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DAC82001: buffer output is always REF voltage 2.5V

user5883945
Expert 1976 points
Part Number: DAC82001
Other Parts Discussed in Thread: OPA189,

Tool/software:

Hi

The customer's circuit reference datasheet uses OPA189 buffer;

During testing, OPA189 output was fixed at REF voltage 2.5V;

Customer added a resistor 6M Ω to the VOUTA pin2 of DAC82001 and pulled it down to GND, which then returned to normal;
What could be the reason for this？

  • 0
    TI__Genius 11850 points

    Hello,

    I'd appreciate more information on what you are seeing. What is the DAC output? Is the DAC output changing when you add the 6M Resistor? 

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Expert 1976 points

    Hello,

       As mentioned earlier, using DAC output as the test point is equivalent to connecting a resistor to GND. At this point, it returns to normal and the problem cannot be reproduced.

  • 0 in reply to user5883945
    TI__Genius 11850 points

    Hello,

    It seems like there may be an issue with how the DAC output works with the buffer. I'll talk with some designers to see if we can figure out why putting a pull down seems to fix it.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    TI__Genius 11850 points

    Hello,

    If you disconnect R103, does the DAC update correctly? 

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Expert 1976 points

    Hello,

      As mentioned earlier, it is impossible to measure the DAC output. When a measuring instrument is connected to the DAC output, it is equivalent to connecting a high resistance value to GND, and the output returns to correct. So it is impossible to capture abnormal data.

    If R103 is disconnected, the operational amplifier has no input and the output of the operational amplifier cannot be measured.

  • 0 in reply to user5883945
    TI__Guru 55163 points

    Hi,

    The DAC82001 features an unbuffered output, and as such, not very sensitive to any load.  If you are seeing intermittent failure that resolves with adding contact or resistance, I would suspect some kind of solder or connectivity issue.  Does this issue exist on multiple boards? Have you tried re-soldering a device to see if it still has the issue? Is your communication with the DAC consistent? That is to say if you enable the output to work correctly (adding a probe) can you set the output to be a specific voltage?