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DAC161S997: use of two DAC161S997 for the transmission of temperature and humidity

user6508816
Prodigy 20 points
Part Number: DAC161S997

Hello.

I am trying to transmit temperature and humidity through 3 wires, +LOOP, -LOOP_temperature, and -LOOP_Humidity.

Individually, they work perfectly, but when I connect them in parallel, they don't work. The current outputs are regulated:

For example, if I want 9mA to flow through the temperature loop and 16mA through the humidity loop, the result is that I have two equal currents of 16mA.

It always equalizes more or less to the higher output.

I have attached my schematic.

Is it impossible to use 2 DAC161S997 with the same LOOP+ and the same ground? How should I connect them to make them work correctly?

Regards.

  • 0
    TI__Guru* 93035 points

    Hi,


    The problem you are seeing is definitely coming from the two devices having a common ground. To control the loop current, the ground basically set to a voltage above the OUT pin that is connected to LOOP-. Here is the simplified schematic at the front of the datasheet.


    The DAC current drives current into the 80kΩ resistor and that current is gained up by the op-amp driving the 40Ω resistor. The 40Ω resistor sets the current of the loop. Because of this, the voltage between COMA (or ground) is variable to the voltage at OUT. This is required to set the loop current.

    For multiple devices, you would need isolated grounds between the two DAC161S997s.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Damn it's true, I had not seen it thank you very much.

    So, my problem is unsolvable? I need to transmit the two 4-20mA variables and I only have 3 wires, one for the positive of the source and one for the loops.
    But with what you have explained to me, each intensity will cause a different voltage drop in each resistance and therefore the reference of the circuit will change...

    Is there an integrated circuit or a circuit that I can use for this problem? an IC that has two outputs?
    Is it impossible to solve this problem with only 3 wires?

  • 0 in reply to user6508816
    TI__Guru* 93035 points

    Hi,

    I'm not sure I have a good simple solution. Normally, you could have two separate DAC161S997 transmitters but you wouldn't share the common ground. Using a common controller, you would just insert some digital isolation for the communication. If you think of this as two isolated solutions, it's a bit easier.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Would it be possible to use 2 LDOs? one for each DAC and separating the 2 COMA pins and joining the COMD in for the microcontroller?

    Could you somehow play with the COMA and COMD pins of the DACs so that there would be no problems, and avoid the use of isolations?

  • +1 in reply to user6508816
    TI__Guru* 93035 points

    Hi,

    The problem is that COMA and COMD should be the same voltage potential. You can't just separate COMA and COMD. I think that isolation is the only solution. In the end, you need the LOOP- to go below COMA and COMD independently for each device and COMA and COMD should not be separated.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    ok thanks a lot for the help