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SN65HVD73: Differential Paris Impedance

Shubham patil
Intellectual 520 points
Part Number: SN65HVD73


Hi Team,

We are planing to use this SN65HVD73 for RS485 .

They are two differential Paris signal can please suggest how much impedance need maintain this two differential paris

If possible plz share the Layout guideline for this   

What is spacing between two paris.

Thanks

Shubham  

  • 0
    TI__Guru 62920 points

    Hi Shubham,

    A typical target for both PCBs and cables used for RS-485 is to maintain a differential characteristic impedance of 120 Ohms.  Other values can work, but this is the most common impedance and aligns with the load assumptions used in characterizing transceiver performance.

    The impedance of a PCB trace depends on a few factors like the width/spacing, the height above a reference plane, and the permittivity of the dielectric material used.  Most PCB layout tools will feature a trace impedance calculator that can be used to tune the trace geometries based on an impedance target.  If your tool does not have such a feature, you can find many free calculator utilities online - just search for "PCB trace impedance calculator."

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    Intellectual 520 points

    Hello Max,

    Thanks for fast response.

    we know about the PCB trace impedance we can do that one 

    But we need to know that from your side it is hard requirement for 120 Ohms impedance????

    If this is not a hard requirement what will impact on performance.

    it is possible for you to share spacing between two differential pair signal .

    we need spacing between P and N signal.     

    Use attached image 

  • 0 in reply to Shubham patil
    TI__Guru 62920 points

    That figure in the datasheet is just a simplified diagram, it is not a required layout and was not intended to show exact trace geometry.

    However, 120 Ohms impedance is not a hard requirement for the transceiver.

    For optimal signal integrity, it is typically best to make sure that the impedance is matched between the PCB traces, the transmission cable, and the end-of-line termination resistances.  This is because reflections can be generated at any interface between dissimilar impedances that is encountered by signals propagating through the network, and these reflections can interfere with the intended signal.  However, since the routing distances on the PCB tend to be fairly short compared to the lengths of cables used, the effect of the PCB impedance on signal integrity is typically very minor.  Whether you had a 50-Ohm impedance versus a 200-Ohm impedance on these traces you are unlikely to see much of a difference.

    If you are talking about changing the overall impedance of all components (including not just PCB but cabling and termination as well), then note that this is OK as well but may affect the overall signal amplitude (lower impedance creates higher loading and thus a smaller transmitter output).

    Regards,
    Max