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82 DESIGN007 MAGAZINE I JULY 2021 In recent years I have been getting many questions about PDN filters from my course participants and from friends, colleagues, and even from strangers. Long gone are the days when the essence of power distribution design recommendation was "place a 0.1 µF bypass capacitor next to each power pin." Power dis- tribution networks used to primarily contain wires, traces, planes, and parallel bypass capac- itors, but very few had systematically designed filters. Just for clarifying the nomenclature, Figure 1 defines what I call a parallel PDN and a PDN filter. For the purposes of this article, the differentiation between a parallel PDN and a PDN filter is the intentionally included series resistive and/or inductive element in the PDN filter. e figure shows a very simple implementa- tion of a PDN filter. In actual circuits we may have much more complicated circuits. We may have higher order filters with cascaded sec- tions, filter sections connected in branching topologies, and we may also have filters that have to filter in "reverse direction," preventing the noise from spilling out from the load con- nected to the output of the filter. Or maybe we need filtering in both ways. In filter circuits like the ones in Figure 1, the usual question I get is: How do you describe the filter? Should we use transfer impedance (Z 21 ) or scattering transfer parameter (S 21 )? As I will show, in a lot of applications, none of these two would serve us well, we will need something else: a voltage transfer ratio. Ask The Experts— PDN Filters Quiet Power Feature Column by Istvan Novak, SAMTEC Figure 1: Definition of PDN filters.

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