A common design technique for power distribution networks (PDN) is the determination of the peak distribution bus impedance that will assure that the voltage excursions on the power rail will be maintained within allowable limits, generally referred to as the target impedance. In theory, the allowable target impedance is determined by dividing the tolerable voltage excursion by the maximum change in load current. Measuring in the time domain offers a large signal measurement solution, however, this test method is much more difficult to perform because the ability to control very high speed current steps is challenging, and may not even be possible. The time domain measurement result can also be easily misinterpreted. This article focuses on the fundamental flaw of using target impedance as an assessment method. Using simple, lumped element models and both frequency domain and time domain simulations key issues are highlighted. A high performance optimization simulator (ADS) is used to determine the best- and worst-case voltage excursions for a given tolerance.
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