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Original question:

DP83867IR - center tap of RJ45 with pulse trans

DP83867IR: Center tap connection to Vdd

TISL
Expert 5120 points

Part Number: DP83867IR

Tool/software:

Dear E2E Support,

Our documentation confirms that connecting the magnetics center taps to a supply voltage is not necessary; but is there a limitation or any reliability issue if it's connected indeed to Vdd ?

Regards,

  • +1
    TI__Mastermind 26771 points

    Hi, 

    Because DP83867 is a voltage mode driver, we connect center taps to GND to provide the proper voltage reference. Connecting to Vdd will not provide the voltage reference needed for the voltage mode driver. 

    Best,

    J

  • 0 in reply to J
    Prodigy 160 points

    Hi,

    I am not sure I understand the answer.

    To clarify our situation:

    the center tap (PHY side) is decoupled to ground using a 100 nF cap as recommended (§8.2.1.1 of the datasheet).

    It is also directly connected to VDD as for a current mode line driver.

    But it seems that the DP83867IR has a voltage mode driver architecture. So we are wondering if the connection to VDD of the center tap may affect the device operation, reliability... Please advise.

    Regards,

    Nicolas

  • 0 in reply to Nicolas PLUMEREL
    TI__Mastermind 26771 points

    Hi Nicolas,

    Connecting the center taps to VDD will affect the device operation possibly not seeing the correct output on the copper side.

    Best,

    J

  • 0 in reply to J
    Prodigy 160 points

    Hi J and thanks for the quick answer.

    We observe that the differential output voltage is:

    +/-900mV when the center tap is connected to VDD

    +/-1V when it is not (expected levels)

    We do not observe any particular signal distortion when the center tap is connected to VDD.

    The datalink operates properly (no bit error at all) in both cases. Note that the length of the medium (backplane + cable) is pretty short (less than 50 cm) in our application, which ensures little attenuation.

    So the connexion of the center tap to VDD does not seem to prevent the datalink from operating reliably.

    But we are wondering if keeping our design as is may damage the DP83867IR in the long term: the driving stage of the part is more or less shorted to VDD through the transformer primary winding, possibly sinking too much current...

    Any opinion?

    Regards,

    Nicolas

  • 0 in reply to Nicolas PLUMEREL
    TI__Mastermind 26771 points

    Hi Nicolas, 

    We have not validated this combination internally so we cannot guarantee any long term effects. If you choose to go outside of the bounds of the datasheet recommendation, the validation responsibility for the performance of the device falls onto the customer. 

    Best,
    J