Techniques for Power Supply Noise Measurements

By Art Porter

November 20, 2006

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Due to the trends of higher switching speeds, faster slew rates, more active pins per package, and smaller signal swings, power supply noise has risen to a very high level of concern in new digital designs. Real-time oscilloscopes are commonly used to measure power supply noise. This article illustrates techniques for analyzing power supply noise by means of an example, and discusses selection and evaluation of tools for power supply noise measurements.

The Problem

Three trends conspire to elevate power supply noise to a very high level of concern in new digital designs:

• Higher switching speeds and slew rates.
• More active pins per package.
• More total gates on each IC all lead to larger and faster switching current demands on power supplies.

Increased active pin counts are often the worst offender, as pin drivers have much higher current demands than internal gates.

At the same time circuits are becoming more susceptible to power supply noise. Decreased unit intervals mean shrinking timing margins. Reduced signal amplitudes translate to reduced noise margins.

As with all engineering problems, understanding the problem and having accurate and precise measurement data to characterize the problem are essential to solving the problem.

Insights to "Noise"

Before we go any further, let's consider where noise comes from. Ideally there wouldn't be any noise on your power supplies. How did it get there?

In addition to simple Gaussian noise that arises due to unavoidable thermal processes, which by the way is usually NOT the dominant source of noise, almost all noise on power supplies comes from one of two sources.

Switching power supplies create their own undesired noise, usually at harmonics of the switching frequency or coherent to the switching frequency.

When gates, and especially output pin drivers, switch, this creates transient current demands on the power supplies. This is usually the primary source of noise in most digital circuits.

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