By Brian Hostetler – HPE, Inc.
This document explains the measurements made on a few different power rails on a printed circuit board (PCB) assembly. The primary task was to measure the impedance of a 3.3 V rail named VDD_3V3_S0. This rail needed to have low noise/ripple and provides 20.7 A to its load. It is a single-phase current-mode buck regulator fed from a 12V source.
A prototype Picotest 2-port probe was selected to make the impedance measurements. Ideally, the impedance of any rail is most important at the load. However, since the primary concern on this rail is the low-frequency impedance (e.g. control loop resonance), the impedance can be measured near the VR instead of at the load. In some scenarios, this can be easier and require minimal modifications to the PCB....
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