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DAC161S997: Not the right Current

Yannick Burkhalter
Prodigy 100 points
Part Number: DAC161S997
Other Parts Discussed in Thread: DAC161P997

Hello everybody

For a project in my company I am using the DAC161S997 to simulate a 4 to 20 mA interface. The Problem is that there is always the same current of 11.7 mA and this current depends on the resistor in the loop. I have used the circuit from the DS for the typical application. Another thing is that i don't measure any voltage at the base of the pzt3904 or over the resistor RE (In my scheme it's R10). I have configured the DAC with a write to the Err_config register with the value 0x035E  and if I read the status_reg the DAC returns 0xE0 which should be totally fine.

  • 0
    TI__Guru 53283 points
    Hi Yannick,

    I suspect that you have something sinking current on in the circuit. If no current is flowing through R10 then there must be another path for the bus supply current. Note that the bus voltage needs to be independent from ground. By that I mean that the bus supply should be connected to J16-2 and J16-1, were the low side of the supply (connected to OUT, pin 9) can be a lower potential than ground. If the low side of the 12V supply is connected to GND the DAC161S997 will not be able to regulate current. Otherwise I would check to see if you have any issues with the board - shorts, bad solder joints, bent pins, etc..

    Let me know if that resolves the issue.

    Thanks!
    Paul
  • 0 in reply to Paul_Frost
    TI__Guru 53283 points
    Hi Yannick,

    I reviewed your thread that you had with Kevin 'The Duke of DACs' Duke, and I think you still might have an issue with your supplies. I think the best way to think of it is like this: The low side of your bus supply is not the ground of the DAC161P997, it is a lower potential than ground.

    I saw that you removed the ground connection from the OUT pin of the DAC161P997, can you share your power portion of your schematic?
  • 0 in reply to Paul_Frost
    Prodigy 100 points

    Hello Paul

    I don't understand 100 % what you meant but it might be the problem with the power of the loop. I have two DC/DC regulators on my PCB and both Grounds are connected.

    Here is also the scheme of the 12V power supply.

  • 0 in reply to Yannick Burkhalter
    Prodigy 100 points

    Hello Paul

    I have now changed my scheme again and i have a separate power supply for the loop. But now I don't measure any current and I still don't measure a voltage over R10 or R39. Might it be a problem, if the power supply for the DAC itself isn't driven by the loop power supply? Or is it possibly just the power supply of the loop? (Traco Power TEM 3 2412N)

  • 0 in reply to Yannick Burkhalter
    TI__Guru 53283 points
    Hi Yannick,

    Is the 3.3V supply derived from the 24V DC supply or from the bus voltage? It should not really be a problem either way. Please confirm that you see 12V across the bus supply. R39 is probably too large of a resistor to be useful here, try replacing it with a smaller value, maybe 100Ω. If you do not measure any voltage across the R39 then there is no current in the loop and I suspect that there is an issue with the circuit. It might be useful to confirm how much current is being sourced by the 3.3V supply.

    Paul
  • 0 in reply to Paul_Frost
    Prodigy 100 points
    Hi Paul,

    The 3.3 V supply is derived from the 24 VDC supply. And yes at the output of the bus supply and over the transistor i can measure 12 VDC. I have exchanged R39 with a 100 Ohm resistor but i still can't measure any voltage over it. I have measured the current from the 3.3 VDC supply for the DAC and I've measured a current of 0.1 mA.

    Yannick
  • 0 in reply to Yannick Burkhalter
    TI__Guru 53283 points
    Hi Yannick,

    Has these changes all been implemented on the same board? I wonder if at this point it is reasonable to suspect that this device or the circuit has been damaged with all the rework and testing.

    If you source the 3.3V externally do you see any change?

    Thanks!
  • 0 in reply to Paul_Frost
    Prodigy 100 points

    Hi Paul

    Yes, all these changes has been implemented on the same board. The chance that anything got damaged is really high. But i don't think that the DAC is damaged because i can still write and read data from it. But it still might be.

    I have testet the circuit with an external source. It doesn't make a difference.

    Maybe i have to try another method to successfully build my 4-20 mA loop.

    Thank you for your help!