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ONET1141L Non Standard Load Impedance

Carissa Pocock
Prodigy 130 points
Other Parts Discussed in Thread: ONET1141L

Hello-

I am looking at using the ONET1141L part and I would like to drive a non-standard load impedance (closer to 25 ohms than 50ohms) and I was wondering what kind of impact that would have on the 2Vpp maximum output voltage.  Ideally I would like to use each output (OUT+ and OUT-) to each provide a 0.4-2.4V voltage to a 25 ohm load where OUT+ and OUT- and 180 degrees out of phase from each other.

I see in the datasheet that there is an internal 60ohm impedance on each of the output (OUT+ and OUT-) drive lanes and that the design is optimized for a 50 ohm load. Is this depiction fairly accurate and could I essentially assume that the output voltage would be approximately halved when the load impedance is halved?

Thank you in advance

 

  • 0
    TI__Genius 16150 points
    Hello Clarissa,

    The internal back termination of 60 Ohm are tied to VCCO. The OUT+ and OUT- output pins are designed to drive a 50 Ohms load and to obtain 2.0Vpp máximum output voltaje. Hence, if the load is not matched to this impedance (50 Ohms) the system will have reflections and losses affecting the signal integrity of the device.

    I hope this helps.

    Best Regards,
    Luis Omar Morán Serna
    High Speed Interface
    SWAT Team
  • 0 in reply to Luis Omar Moran
    Prodigy 130 points

    Luis-

    Thank you for the response. Yes, I do understand that the ONET1141 is designed for a 50ohm termination impedance and that a 25 ohm load will have reflection and losses. I also understand that the back termination of OUT+ and OUT- is 60ohm. What I would like to better understand is how will the ONET1141 be impacted.

    How accurate is the simplified block diagram on page 2 of the datasheet? Does it correctly depict the output driver or is there an additional buffer that I am not seeing?

    If the output could be modeled by a source voltage and source impedance driving a load, what would those values be? On Page 3 of the datasheet, VCCO is specified as 3.3V +/- 10%. If I assume I can model the output stage source as VCCO and that the source impedance is 60ohm, the math doesn't work out to have a 2.4V peak voltage with a 50ohm load. Is there something in the model I am missing or you could help me understand to get a better idea of how the ONET1141L and my non-standard load would be impacted?

    Thanks,

    Carissa

  • 0 in reply to Carissa Pocock
    TI__Genius 16150 points
    Hi Carissa,

    Basically the ONET1141L output can be modeled as a simple voltage source with 60ohm source impedance and a single-ended voltage swing of between 400mVpp and 4000mVpp (unterminated).

    Regards,
    Luis