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Original question:

SN65HVD72: SN65HVD72

SN65HVD72: SN65HVD72

Charles Corris
Prodigy 20 points
Part Number: SN65HVD72

According to a previous discussion, Tyler Townsend answered "It looks like these two waveforms show two different devices / drivers. The SN65HVD7x RS-485 transceivers do not have a built in offset as shown in the second scope capture". Yet when I simulate the chip in Pspice for TI, I get a waveform that shows an offset?

  • 0
    TI__Guru 51215 points

    Hi Charles,

    Can you please specify what you mean by offset? I believe this is referring to the common mode or average voltage of A and B, but I would like to be sure.

    The simulation you show here has a common-mode of around 1.4V as A and B drive between 2.3V and 0.5V for a differential of 1.7V. This is fairly typical and what we'd expect based on the datasheet specification of Vcc/2 for common mode and differential output of 1.5V - 2.0V across a load. 

    For the previous post, we see that the first and third examples have common-modes of nearly 2.0V, and differentials of 1.5V. These are also consistent with datasheet specifications. Also keep in mind that the Vcm measurement here can be impacted by ground shifts, so where the scope ground is connected relative to the transceiver can have an impact here. This may explain why the 2.0V common mode is larger than what we see in the simulation. 

    For the second example in the previous post (waveform with smaller differential), the high-level output appears much lower on the A and B channels while the low-level output remains consistent with the other waveforms. This lower high-level output decreases the common mode (1.6V) as well as the differential output voltage (0.8V) of the resulting signal. While this common mode is still within specified ranges, the differential voltage is too low. Because the data of an RS-485 signal is dependent on this differential, this can significantly impact signal integrity and should therefore be addressed. Typically we would see a decreased differential as the result of an overloaded bus with less than 54-ohms between the A and B lines. In this case, this doesn't appear to be true since another transceiver is able to drive the same bus conditions with a valid differential of 1.5V. It is for this reason we suspected that the device with the smaller amplitude may either not be operating correctly or is not the device specified by the SN65HVD72 datasheet. If you are able to provide lot trace codes (included on shipping label) for these two sets of devices or the top-side markings for them, we can look them up in our system and confirm that the codes match our records. 

    Let me know if there are other concerns you have and if you have any more questions. 

    Regards,
    Eric Schott