AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
Public
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1.1.1. FDTIM Decoupling Concepts

The key concept of the FDTIM method is the determination of the target impedance (ZTARGET) for the power rail under consideration. A reliable decoupling strategy warrants for the effective power rail impedance (ZEFF) to be lower than the ZTARGET from a few KHz up to the highest frequency of interest (fTARGET). The following figure illustrates this concept, where the solid horizontal blue line is the ZTARGET and the dotted vertical brown line is the fTARGET. The solid red ZEFF line has been designed by using various decoupling and bulk capacitors such that its impedance remains below the ZTARGET from DC up to fTARGET. With this design, power integrity is achieved from DC up to the target frequency of decoupling.

To design a reliable decoupling scheme using the FDTIM, perform the following calculations:

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