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DS90UB954-Q1: Question regarding power over co axial network.

nithish kumar
Prodigy 175 points
Part Number: DS90UB954-Q1

hi!

According to the datasheet of DS90UB954-Q1, The absolute maximum input voltage to the FPD_Links range from 1.35V to 2.75V. But in the Power over coaxial cable section, the coaxial cable voltage is pulled up to 9V! (VPOC_RX) and the same coaxial line feeds to the FPD Link lines just through a Coupling Capacitor. So my question is, Won't it destroy the DS90UB954-Q1 IC? since the data lines are pulled to a higher voltage than absolute maximum value.

  • 0
    TI__Guru** 103215 points

    Hello Nithish,

    PoC is a DC voltage and you will note that the DC voltage is applied on the line after an AC coupling cap. Capacitors act as an open to DC, so the 9V will not reach to the RIN pins. 

    This circuit is commonly referred to as a "bias tee" https://en.wikipedia.org/wiki/Bias_tee

    Best Regards,

    Casey 

  • 0 in reply to Casey McCrea
    Prodigy 175 points

    Thanks! But ,if we pull up the line to 9V, won't the signal have a logic level voltage of 9V? Will it not damage the IC since the signal is in the form of pulses and capacitor allows it to pass? please correct me if I have made a mistake in understanding this one.

  • 0 in reply to nithish kumar
    TI__Guru** 103215 points

    Hello Nithish,

    From the 954 side which is usually the side sourcing the power, the DC voltage is sourced from the other side of a inductor/FB network which means which PoC voltage is switched on, the inductive components will significantly slow down the transient voltage seen at the AC coupling cap and subsequently the voltage seen at the pins of the device. 

    On the opposite side where the camera plugs in could be more stressing if the cable is hot plugged because the DC voltage is already sitting on the cable. So it would not be recommended to hot plug the cable into a serializer module for this reason, however in reality there is also inductance in the cable, connector, PCB trace, etc. which limits the edge rate of the voltage transient during a hot plug event and it is very rare to see transient voltage high enough to damage the device this way too. However this depends on the PoC voltage applied, the cable, PCB, etc. 

    At the end of the day this circuit is has been commonly used in production platforms with these devices for many years already without issue. 

    Best Regards,

    Casey 

  • 0 in reply to Casey McCrea
    Prodigy 175 points

    Thanks a lot! This answered my question.