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SN65C1168: Transceiver IC powered with two different power supply

Anoop kumar N
Intellectual 908 points
Part Number: SN65C1168

Hello Experts,

I have a DSC card stacked in Peripheral interface card (PIC) both having common power supply 5V and GND. The transceiver IC placed in the PIC is powered by this same 5V and the signals are connected to the transceiver IC in mother board through FRC ribbon cables. But in mother board, The 5V from the PIC is passed through a emi filter and this new5V and newGND powering the transceiver ic in mother board. The gate driver ic also powered from this new5V.

1. Does this connection can make any ground elevation ? The CM choke have a 0.5mOhs DC resistace.

2. Is it necessary that both Transceiver ICs shall powered with same ground.? if i replace the common mode choke with a 5V to 5V isolated DC power supply, does this transceiver circuits works ? 

Regards 

Anoop kumar 

  • 0
    TI__Mastermind 47237 points

    Hi Anoop,

    1. Yes this system could have a ground potential difference - the cm choke resistance + wire resistance multiplied by the ground current will introduce a ground shift. Just as a quick example - this may not reflect the system - but just to explain the concept - if you have 1A of current between the sources at 0.5mOhm that is a shift of ~500uV which is very small and you could assume is negligible.

    2. No - RS-422 devices do not need to be on the same ground plane. The A/B pins (receiver) have a recommended common mode range of -7V to 7V - i.e. the ground can have a significant shift  from driver to receiver without issue. As long as the shift doesn't cause the incoming data signal to violate the abs. max conditions it shouldn't cause data. Essentially the ground shift adds a DC level to the data signal -if the data signal + shift doesn't violate spec it should be fine. The driver doesn't have the same common mode range rating - but since its driving the signal it shouldn't be that large of a concern. 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 908 points

    Thanks. What do u mean by driver not having same common mode voltage rating?

  • 0 in reply to Anoop kumar N
    TI__Mastermind 47237 points

    Hi Anoop,

    In RS-422 devices, such as this one, only the receiver pins are rated for -7V to 7V input common mode ratings. Driver pins do not have this rating. This isn't a concern since in RS-422 systems there should only be 1 driver per bus - so all the voltages on the bus should be generated by the driver w.r.t. its local ground connection - so the common mode protection isn't necessary on the drivers - but its something that I want to point out as its an important distinction, but in the vast majority of applications its not really an issue. The input common mode ratings are there specifically to deal with ground potential differences so its assumed that the driver output won't have a ground potential difference when it sends out a signal - but the receiver could.

    Please let me know if you have any other questions.  

    Best,

    Parker Dodson