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DAC161P997: how to solve the problem of output current drift due to temperature changes

Amy Luo
Expert 6895 points
Part Number: DAC161P997

Hi team,

The external BJT of DAC161P997 is BCX56-16 (JCET ) and the 20Ω resistance is 10ppm, 0.1%. After 4mA and 20mA verification at room temperature, and then test 4mA and 20mA at - 20 ℃, it is found that the 4mA maximum drift is 10uA, and the 20mA maximum drift is 20uA. How to solve the drift problem? Do you have any suggestions?

Best Regards,
Amy

  • 0
    TI__Intellectual 1570 points

    Hello Amy,

    The temperature drift performance of the part is mostly determined by the DAC and it's internal components. The 20 ohm resistor does not affect the drift that much.  The current error limits in uA are shown in the data sheet electrical characteristics table and align with your measurement. 

    What kind of accuracy is required for your application? Are you able to calibrate your system at room temperature?

    Regards

    Reza

  • 0 in reply to Reza Abdullah
    TI__Expert 6895 points

    Hi Reza,

    Thank you for your reply.

    The accuracy requirement is less than 1/1000, that is, 16uA. The system was calibrated at room temperature.

    Do you mean that if I choose 10 ppm, 0.1% 20 ohm resistor, it has little effect on the temperature drift, or no matter what kind of resistance I choose, it has little effect on the temperature drift? I wonder if the output accuracy is related to the external BJT and 20 ohm resistance.Could you clarify that.

    Best Regards,
    Amy

  • 0 in reply to Amy Luo
    TI__Intellectual 1570 points

    Hello Amy,

    I'll use figure 21 of the datasheet to explain.

    In this figure, the loop current is regulated by the combination of the DAC & buffer combination. Hence the accuracy is mainly determined by the DAC accuracy, the offset of the output buffer, and the gain ratio between R1 & R2. Notice that all these components are on chip. The part is designed and tested to meet total accuracy spec (TUE) of +/-0.23% as shown in the datasheet.

    Re (the 20 ohm resistor) is only used as emitter degeneration for the BJT to improve the loop stability. You can even replace it with 22 ohm and the loop current will be unaffected.

    It is important that the loop has sufficient gain under all conditions and as such, the BJT needs to be rated correctly. I suggest a minimum beta of 40 for the BJT and VCE minimum rating should be the loop supply voltage.

    Regards

    Reza