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38 DESIGN007 MAGAZINE I MARCH 2020 down in a via field; skew in a differential pair; or a gap in the return signal path. Generally, the reflected noise level should be kept below 10% of the voltage swing, but this depends on the noise budget. In more conservative de- signs, the noise budget may be as low as 5%. There are numerous ways to terminate trans- mission lines, but the most common methods fall into three categories: 1. Series termination 2. End termination 3. Differential pair termination 1. Series Termination Series termination (aka source termination) is excellent for point-to-point routes—one load per net. It works well for traces that are elec- trically short and is used to fan out multiple loads radially from a common source (star routed) without affecting other circuits in the network (Figure 1). Impedance back-matching slows down the rise and fall times and reduces the ringing (over/undershoot) of clock drivers. A pulse is launched from the source, but due to the voltage divider network formed by the source impedance, series terminator, and the trans- mission line, only half the amplitude appears at the transmission line. As this half voltage pulse reaches the load, it is instantaneously reflected back along the trace. This reflected Figure 1: Series termination (back-matching). Figure 2: Matching the Spartan 6 driver to the transmission line (iCD Termination Planner).

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