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LM25066 / current sense offset

Other Parts Discussed in Thread: LM25066, LM5066I

Hi,

Could you tell me about LM25066 offset.
Offset occurs by internal AD conversion in a current sense.
(When customer change current to 0A→0.2A, values of AD increase.)

Customer changes B of the following expressions and copes.

However,0.2A is added to data in 0A condition.
Therefore the customer is troubled.
Is this offset specifications of LM25066?
Is there any solution?

Best Regards,
Yusuke/Japan Disty

  • Hi Yusuke-san,

    I will be revising the datasheet to help explain this section better.

    In the meantime, please take a look at the revised explanation on the LM5066I datasheet, which is a very similar device (high voltage vs. low voltage):

    Section 9.5.5 on Page 41

    The key sentence which summarizes is: "It is not considered good practice to take measurements on one board and use the computed coefficients for all units in production, because the RSNS and the LM5066I on a given board are randomly chosen and do not represent a statistical mean. It is recommended to either calibrate all boards individually or to use the recommended coefficients from Table 48."

    So yes, if you use the recommended coefficients for the LM25066 then you will see an offset in the 0A condition. However, using these coefficients will produce good accuracy as current levels increase, closer to the full scale range.

    The only way to calibrate out this offset would be to determine the coefficients empirically following the guide mentioned above (Section 9.5.5 in the LM5066I datasheet), however this would not be recommended due to Rsns, LM25066 and PCB variance.

    Thanks,

    Alex

  • Alex-san,

    Thank you for your kind support.
    May I use the coefficient of LM5066I for LM25066?



    If the offset is not disappear with this factor,
    Is it impossible to carry out the measurement of 0A?
    Could you tell me a little more in detail?

    Best Regards,
    Yusuke/Japan Disty
  • Hi Yusuke-san,

    No, the LM5066I coefficients would simply give you inaccurate results.

    What I meant is that you can follow the procedure listed in the "Determine Coefficients Empirically using Linear Fit" to determine optimal coefficients for a specific PCB:

    www.ti.com/.../lm25066.pdf
    Page 48

    However, this may produce good results for one specific IC + PCB, but not for another.

    Otherwise, we would recommend the coefficients used in the datasheet which will have an offset at 0A, but will be more accurate towards full-scale range (higher currents).

    Thanks,
    Alex
  • Alex-san,

    Thank you for your support.
    I told the contents of Data sheet Page 48 and PCB issue to the customer.
    Let me ask you one more thing(about your following advice) .

    >this may produce good results for one specific IC + PCB, but not for another.

    Is the numerical value of specific IC listed in a data sheet?
    Could you tell me about internal amp offset?
    Customer wants to know the factor of LM25066 side.

    Best Regards,
    Yusuke/Japan Disty

  • Hi Yusuke-san,

    It is a non-ideality within the internal ADC. It's value is not listed and may vary from IC to IC. This is how the recommended coefficients were determined: by performing characterization testing of several devices and coming up with coefficients which will work best when operated closer to the full-scale range (around 20mV of Vsns), but as a result, readings are less accurate at low-scale range (at 0mV).

    Thanks,
    Alex