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Unused High Speed LVDS output

I want to know how to treat unused LVDS out put if it should be open or some termination.

This is general question and please answer for following example case.

  -Clock distribution or multi channel LVDS buffer, and unused LVDS output is working at 1-2GHz.

Please answer about influence for EMI and used channel jitter ........

Regards. 

  • Hi Tony,

    In general, it is best not to leave unused outputs unterminated, in order to reduce possible system noise. For an LVDS output pair, a 100 ohm load between the differential signals is the proper way to terminate the output. The driver, by specification, is current driven and this will create a known balanced state for the drivers. Leaving outputs unterminated may lead to increased noise or jitter on the used outputs.

     Regards,

    Mike

  • Hello Mr. Mike,

    Thank you for your answer.

    Regards.

  • Hello,

    I understand your answer for the specific situation described but I guess it is not generic.

    I plan to use several SN65LVDS179D transceivers but 4 of them only as receivers. I left LVTTL input and LVDS outputs OPEN. From what I've seen, the internal fail-safe circuit will create a fixed output but with no load -> no current.

    Can you confirm it's a safe choice ? Can you give me the expected Icc_max in this configuration ?

    Thank you very much !

  • HI Thomas,


    The fail safe circuitry is only implemented in the receiver direction so this should not affect the devices operation in the driver direction. From the data sheet it looks like the driver is pulled to ground through a 300k ohm resistor which should help prevent the driver from switching, so you will be OK with leaving these unconnected. It is OK to leave the driver outputs disconnected.

    Unfortunately I do not have any data supporting the ICC max number in this conditions. You would have to assume that the internal circuitry is going to consume some power as there is no way to disable the driver but since you are not actively switching it should be something less than the typical value of 9mA.