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31 August 2020

Fundamentals of Power Integrity: Characterizing PDN Noise

Figure 1. Noise tolerances for embedded system
components are becoming ever tighter.
Power integrity concerns maintaining the quality of power from generation to consumption in an embedded system. “Good” power integrity could be defined as having noise levels that are within tolerance. This short series will focus on characterizing noise on your power delivery network (PDN), with the goal of knowing where you must adjust your design to meet those tolerances.

Why do we care about voltage rail noise? As electronic designs strive for ever lower power consumption, power rails already carry very low voltages, often 1 V or less. Components like RF receivers, ADCs and DACs can be affected by noise of less than 1% of the rail value (Figure 1). This means noise tolerances can be as tight as single-digit millivolts, which is why power integrity takes up considerable validation time in labs.

Figure 2. A typical modern embedded system includes a power delivery
network (PDN, yellow blocks and traces), which must be engineered
so that system noise remains within acceptable tolerances.
Figure 2 shows a common topology for a modern embedded system, which we’ll use to illustrate power integrity concepts. A board-level PDN consists primarily of:

  • Voltage regulation modules (VRMs) that convert the bulk supply to DC current for consumption by devices
  • Board interconnects--the traces and planes that distribute power from the VRMs to the devices that consume power
  • Capacitors

Power integrity is most commonly addressed at the board level, but there are applications where it is desirable to look at the power integrity of the on-die distribution network inside an integrated device. (At the end of this post are links to previous posts discussing on-die power rail measurements.) Although these are usually considered separate realms, as we’ll show, there are cases where on-die effects can cause board noise, and vice versa.

First, let’s categorize the types of noise we expect to encounter in a PDN. Whether discussing on-board PDN or on-die PDN, noise can be placed into three broad categories:

  • Self-aggression noise—noise inflicted by a PDN component on itself through normal operation
  • Board pollution—noise coupled onto board packages and interconnects
  • Mutual aggressors— crosstalk coupling from the PDN onto devices

Each type of noise requires distinct measuring techniques, which we’ll describe in our forthcoming posts.

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