and that receiver must be located at the very end of the transmission line • Subtracting the source imped- ance from the trace characteris- tic impedance gives the required series terminator value Figure 5(a): Single resistor differential termination. Figure 5(b): Differential center tapped termination.