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40 DESIGN007 MAGAZINE I FEBRUARY 2020 In last month's column, I looked at why in- terconnect impedance is so important to the performance of our system. I also mentioned that impedance is defined in both the time and the frequency domains. In the time domain, we need to consider the impact of the inter- connect impedance on the propagation of elec- tromagnetic energy. However, in the frequency domain, the AC impedance needs to be kept below an acceptable level to prevent excessive radiation. Decoupling capacitors are generally spread throughout the power distribution network (PDN). In the past, we thought that it was only necessary to add a few capacitors to each IC power pin to stabilize the power at that device. That worked fine at low frequencies. Think- ing in the time domain, you can say that de- coupling capacitors store and supply charge to the loads until the power supply can respond. However, in the frequency domain, decoupling capacitors also lower the impedance at differ- ent frequencies to help meet the AC impedance target. Thus, there are two distinct functions of capacitors that work in unison but in different domains. In this month's column, I will look at impedance in the frequency domain and its impact on EMI. The PDN acts like an on-board ecosystem (Figure 1). If one small part of the PDN were to change, then the entire network performance may be impacted. This is particularly notice- able when mounting, loop, and spreading in- ductance are added to the profile of the capaci- tors. Inductance has its greatest influence in the high-frequency band, where odd harmon- The Impact of PDN Impedance on EMI Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: The PDN acts like an on-board ecosystem.

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