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DP83822EVM: Hardware design for using an integrated connector with the DP83822RHBR

Boris liu
Prodigy 230 points

Part Number: DP83822EVM

Dear Technical Support Team

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  • 0
    TI__Guru**** 189101 points

    Hi,

    As an alternative to the discrete magnetics modules, Ethernet connectors with all the required magnetics integrated into their housing (Magjack) can offer a quicker and easier design process, lower component count, lower assembly cost and often, smaller footprints than using a separate module. Reliability is improved as there are fewer solder connections in the design. But with the cores are relatively small and close together which can result in relatively high levels of crosstalk, and possible EMC susceptibility. 

    For the selection of Magjack, same component requirement applies as the discrete transformer. Please see this E2E FAQ on the transformer spec.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 230 points

    HI.David

    Thank you for your reply. It was very helpful. I have compared the performance of the two transformers. The 74980101200 is an RJ45 integrated with magnetic components, while the HX1188FNL is the isolation transformer used in the EVM.

    From the figure below, the comparisons of turns ratio, inductance, insertion loss, and return loss are shown in the table. Based on the comparison, I believe that from the perspective of magnetic component performance, replacing the HX1188FNL with the 74980101200 should not be an issue.

    As for the design of the resistive and capacitive components (75Ω and 0.001uF), what aspects should I consider or what performance comparisons should I make?

    For the differences in RC (resistor-capacitor) design diagrams between 74980101200 and HX1188FNL, please refer to the previous conversation content.

  • +1 in reply to Boris liu
    TI__Guru**** 189101 points

    Hi,

    The 75ohm is part of the Bob Smith Termination, The Bob Smith termination is used to reduce interference caused by common-mode current flows as well as susceptibility to interference from unused wire pairs on the RJ45 connector. For each wire pair, a 75 Ω resistor is connected to form a "star point”, the whole circuit is then galvanically isolated and connected to the chassis ground by means of capacitors. 

    On the capacitor, Ethernet standard requires 1500 RMS VAC isolation which is approximately 2200 peak DC. So the capacitor on the Bob Smith termination voltage rating value needs to be at least 2 kV. The capacitor helps in filtering the noise residue managing to leak out through the transformer.

    Thanks

    David