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DAC161S997: HART communication problem under 5 mA in current loop

Ivan Bastos
Prodigy 30 points
Part Number: DAC161S997
Other Parts Discussed in Thread: ISO7141CC

Hi,

We have developed our 4-20 mA loop current module that uses the DAC161S997 and it is working fine.
We have been able to communicate using the HART protocol, however with a critical limitation. When the current loop is below 5 mA, the HART signal is severely attenuated, we can only communicate within the range of 5-20 mA, which is not acceptable in most industrial processes.
Could you give us any insight on what is the cause of the problem or a suggestion regarding the circuit connections to our module. Please find in the attachments the circuit diagram of our module.We are using a fintHRT T210 HART interface module directly into the 4-20 loop (pins 2 and 5 from CN1).

Thank you.

DAC161S997_HART.pdf

  • +1
    TI__Guru* 93975 points

    Ivan,


    The schematic you show is similar to what we show in the simplified schematic, shown on page 1 of the datasheet:

    In your schematic the loop power comes from the L78L24ACUTR, and the LM2936MX-3.3 powers the device. Note that we do have some differences in what we call LOOP+ and LOOP-, but the way the schematic operates is about the same.

    Part of the problem may be what is not shown on the schematic. What else is the LDO driving? If it takes more than 4mA, then the DAC161S997 will not be able to control the current and the loop may not be able to go to low values (5mA for example, in your circuit).

    Check the current consumption for your circuit and see if it does take more current than expected.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Joseph,

    Thank you very much for your suggestions. It was spot on. The missing part of the circuit is an ISO7141CC which isolates the SPI interface from our field transmiter and it is being supplied by the LDO.

    I removed the ISO7141CC, connecting the SPI interface directly on the transmitter and it worked fine. Now I supose I will have to find another solution regarding the isolation, but that is another problem.

    Thank you for your support.