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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Limits on the Y-Capacitance in Magjacks
The basic purpose of Y-caps is EMI suppression. It is a path for common-mode noise to flow into the chassis and away, thus preserving signal integrity, since we know that data signals exist in the differential domain, not in the common-mode domain.
Should we make the Y-caps big or small? Perhaps for EMI reasons we would want to make them as big as possible, but that is not a good solution overall. For example, for ensuring isolation, we have already realized that if the Y-cap is too big, its impedance is very small, so a larger voltage will appear across the termination (blocking) caps (Cx). If we made Cy equal to 22 nF in Fig. 9.11, the voltage on Cx would, by simple divider action, be 2500 V/2 = 1250 ...

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