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THVD1550: How to calculate the signal attenuation?

Yuan Tan63
Expert 3280 points
Part Number: THVD1550

Hi,

customer chose THVD1550 as 485 interface in CNC system.

the port rate and distance are known.

he concerns about the accurate degradation through transfer, could you please tell me how to calculate the attenuation amplitude?

is there any document about this issue?

thank you!

  • 0
    TI__Mastermind 25705 points

    Tan,

    It's a good question. What's the communication data rate in CNC system, and what's the distance? If it's high speed communication (like 20Mbps), usually it's the load capacitance that brings down the amplitude. The longer the cable is, the more it will attenuate the signal. We have an app note that collects all kind of data for THVD1550 with various speed and distance conditions. You can take a look. Please let me know if you have any questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Expert 3280 points
    Hi Hao,

    the port rate is 8M, and the distance is 30m.
    thanks for your answer, but I cannot find the result for 8MHz and 30m.
    could you help me with that?
    Thank you!

    Yuan
  • 0 in reply to Yuan Tan63
    TI__Mastermind 25705 points
    Yuan,

    What cable do you use in the application? From the other existing data, the signal should not be attenuated much with such a condition. I'll get some data for you.

    Regards,
    Hao
  • 0 in reply to Hao L
    TI__Mastermind 25705 points

    Yuan,

    Here is the data I took. The setup is two boards with a 120Ohm termination each. The signal is 8Mbps PRBS data over an 100ft cat 5 cable.

    This is the differential signal before the cable

    This is the signal after the cable

     

    The amplitude of the voltage drops from 2.54V to 1.76V, which is about 70%.

    Please let me know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Expert 3280 points

    Hi Hao,

    Thank you so much!

    the data is very useful!

    I want to know that can this result fan out on other 485 transceiver?

    I mean, if the customer chose a 3.3V one, will he get 3.3*0.7=2.31V?

    if the SCI is TTL, the signal cannot be recognized, right?

    Yuan

    Yuan

  • 0 in reply to Yuan Tan63
    TI__Mastermind 25705 points

    Yuan,

    Please notice the difference between logic signal (digital signal) of Rout, REB, DE, D and bus signal (RS485 signal). The attenuation applies to the bus signal only since it's transmitted over a long cable. You can see the difference in digital signal by the jitter, but the signal is always rail to rail.

    A simple way to evaluate the attenuation of high speed communication is by looking at the frequency response of the cable.

    With cable length and data rate, you can find the estimated loss in dB, then translate it in voltage.

    Please let me know if this makes sense to you.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Expert 3280 points

    Hi Hao,

    you are so nice. Thank you !

    Do you mean that the attenuation is only relevant with cable kind, cable length and  port rate? while the transceiver, communication mode influence jitter?

    does jitter influence attenuation?

    Yuan

  • 0 in reply to Yuan Tan63
    TI__Mastermind 25705 points

    Yuan,

    Yes your understanding about the factors impacting attenuation is absolutely right. I'm not sure what communicate mode you referred to. Do you mean transmitter mode or receiver mode? Jitter and attenuation are somehow related but they are not always dependent on each other. Jitter is more about timing (AC) while attenuation is more about amplitude (DC). Please let me know if you have more questions.

    Regards,

    Hao