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This part apparently supports ESD protection of eSATA. But SATA interfaces are AC coupled. This means their common mode voltages could well be centered around 0V, with negative voltage swing. On the diagram for this ESD part, I can see that there will be a problem with it clipping any negative voltage swing below -0.7V.
Am I understanding this right? How can this part possibly support eSATA? The other interfaces such as USB3.0 and DisplayPort are also AC-coupled. Am I missing something?
Most differential RX circuits by their nature require that the amplifiers have CMFB (Common Mode Feedback) indeed, differential amplifiers are mostly useless w/o this. So the chip automatically compensates for this itself. Some chips will have the CMFB pin available for monitoring or external capacitance but that is not strictly necessary. Of course on startup and plug/ un-plug situation this is disturbed, which can mean that the signals hit against the ESD rails. But the higher level protocols of re-transmit deal with this situation.
Additionally, one some advanced differential signalling situations, there is active equalization upon startup, so this packet corruption at this level is understood and handled in this case.
eSATA has a common mode DC float voltage and each driver is normally between the range of these diodes. -0.7 ~5+V. Although you are correct, SATA has a RLL code that is DC neutral and MAY be AC coupled, and then these ESD diodes will not work. AC coupling could be done after line receiver not before.
