The PCB Design Magazine

PCBD-Aug2017

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42 The PCB Design Magazine • August 2017 As IC die sizes continue to compact due to demand for smaller and faster technology, and as switching speeds continue to improve, rise and fall times are creeping down into the sub- nanosecond realm, a territory previously re- served for microwave engineers. This relentless shrinking trend that perpetuates Moore's Law can create a huge problem for legacy designs as faster switching intensifies signal integrity is- sues. Over the years, as logic drivers have con- tinued to switch faster and faster, problems with ringing, crosstalk and electromagnetic emis- sions (EMI) have become progressively worse. It is a common quandary that established products that have worked flawlessly for years suddenly stop performing reliably, due to a new batch of ICs that is used in the latest produc- tion run. The cause of this problem is rise time shrinkage. Figure 1 illustrates the consequences of three different rise times for the same clock frequency. This example brings home two very impor- tant points. Firstly, for a given layout, faster switching produces spurious signals exhibiting excessive overshoot and ringing. This prob- lem is unavoidable. It can only be prevented, by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA When Legacy Products No Longer Perform BEYOND DESIGN Figure 1: Increased ringing with faster rise times (simulated in HyperLynx).

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