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DAC8760: Maximum output power less than 20mA

Zoe Yang
Intellectual 2210 points
Part Number: DAC8760

Tool/software:

Hi team,

My customer feedback when using DAC8760 the outputs current are less than 20mA maximum. 150 pieces were put into production. After testing 60 pieces, it was found that the output current of 13 pieces did not meet the standard, and the defective rate was 21.7%.
① The abnormal product phenomenon is confirmed as follows: the output current cannot reach the nominal 20ma of the product, and the maximum output current is only about 19.96ma.
② Replacing the resistor and capacitor components in the relevant circuit failed to solve the related abnormality.
③ After replacing the relevant DAC digital-to-analog conversion chips of the three abnormal products, the maximum current of 20ma is output normally.
④ Try to replace the abnormal chip removed from the defective product with the normal product, and the normal product reproduces the abnormal situation that the current cannot output more than 19.96ma.
⑤ Based on the above situation, it is currently preliminarily believed that the abnormality is caused by the DAC digital-to-analog conversion chip.

Can you help advise whether 19.96mA is abnormal? What else should we advise them to optimize?

  • 0
    TI__Guru* 93545 points

    Zoe,


    That is a little higher than what I would expect for the max error on the output. However, I do want to check a few things.

    • What mode are they operating the device in? Is this 0-20mA or 4-20mA?
    • What exact values for 20mA did they measure on these 60 pieces? (Can you send the measurements for these 60 devices?)
    • Did they record the reference values for each of these devices at the same time?
    • How do they have the device configured? Do they use the internal reference? Do they use an external ISET-R?

    Normally, when you set the DAC to 0-20mA mode (or 4-20mA mode) and then set the output to maximum, there are two things that can cause the output to be less than 20mA exactly. First, there is the full-scale error. For the DAC8760 at 25°C, the full-scale error is ±0.08%. That means at 20mA, you may have an error of 0.016mA which would get to 19.984mA.

    Also, this error does not include the reference error. If the reference is low, then the DAC code will be low by the same percentage. If they are using the internal reference, then the error is expected to be ±5mV from the 5V reference, which translates to an added error of ±0.1%. This error added to the above. If this were low as well, you'd get to 19.964mA.

    Again, if both the full-scale error and the reference were both off, then you'd still get an error a little less than what they have measured.

    Ask them about the things I mentioned at the beginning of the post, and report back the results back.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Intellectual 2210 points
    • What mode are they operating the device in? Is this 0-20mA or 4-20mA?
      • 0-20mA
    • What exact values for 20mA did they measure on these 60 pieces? (Can you send the measurements for these 60 devices?)
      • There is no record of this, but it was discovered during production that the control current output could not be greater than 19.96mA before and after calibration.
    • Did they record the reference values for each of these devices at the same time?
      • No
    • How do they have the device configured? Do they use the internal reference? Do they use an external ISET-R?
      • The reference is internal, and the external ISET-R is connected to 15K/±0.1% for testing, and it is also unable to output more than 19.96mA. Currently, it is found that only by replacing DAC8760 can it work normally
    • The attached circuit is what they referred now. 
  • +1 in reply to Zoe Yang
    TI__Guru* 93545 points

    Zoe,

    Using the external ISET-R adds to the error from the tolerance of the resistor. If the resistor is 0.1% high, then the output current would be 0.1% low. This is an additional error on top of the gain error and the reference error. They could probably test this by using a different resistance that is intentionally lower and checking the output again to see that the current is higher.

    If there is an issue, they would need to measure the reference to eliminate that as a problem, and use a resistance for ISET-R that is a higher tolerance to make sure the resistance isn't contributing to the gain error. Even if they don't use the resistance in production, it would help to know how much the resistor tolerance is contributing to the output error.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Intellectual 2210 points

    Hi Joseph,

    Leadshine have verified that the ISET-R resistor was changed from 15K to 10K/0.1% and the test still failed. What else can we suggest?

  • 0 in reply to Zoe Yang
    TI__Mastermind 30168 points

    Hi Zoe, 

    Joseph is out of office until Monday so there may be a delay in his response until then. 

    Best,

    Katlynne Jones

  • 0 in reply to Zoe Yang
    TI__Guru* 93545 points

    Zoe,

    Sorry, I'd missed this post. Can you let me know what happened when they put in the 10k, 0.1% resistor? I would have expected the current range to go from 0-20mA to 0-30mA.

    My point was originally that they could make small adjustments to the ISET-R resistor to get the proper range.

    Joseph Wu