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Interfacing two XTR117 Devices to one MCU

Gaston_Melo_Arg
Mastermind 7675 points
Other Parts Discussed in Thread: XTR117, ISO7241C, ISO722, ISO721

Hi Collin,    

            my application requires two current loop transmitters 4-20mA (Two wire) in the same board. An msp430 will provide one PWM-DAC to each xtr117 to get the desired output.
In the forum I found this thread:

http://e2e.ti.com/support/other_analog/etc_analog/f/247/t/33298.aspx

that is something similar to what I want. But I already know that the problems is the GND point of each channel as is explained in the link. I was thinking to use two iso7241C (What I got here) to isolate each channe, but that according to the iso7241C datasheet, the current draw by each iso is about 8mA. So my question is:

1) this approach is correct?

2) Is there an alternative to the xtr117. What I mean is an IC that already have two 4-20mA outputs?

3) Is there an alternative to isolate the MSP430 and the xtr117, considering the current draw in the loop?


Regards

Gastón

  • 0
    TI__Guru 63185 points

    Hi Gastón,

    I recommend creating a new thread when asking questions on the forum, it's easier for us to find and make sure it's answered.

    Regarding your application, since you didn't mention it I'm going to assume the MCU gets power from somewhere separate from the two XTR117 circuits and is not powered from the XTR VREG circuit.  If that's the case, then it seems that you'll only need to isolate one digital PWM signal for each XTR117, so the 4-channel ISO7241C doesn't seem like the correct fit for this application.  I suggest you use two ISO721 (or two ISO722 for enable/disable function) devices to accomplish this design.  The ISO721 has a quiescent current of only 1mA so there will not be issues powering it from the VREG output of the XTR117.  

    You will connect the VCC1 and GND1 side of both isolators to the same supplies that power the MCU, and connect the VCC2 and GND2 pins of each ISO721 to VREG and IRET of the respective XTR117 devices.  

    Hope this helps!

  • 0 in reply to Collin Wells
    TI__Guru 63185 points

    Hello,

    Also, below you'll find a useful drawing on a similar post that can be used to describe the system you should create:

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/258994/909627.aspx#909627

    2234.8182.Document1 (1).pdf