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SN65LVDS049: Determining characteristic impedance for wiring harness

Wiley Post
Prodigy 10 points
Part Number: SN65LVDS049

Working with a wiring harness manufacturer and had a quick question in regard to the characteristic impedance needed for the transmission lines when using the SN65LVDS049 to send a differential signal. We are sending a SPI differential signal, we know that the wires need to be twisted pair and have a differential impedance of 100 ohms, this is the first time we have used any kind of differential signal and are a little confused about what exactly the characteristic impedance of each individual single-ended transmission line should be. Does 100 ohm differential impedance mean that each line of the twisted pair should have a characteristic impedance of 100 ohms, or should each one have a characteristic impedance of 50 ohms totaling to 100 ohms over the twisted pair?

  • +1
    Guru 257000 points

    The single-ended impedance indeed is about half the differential impedance (the factor becomes less than two for tightly coupled lines). But only the differential impedance matters.

    The SN65LVDS049 is designed for loads of about 100 Ω; you can use cables with a slightly different differential impedance as long as you adjust the termination resistor.