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Design007-Jan2018

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88 DESIGN007 MAGAZINE I JANUARY 2018 Ground Bounce Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Ground bounce, or more precisely, supply bounce, is the voltage produced between two points in the power deliv- ery path. It is fundamentally related to the total inductance of the current path and shared return paths and the instantaneous surge current delivered by the power sup- ply. Once again, we find that induc- tance is the covert enemy of the high- speed PCB designer. It is the primary cause of simultaneous switching noise and electromagnetic radiation. As edge rates continue to increase, the impact of intrinsic electrical characteristics become more pronounced. One of these inherent characteristics is the inductance found in the supply leads of all ICs. In this month's column, I will look at supply bounce and how to minimise the impact on high-speed digital circuits. Ground bounce arises from a common-mode potetial developed between an IC die substrate and the PCB ground return plane and is totally independent of the trans - mission line characteristics. The physical location of the device driver within the IC as well as the number of out - puts that are simultaneously switched, with respect to the common power and ground connections to the die, also has an impact. In addition, ground bounce is associ - ated with the dI/dt (change in current over time) of the output which depends on the switching speed of the driver gate. Ground (GND) bounce is generated by high-to-low transi- tions of the signal, whilst power (VDD) bounce to generated by low- to-high transitions as in Figure 2. The output buffer stage of a CMOS device is an inverter; thus, the outputs are switching low-to-high when their inputs are switch- ing high-to-low and vice versa. It is the current associated with switching the output transistors that generates ground bounce. Note that everything discussed here concerning ground bounce can equally be applied to the opposite effect: VDD bounce. Figure 1: Ground bounce may be tougher to master than a good jump shot. Figure 2: Output voltage and supply bounce.

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