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THVD1452: impedance tolerance

Michiaki Tanii
Expert 4650 points
Part Number: THVD1452


Hi team,

 

Customer would like to understand how much tolerance for the impedance THVD1452 can be accepted at 30Mbps, full duplex communication.

Could you share some data with customer ?

 

Thank you and best regards,

Michiaki   

  • 0
    TI__Guru 74322 points

    Michiaki-san,

    Do you mean the bus impedance tolerance?

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Expert 4650 points

    Hi Eric,

     

    Yes, customer is concerned about the tolerance of the cable and would like to know how much tolerance can be acceptable for THVD1452.

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru 53575 points

    Hi Michiaki,

    Most of our RS-485 drivers are designed and specified for cabling with an impedance of 120-ohms. Matching termination resistance with this cable impedance is needed to reduce signal reflections in the network. This termination resistance provides a load to the transceiver and this load is what the transceiver is specified to drive. Because 120-ohms is a common impedance value, many of THCD1452's characteristics are specified with a load (RL) of 60-ohms (two 120-ohms in parallel), or slightly larger loads (i.e. 54-ohms) to account for loads presented by other transceivers. 

    If cabling with different characteristic impedance is used and termination values are chosen to match, the differential output voltage capability of the transceiver will reflect the resulting load. For instance, if a impedance and termination value of 100-ohms is used, the output differential will be lower than typical because the transceiver will need to provide more current to maintain the same output voltage. 

    Did I understand your question correctly? Let me know if there's anything I can clarify or elaborate on.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 4650 points

    Hello Eric,

     

    Thank you for your kindly explanation.

     

    You mean that what the communication can be ok or not is depending on the load if the impedance of cable has large tolerance ?

    And also customer would like to know if RS485 communication at 30Mpbs can be acceptable without impedance control.

    In my understanding, the case wouldn’t be accepted due to large reflection. Is that correct ?

    And could you share the application report described termination resister to understand the impedance control ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru 53575 points

    Hi Michiaki,

    Yes, most RS-485 drivers are capable of supporting a range of bus loads. 

    The achievable data rate of system is dependant on many variables including the bus topology (where nodes are placed), bus length, transceiver selection, transceiver load, etc. If other variables are accounted for, most systems will be able to tolerate some impedance mismatches without compromising data integrity. 

    What are you referring to when you say "without impedance control"? Does this mean that there will not be termination on the bus? Or do you expect the cable your using to have poor impedance tolerances and may vary? 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Guru 53575 points

    Please see this application report for more information on RS-485 data rate, termination, and bus configurations.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 4650 points

    Hello Eric,

     

    Thank you for your kindly explanation.

     

    I dig into the detail for the customer’s use case as following.

     

    • RS422 communication
    •  Point to Point (1:1)
    • Length : 1.5m
    • Tolerance of impedance : 100 ohm ± 50 ohm
    • Target data rate : 30M bps

     

    With the above condition, can customer achieve the target data rate at 30M bps ?

    If no, how much tolerance can be accepted for the cable ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru 53575 points

    Hi Michiaki,

    The only point where I see an issue might occur is when the bus load is lower than 54-ohms (lowest Vod is specified here). Because it sounds like only the impedance has this variance and the termination resistance could remain above this value, it doesn't sound like it will impact achievable data rates.

    Apart from above, I see no issue with these requirements and THVD1452 will be a good fit here.

    Regards,
    Eric Schott