Published by Signal Integrity Journal, May 5, 2022
I receive many questions about measuring power rail noise: Do I really need to use a power rail probe to measure ripple and noise? Power rail probes are single ended, so does that mean I can only monitor one power rail at a time? Are there other, more affordable options available? In this blog post, I am going to answer these questions. There are plenty of posts on making your own budget probe. I do not recommend this for a variety of reasons, such as uncertainty in the results, lack of calibration and certification data, and safety issues. It is also a poor use of expensive engineering time. Because of this, I only considered off-the-shelf probes in my testing. The questions I get are generally related to mixed signal and application-specific integrated circuit (ASIC) boards, so I’m going to limit our discussion here to power rails of 5 V or lower, with the assumption that we want to see the direct current (DC) voltage on the screen. This definition disqualifies the use of DC blocks, which in some cases are a valuable and viable solution for improved sensitivity or higher voltages. BEWARE, NEVER EXCEED THE INSTRUMENT RATING, EVEN WITH A DC BLOCK. To quantify and compare solutions, figures of merit are defined that can be scored and weighted. For the purposes of measuring power rail noise, appropriate figures of merit include:
Bandwidth / flatness
Common-mode rejection ratio (CMRR)
Noise / signal-to-noise ratio (SNR)
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