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VTTP_100 condition [DP83848I]

Takahiro Ogo
Genius 5940 points

Hi,

I have a question about VTTP_ 100 of DP83848IVV.
The following is DC specification of VTTP_100.



Is this measured differentially after 1:1 transformer?
Or is this mesuared TD+/TD- signal output? 

I think this indicates the specified value after transformer(1:1).
As you know, the IEEE 802.3 specification defines the required 100BASE-TX VOD (Voltage Output Differential).  Per the specification, this voltage must be 1V +/-5%.　So, 0.95 to 1.05V.  the transformer(magnetics) have ±2%.  If TD+/TD- is 0.95 to 1.05, it is fail for the IEEE 802.3 specification. (the VOD becomes 0.931 to 1.01)


If we obseve the following conditions,  the output of RJ45 becomes 0.95 to 1.05V and it pass the compileance test for VOD.
- RBIAS = 4.87 kΩ 1%
- Transformer(Mangnetic) = 1:1  2%

Is this understanding correct?

Best Regards,
Taka

  • 0
    TI__Mastermind 31600 points

    Taka,

    Your understanding is correct.  The transmit VOD (Voltage Output Differential) is typically measured after the transformer.  To make these measurements, we usually use a use a short (1-2") pig-tail cable with 100 Ohm terminations.  The cable would be plugged into the RJ-45 connector.  You can see a picture of the cable I use in the following E2E post:

    e2e.ti.com/.../805219

    As you have noted, it is important to meet the specified tolerances for the RBIAS resistor and the transformer turns ratio.  It is also important to use 1% resistors for the 50 Ohm (or 49.9 Ohm) termination resistors and it is important that the board traces have 50 Ohms impedance to ground or 100 Ohms differential impedance. 

    Patrick

  • 0 in reply to Patrick OFarrell
    Genius 5940 points

    Hi Patrick,
    Thank you for your kind support.
    I undestood!

    I hope the condition is described in datasheet. Becuase Micrel datasheet describs the condition for VTTP.



    That's my hope.
    Thanks!
    Taka