Book Recommendation -- By Eric Bogatin

Power integrity is both an important and confusing topic in today’s designs. This is partly due to the complicated nature of the effects that occur in the PDN, and partly because there is not just one power integrity problem, but many—each with their own root cause and specific design guidelines.

The Printed Circuit Designer’s Guide to… Power Integrity by Example from Mentor, A Siemens Business, explores the specific problem of via-to-via coupling through a cavity starting from the ground floor. When a signal via passes through the power and ground planes that make up a cavity, the vias provide a clean path for the signal current, but leave the return current to find its way through the impedance of the cavity.

This eBook explains a simple way to think about the impedance of the cavity and how the return current flowing through the cavity impedance generates voltage noise that is picked up by every other via passing through the cavity. As Fadi Deek explains, the way to control the via-to-via crosstalk is to control the impedance of the cavity.

Most of the noise is generated in the peak impedances of the cavity, not just the structural resonances that cause impedance peaks. Other important factors include the interactions of decoupling capacitors, their mounting, cavity spreading inductance, and cavity capacitance that can produce even larger impedance peaks.

An important message from this eBook is to know your parallel resonances because that will be where most of your noise comes from. Murphy’s Law suggests that if there are any impedance peaks in your PDN, some customer’s microcodes will drive worst-case signal currents through the cavity with a data pattern right at the peak impedance frequency, possibly generating pathological noise. Use the principles laid out in this eBook to help you avoid this catastrophe in your next design.