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DP83TC812S-Q1: Cable Shielding and grounding

Rahul Khedkar
Prodigy 20 points

Part Number: DP83TC812S-Q1
Other Parts Discussed in Thread: TIDA-020061

Tool/software:

Hello, 

For automotive ethernet 100Base-T1, shield is connected to board ground with capacitor and resistor network having low impedance at frequency of interest (TI reference design 100MHz). 

I am also looking at this application note which specifies on page 3: Shield is not required provided it matches cable specs. 

 "It only requires an unshielded single twisted-pair cable to transmit and receive data at 100 Mbps speeds over communication distances of at least 15 m."

What could be the impact if cable is connected to board ground directly? 

Thanks. 

  • 0
    TI__Mastermind 28442 points

    Hi Rahul,

    Please share which reference design you are looking at. 

    Are you planning to use unshielded cable or shielded cable? Typically 100Base-T1 designs use unshielded cable.

    Thanks,

    David 

  • 0 in reply to David Creger
    Prodigy 20 points

    Dear David, 

    Please see points below: 

    1. I am referring to TIDA-020061. Design uses C19 with MPN GRM31BR73A102KW01Lwhich has lowest impedance in 100MHZ to 300MHZ. 

    2. Our Product requires connector which is not rated specifically for 100Base-T1 application. Length of pins will be much lesser than Wavelength / 10, if we consider signal integrity criteria. Is this right approach? 

    3. Noted that 100Base-T1 use unshielded cable. I am trying to offset some of the effects caused due to connector mismatch by using shielded cable and using right shield termination on board. Please advise if using shielded cable is overkill. 

    Thanks.

     

  • 0 in reply to Rahul Khedkar
    TI__Mastermind 28442 points

    Hi Rahul,

    Section 96.7 of the IEEE802.3bw standard (100Base-T1) defines the required link segment characteristics.

    Characteristic impedance of cable and connector but be equal to 100 ohms. 

    Shielded cable can be used with 100Base-T1 PHYs. This is generally not chosen due to cost and weight concerns, but the design can work fine with it.

    Thanks,

    David