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TLK1221: Differential termination of TX.

Paul Connell
Prodigy 30 points
Part Number: TLK1221

I'm getting signal amplitude about 2x expcted amplitude when terminated with 100 ohm differential.

I have tried various configurations, including connecting TX lines backto RX lins of TLK1221 with the termination shown in data sheet figure 12 with the same result.

In order to get the expected signal level is to terminate with 50 ohms diffrential, not 100 ohms as shown on the datasheet  figures and the implemented on the Eval board.

Can someone confirm the TXn and TXp single ended impedance and voltage launce amplitude?

Thanks

Paul Connell Crestron

  • 0
    TI__Genius 16150 points

    Hi Paul,

    Have you tried the configuration of our TLK1221EVM (Page 9B)?

    www.ti.com/.../sllu100.pdf

    Output impedande value is not specified in datasheet but it should be in the 30 - 50 Ohm range. As well, V(CM) Receiver input voltage requirement, VID = | RxP - RxN |.This parameter refers to RX input differential peak voltage. Are you getting 2x expected value?

    Best Regards,
    Luis Omar Moran
    SWAT Team

  • 0 in reply to Luis Omar Moran
    Prodigy 30 points

    Hi Luis-

    Thanks for the reply -

    If you are referring to the first page of the schemaitc, yes. With TX feeding back to RX I have 1700mVpp across R1, not the 850 I am expecting.

    Regards,

    Paul

  • 0 in reply to Paul Connell
    TI__Genius 16150 points

    Hello Paul,

    Sorry for the confusion. The value specified in datasheet VID & VOD reflects the absolute value of the subtraction of signal positive - signal negative.  The plot you are showing above reflects VODpeak-to-peak. Even in figure 7 of datasheet you can see the range from ~-1V to ~+1V (~2Vpp). So, the value you are seeing is into the specified range.

    Best Regards,

    Luis

  • 0 in reply to Luis Omar Moran
    Prodigy 30 points

    Luis -

    There is one word use I don't understand that prevents my understanding of your answer: the word "rest" in:

    "absolute value of the rest of signal positive - signal negative". I don't know what that means.

    I do not have the diagram that you show here in my datasheet, so VOD(p) , VOD(d), VOD(pp_d) have no definition for me.

    What I need  to know is what voltage I should expect if I terminate the TX with 100 ohm differential termination.  I expect 850mVpp (typ), I am getting twice that.

    The FPGA that is receiveing this signal is expecting 850mV typ and can only handle 1100mVpp max, I will damage the part with 1700mV.  The FPGA has 100 ohm ODT.

    Thanks

    Paul

  • 0 in reply to Paul Connell
    TI__Genius 16150 points
    Paul,

    I confirmed with the design team of the device and we a re right, Vod = |TXP-TXN| where the || represent absolute value. This mean that the spec is the amplitude of the differential output TXP-TXN. If the plot is of TXP-TXN it should swing between +850mV and -850mV and give a peak-to-peak value of 1700mV.

    Could you provide the model of you FPGA to double-check the compatibility with the TLK device?

    Best Regards,
    Luis