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SN75LVDS9637: Noise immunity

Ian Benton
Prodigy 100 points
Part Number: SN75LVDS9637

Tool/software:

Does LVDS have better noise immunity than RS485?

(Your little video just said that they were both good)

I would expect RS485 to be better because of bigger signals.

  • 0
    Guru 280000 points

    Yes, RS-485 uses higher voltages and currents, so it has more noise margin. But LVDS uses less power and allows higher speeds.

  • 0
    TI__Genius 14681 points

    Hi Ian,

    Good question. Clemens makes good points, RS-485 will have more noise margin, while LVDS uses less power and can achieve higher data rates.

    The noise immunity will likely depend on your implementation. For example, if you are looking to use RS-485 in half or full duplex implementations, you would be comparing the noise immunity with MLVDS devices configured in half or full duplex implementations.

    Here is an interesting application note that does a comparison of noise immunity of 3.3V RS-485 with 3.3V M-LVDS: TIA/EIA-485 and M-LVDS, Power and Speed Comparison. It notes that “Devices compliant with the M-LVDS standard do not provide as much noise immunity as RS-485, but these devices can provide signaling rates that exceed RS-485 capabilities.”

    Also keep in mind that other factors impact noise immunity, such as cable construction and shielding.  

    Regards, Amy

  • 0 in reply to Amy
    Prodigy 100 points

    I have two PWM signals that I want to get from one side of a cabinet to the other. No more than 20kHz, but the inside of the cabinet is noisy (lots of power control)

    I would rather not have to find enough current to run a DS9637 at the receive end. A 65LVDS9637 would suit the power supply rather better! The circuits would be identical, just a simplex system and a drop in replacement of one IC for the other. 

    I was wondering if it would compromise the signal. 

    Obviously, the voltage reduction from 3V to 0.3V would suggest 20dB worse immunity, but decades of development of the circuitry since the DS9637 arrived on the scene might make up for it.

  • 0 in reply to Ian Benton
    Guru 280000 points

    For differential protocols like LVDS and RS-4xx, common-mode noise is ignored. (This assumes that the outside noise affects the two differential lines in exactly the same way. You should use twisted-pair cables. I do not know how much shielding you can afford.)

  • 0 in reply to Ian Benton
    TI__Genius 14681 points

    Hi Ian, 

    RS-485 will have greater noise margin - 4x greater than LVDS. The question to use RS-485 or LVDS will depend on your application, especially around your expected cable length. If the cabling is not too long and a ground shift greater than +/-1V is not expected, then LVDS will work.

    Here is a helpful explanation of LVDS common mode and ground shifts (page 3): An Overview of LVDS Technology (ti.com)

    Regards, Amy