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THVD1550: At a rate of 10Mbps, 100 feet of cable length, its signal jitter

Rhea Gao
Expert 2580 points
Part Number: THVD1550
Other Parts Discussed in Thread: SN65176B, TVS1400

Tool/software:

Hi,

What is the signal jitter of THVD1550 at a speed of 10Mbps and a cable length of 100 feet? Is it less than 20%?

According to the documentation, the cables are all 1,000 feet long. Please help to confirm.

Thank you~

  • 0
    TI__Genius 13642 points

    Hi Rhea,

    I see that you tried to upload an image, but it is not showing up properly. For future reference, uploading a Powerpoint or spreadsheet file with the images will work better.

    Figure 24 in the datasheet shows the general relationship between the cable length and data rate. Based on that graph, 10Mbps at 100ft should have less than 20% jitter assuming all datasheet recommendations are followed properly. 

    Best regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Expert 2580 points

    Hi Ethan,

    Sorry I forgot to add picture information.

    I also found this document information.

    Thank you for your support！

  • 0 in reply to Rhea Gao
    TI__Genius 13642 points

    Thanks for sharing that Rhea.

    As you mentioned, that document only tests for the longer transmission lines >1000ft to show that RS-485 can transmit at those distances. RS-485 also supports all smaller ranges of cable length.

    I do not see any major concerns transmitting 10Mbps over 100ft. I have highlighted Figure 24 where you would be transmitting:

    I would be happy to review any schematic or layout to ensure that the design minimizes noise and jitter from the bus. Additionally, 2 of our RS485 Half Duplex EVMs connected over 100ft of impedance matched cable could be used to test your particular setup.  

    -Ethan

  • 0 in reply to Ethan Sempsrott
    Expert 2580 points

    Hi Ethan,

    The customer will change the difference RC parameter down to THVD1550 on this SN65176B schematic.

    Update will be sent again.

    Thank you ~

  • 0 in reply to Rhea Gao
    TI__Genius 13642 points

    Hi Rhea,

    I apologize for the delay. Here are my comments:

    • There are failsafe resistors that are not needed for THVD1550 (1K Ohm near the 100pF caps). SN65176B requires an external failsafe configuration, but THVD1550 has it internally so those resistors are no longer necessary.
    • What are the 10 Ohm resistors intended for? (near the 100pF caps) If these are for surge protection, they are likely not needed since you have TVS diodes placed. Series resistors will decrease the drive strength, and these can be left out unless directly needed.
    • I believe those ESD diodes SMBJ5.0CA have a 5V working voltage rating. I recommend a working voltage of 12-15V since the common mode range for THVD1550 is +/-15V. A device like TVS1400 would work better: https://www.ti.com/product/TVS1400
    • Another note is that the 100pF on the A/B bus lines may not be needed. The TVS diodes typically have 80-200pF already and adding to much capacitance will decrease the signal quality at high data rates. 

    Let me know if that helps or if you have any other questions,

    Ethan