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Original question:

DAC8742H: Hardware changes to use HART and Field bus

DAC8742H: Using with 4-20mA receiver circuit

Remya R
Prodigy 180 points
Part Number: DAC8742H
Other Parts Discussed in Thread: TIDA-060020

Hi,

We are using DAC8742H HART Modem with 4-20mA receiver circuit.

The HART Output is coupled to the 4-20mA analog input circuitry.

We are referring TI Designs: TIDA-060020.

In this application note says that the HART specification requires a load resistance between 230 Ω and 600 Ω for the functioning and illustrated as below diagram

In our design, the loop current firstly enter into our power supply module. So R1 will be our power supply which can be assume as a variable resistance.

So could you please tell is there any problem will occur in the functioning of the HART circuit due to this variable load?

Also please share is there any other reference designs available for this kind of application.

  • 0
    TI__Guru 53433 points

    Hi Remya,

    The image you shared did not seem to get attached correctly.

    I am not quite sure what you mean when you say "the loop current firstly enter into our power supply module." Can you create a figure to illustrate what you mean?  The resistor, R1, would be used by the ADC to sense the current loop value.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 180 points

    Hi Paul,

    Could you please check the attached diagrammatic representation of our design.

    As shown in the diagram,

    When I compared my design with figure 4 of TIDA-060020, R1 is our power supply and R2 is our current sense resistor.

    So the total load resistance will become the sum of these two.

    But in our case, one is varying resistance( power supply). Is it affects HART transmission?

    Could you please analyze and reply?HART.docx

  • 0 in reply to Remya R
    TI__Guru 53433 points

    Hi Remya,

    This does not look like the return current would flow through the shunt at all.  I think you need something that looks more like this:

    The current path would eventually make its way to GND on the supply source side of the board.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 180 points

    Hi Paul,

    I think you mis understand my blockdiagram.The power supply I shown in the block diagram is a 3.3V DC DC converter which takes the loop supply as input and gives 3.3V output to power the rest of the circuitry in my design. Please check the modified block diagram I attached.3173.HART.docx

  • 0 in reply to Remya R
    TI__Guru 53433 points

    Hi Remya,

    I think the biggest issue is that you do not have your sense resistor in series with the loop.  In addition, the HART modem should be connected with a series resistance as well.  

    Please take a look at this diagram.

    Thanks,

    Paul