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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Measuring PSE Port Voltage
The IC ground is the Source terminal of the PSE’s low-side N-FET (or the other end of the sense resistor connected to it, if any). That is rightfully the PSE-system ground plane too (not to be confused with the overall system ground which is on the host side). Some engineers however prefer to call the upper rail (“48V”) as the PSE’s system ground, in accordance with the old telecom convention of positive ground. But as explained in Chap. 1, that has no relevance to PoE, so it makes more sense to have the control IC’s ground as the PSE system ground—why have multiple ground planes?
But if we place the oscilloscope ground clip on this PSE system ground (Source terminal), we do not see what we really want to monitor: ...

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