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DAC161S997: unable to control the loop current through DAC161S997 IC

Sai Krishna
Prodigy 30 points
Part Number: DAC161S997
Other Parts Discussed in Thread: LM2936,

I developed a 4-20 mA, loop powered transmitter circuit using DAC161S997. The loop is powered using LM2936, which is generating 3.3Volts. i am able to set/control current upto 6 mA, after that the current is increasing exponentially, with change in the input/set control current. The below is the observed data,

Observed Current Set Current
2 0
2 1
2 2
3 3
4.1 4
5.1 5
6.1 6
7.4 7
8.8 8
10.3 9
12.4 10
16.1 11
22.5 12
32.9 13
47.5 14
67.6 15

Used the same schematic which was described in the datasheet.

I am unable to understand why i am unable to control the current after 6 mA.

Could you please suggest, where i need to look further, so that i can rectify the issue.

  • 0
    TI__Guru 53573 points

    We will review this and respond when we are back in the office on Tuesday. For now, I would advise you to confirm that your loop power supply can "float", meaning that the negative or low side of the power supply can be completely independent from the local ground created by the DAC161. Recall that loop- must be a lower potential then COMA and COMD, so the low end of the supply needs to be completely independent. You might also need to take care that you are not connecting the local ground to Earth ground through other equipment such as an oscilloscope.

  • 0
    TI__Guru* 94285 points

    Sai,

    I would also check to see if the current is oscillating. The DAC161S997 controls a feedback loop that sets the current. Because the current starts out roughly correct and the error gets larger through different settings, this might indicate an oscillation. I would use an oscilloscope to view the current through a test resistor.

    Joseph Wu

  • 0
    Prodigy 30 points

    Jospeh,

    DO i need to check the current through the test resistor.

  • 0 in reply to Sai Krishna
    TI__Guru* 94285 points

    Sai,


    How are you measuring the current right now? Does your power supply have a current reading? I was suggesting a resistor because it is similar to the functional block diagram on page 8 of the datasheet. The resistor is the loop receiver, and you could use 250Ω resistor as long as the supply is high enough. You might be able to use the resistor at the emitter side of the transistor, but that would be a rather small voltage, depending on what resistor value you use there.

    Again, you want to be able to see if the current loop is oscillating. You would definitely need an oscilloscope to view that.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 94285 points

    Sai,

    Can you provide a schematic? I know you are using a schematic from the datasheet, but it may be helpful to have more specific information about parts and resistor and capacitor values.

    Joseph Wu