An Overview of PDN Impedance Analysis

The PDN (Power Delivery Network) in a PCB has multiple elements that contribute to its impedance. Power rails on a PCB are part of the structure of your PDN. At zero frequency, the impedance is only the DC resistance defined entirely by the conductivity and geometry of all conductors in the PDN. Above DC, the impedance can be a very complicated curve in the frequency domain due to the complex geometry of the conductors in a PDN. Just as is the case with a typical high-speed interconnect, a PDN will have some impedance that defines how the electromagnetic field interacts with the structure.

Whenever a component in a PDN on a digital PCB switches states, the component will draw a burst of current into the PDN during the switching event. Just like other systems with some reactance, this can lead to an oscillating voltage seen at all components on the PDN, depending on the form of the PDN’s transfer function and impedance spectrum.

When the component stops switching, there is a transient decay back to the nominal DC voltage supplied to the PDN. The impedance will determine how the drawn current waveform produces a voltage fluctuation seen at the downstream integrated circuit, just like the driven and transient oscillatory responses seen in an RLC circuit.

Cadence Sigrity OptimizePI technology does a complete AC frequency analysis of boards and IC packages. OptimizePI – PDN Impedance Checking is a dedicated workflow to quickly verify if the PDN impedances for devices meet the target impedances, and if the PDN has high resonances which may cause EMI issues. OptimizePI will calculate impedances at IC devices and EMI observations by fixing all capacitors.

Watch the video here to learn more about PDN impedance analysis in Cadence Sigrity OptimizePI tool.