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DAC8760: Need help The Current out put of DAC8760 is going to be wrong after use for a few days

Part Number: DAC8760

I use DAC8760 for design 4-20 mA output, and max load is 500 ohm. It have good accuracy and precision ,but after a few day current output is going to be wrong. Current output should be 12 mA at 500 ohm but the real output is too much less than 12 mA, some time it is going to 7 mA. Then I check current output again with Multi meter and on load 500 ohm, the result Current output is good. So I think It can't drive a current output with 500 ohm, then I test it with load 100 ohm, and 250 ohm the result is same as 500 ohm.

I fix my control board by replace with new DAC8760 in the same PCB, It is going to be good.  Is DAC8760 broken? Why it is broken? How to prevent this problem?

  • Nakorn,

    Can you explain more what happens when you check the current with the DMM? From reading the first post alone I don't totally follow. Are you saying that you check the output current with a DMM on the same unit / board and essentially are only changing the load conditions by applying the DMM and in that process the current goes back to the expected value?

    Could you share a schematic at least covering all adjacent circuits for the DAC8760?
  • Hello,

    Any updates on this topic?
  • Thank you, Kevin. This is my circuit. R-Volt and R-Current are select output type. 

    In this case i assembly only R-current. First I use DMM measure a mA output on terminal output with out load, current output is 13.03 mA, Next I do it again with load 500 ohm, current output is 4.44 mA. please see picture below. 

    Measure With out load

       

    Measure with load 500 ohm.

    Thank you.

  • Hello,

    What input code is applied to the DAC in this case?

    Can you measure measure the +18V rail when the unit is loaded with 500 ohms?

    Also, what is the thermal pad connected to?

    I just want to clarify the situation...

    - Initially the board / unit accurately source the 12mA current into the 500 ohm load
    - After some time, the 12mA current decays to ~ 50% the expected value
    - You mentioned "...then I check current output again with Multi meter and on load 500 ohm, the result Current output is good..." exactly what does this step indicate?

    It seems like after the couple of days of running units are permanently affected and unable to drive any load with an accurate current.
  • Hi Kevin. Thanks you for you post.

    The Voltage apply to +18V rail is 16.11 to 16.28Vdc. I have two testing, first is a result of the damage module, and the second is result of the new module that replace the damage module on the same controller board.   Please see a picture below.

    The damage module, an expect output is 12 mA, measured output with no load is 12 mA, and Avdd is 16.24 vdc.

    The damage module, an expect output is 12 mA, measured output with load 500 ohm is 4.53 mA, and Avdd is 16.11 vdc.

    After testing the damage module, I replace it with a new module and the result show below.

    The new module, an expect output is 12 mA, measured output with no load is 12.00 mA, and Avdd is 16.27 vdc.

    The new module, an expect output is 12 mA, measured output with load 500 ohm is 12.00 mA, and Avdd is 16.27 vdc.

    I think the DAC8760 on first module is damage. Is it correct?

    In fact, I try to know a situation that why it is damage?, how this problem can happen? So I apply a wrong signal as 24 Vdc to output pin of DAC8760, but it still ok after apply 24V for hours. I don’t have any ideas for this. How to prevent this problem? If you have any ideas please let me know. Thanks you Kevin

  • Nakorn,

    With the information covered so far in this thread I cannot say for sure what is damaging the units. The power supply rails look to be the same before and after damage, or on a good and bad board, so it would appear that something about the output stage itself is being damaged. I would suggest two more measurements on a bad board to help confirm that it's the output stage:

    - Read back the DAC data register to make sure that it matches the expected contents. Just to make sure that somehow the digital data hasn't been corrupted
    - Measure the internal reference to check whether the reference has been damaged versus the IOUT output stage.

    Also in my previous post I inquired about the thermal pad. Can you indicate what that is connected to?

    The first post indicated that the boards were initially good but fail over a period of a couple of days. Are the units measured multiple times over those couple of days? Is the decay gradual or all of the sudden the units begin to show these symptoms?
  • Hi Kevin

    Sorry, I can't read back the DAC data register because it don't connect to MCU MISO pin, upset :(. It can write data only.

    The internal reference is 5 V for all (good and damaged), and the thermal pad doesn't connect to other pin, but It have a solder pad. 

    Are the units measured multiple times over those couple of days?

    Yes, it install to a control box and use many hours a day.

    Is the decay gradual or all of the sudden the units begin to show these symptoms?

    I don't know. My customer is operator only, they don't know electronic enough. So they can't give me more information.

  • Hi Nakorn,

    Apologies for the lack of replies here. Given the information we have in hand, I still cannot understand what is actually creating the damage to the units and without that it is difficult to suggest how to prevent the damage.

    Have you actually damaged a unit yourself or are all of these returns from a customer?
  • Nakorn,

    Checking in once again. If you have managed to damage one yourself, understanding how that happened would be a great help at this stage.