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XTR116: Can multiple digital isolators share the same communication bus (i2c/SPI)?

Joel T
Prodigy 10 points
Part Number: XTR116
Other Parts Discussed in Thread: XTR300

Hello,

I want to integrate three 2-wire transmitters, each with its corresponding DAC and digital isolator, and connect the isolator to an MCU via an interface bus.

Question: Can I share the same bus for the MCU and three digital isolators instead of having separate buses for each digital isolator?

Regards,

Joel

  • 0
    TI__Mastermind 18957 points

    Joel,

    1. In most cases the answer to this is no.  You should not connect multiple 2-wire transmitters to DACs and digital isolators.  Below are the reasons why this cannot be done:
    2. A 2-wire transmitter is powered by the loop.  This means that the REG pin on the XTR116 is designed to power all circuitry associated with the XTR116.  The low power case is when the transmitter is outputting 4mA.  In this case the current that the REG pin can deliver is 4mA - IQ(XTR116).  Normally, the REG supply is used for a small simple circuit such as an amplifier and sensor.  The DAC, digital isolator, and other circuitry would all need to be powered on less than 4mA.
    3. A 2 wire transmitter has a return pin that acts as a system ground for all other components.  The voltage on the return pin for each XTR will be different from each other and will depend on the output current.  Connecting multiple 2-wire XTR devices return pins together will create a bus conflict. 
    4. The two wire transmitters are intended to be a single self-contained sensor transmitters that are powered by the loop (see figure 8-1).
    5. The way you typically accomplish connecting multiple transmitters to a single interface bus would be to use a 3 wire XTR such as XTR300.  This type of module is not loop powered.  Power for this type of circuit comes from the V+ and V- power supply, so the power for the other circuity is also powered by the V+ and V- supply which eliminates the max 4mA limitation that the loop powered systems have.  Also, the 3-wire systems have a true ground and not an Ireturn.  Thus, you can connect the ground of multiple XTR300s together.
    6. This video ( https://www.ti.com/video/6221985839001 ) covers the operation of 2-wire transmitters.  At time stamp 11:10 an application where two 2-wire transmitters are used with an isolator.   The video shows how the return pins cannot be connected together.  It also shows how two independent isolators can be used where the return pins are not connected together.  Note that for this case the DAC and isolator are still powered by the loop and so they can't consume more than 4mA.  So, you can possibly do something similar to what you want to do, but you cannot share on bus.  I think in almost all cases like this it makes more sense to use a 3 wire transmitter.

    I hope this helps!

    Best regards, Art