High-speed circuits are used more or less everywhere in electronic applications today. As a result, the importance and the mechanisms of impedance (here in the meaning of PCB trace impedance - typically referred to as “characteristic impedance”) for signal integrity have been widely discussed and seem to be generally well understood by PCB designers.
Simply put, PCB trace impedance is a measure of the resistance that a circuit opposes to a current once a voltage is applied. So far so good, but the concept of impedance is also used in PCB design to describe the behaviour of power-distribution systems/power distribution networks (PDS/PDN). And this PDN impedance is becoming more and more of a headache for PCB designers as IC vendors are defining increasingly tight so-called ‘target impedance limits’ that a design must meet (just a few milliohms over a broad frequency range).
You are not sure what the term PDN impedance actually means for you and what you have to pay attention to when designing a PDN? Let’s take a look at what PDN impedance and target impedance are and let me take a stab at explaining their importance for the design of modern high-speed digital boards.