AD4114 calibration

Hi

  The measurement error of AD4114 still exists when we use it. Our measurement error is about 60mV, We requires it to be within 15mV. We suspect it is calibrated problem; Since the formula we use for calibration is our own attempt to convert,  How to confirm the values of formula OFFSET and GAIN as below relationship?

  Is there any way to calibrate AD4114 by non-automatically ?  thanks 

 

  • +1
    •  Analog Employees 
    on Mar 30, 2021 4:17 AM

    Hi, 

    Do you consider using ADC system calibrations? or you just really wanted to adjust the offset manually? I would recommend to perform system offset and gain calibrations once and then read the offset and gain calibration registers. You can then use the same offset and gain register to the other channels or devices. However, there are some variations from part to part so you can't expect the same performance from each but at least it is expected to minimize the errors. 

    Just to add info regarding the above equation. The above equation show the calculations associated with the offset and gain for bipolar modes of operation. You can manually adjust the gain and offset register via their own registers as the ADC provides access to the on-chip offset and gain calibration registers to allow the microprocessor to read the device calibration coefficients and to write the stored calibration coefficients.

    • The 0.075 number reflects the attenuation of the analog input to 0.1 then to 75% before the offset and gain coefficients are applied. This is done to avoid modulator saturation after applying the offset and gain corrections. This number will vary slightly from part to part because of manufacturing tolerances.
    • The value 0x800000 is the default offset coefficient. In the Offset_Reg each bit is equal to 1LSB.
    • The additional 0x800000 in the bipolar equation is to implement the offset binary that is used in bipolar mode.
    • The 0x400000 along with the gain coefficient invert the 0.075 scaling.

    If we work through an example here, where we assume there is no offset and no gain correction applied to the ADC output codes, where the offset coefficient of 0x800000 and gain coefficient of 0x555550. The offset correction is 0x800000– 0x800000= 0. The gain coefficient of 0x555550 and the fixed value 0x400000 together give a value closer to 1/0.75. So, the attenuation (0.75) is reverted here.

    But please note that the ADC does all this processing internally, therefore the main conversion of codes to associated voltages should be use in the final results. 

    Code = 2^ (N – 1) × [(0.1 x VIN/VREF) + 1]

    Thanks,

    Jellenie

  • Hi 

      Thank you for the suggestion, We hope to ADC system calibrations.  We will try again. thanks 

  • 0
    •  Analog Employees 
    on Apr 5, 2021 1:45 AM in reply to Aggil

    Okay Thanks, Just let me know if you encounter any issues. Please take note that there's a limitation in terms of range when performing system calibration. The datasheet provide a complete details. 

    Regards,

    Jellenie

  • Hi Jellenie,

     We meet a calibration issue, When we  used the formula in the datasheet.

    Data = ((0.075 × VIN/VREF) × 223 – (Offset − 0x800000)) ×(Gain/0x400000) + 0x800000

    When changing the values of the GAIN and OFFSET registers, The value of AD4114 calculated by this formula is unchanged, please help us to have a look! 

    The code for our calculation is: Data = (double)((((double)iData) - 0x800000) / ((double)iGain / 0x400000) + ((double)iOffSet - 0x800000)) / (251658.24);

    Where,

     iData: AD4114 converts out of code

    IGAIN: Read back to the GAIN register

    IOffset: Readback to the OffSet register value

  • 0
    •  Analog Employees 
    on Apr 13, 2021 1:54 AM in reply to Aggil

    Hi, 

    Apologies, I may misunderstood what you are saying above. Can you provide more details on what are you trying to do please? 

    But when calculating the data output code you only need to use the standard equation as the ADC perform the calculation/processing of the calibration coefficient internally. So you only need to change the Gain and Offset registers by either performing system calibration mode or by writing to these registers. 

    So for example, try to perform system offset calibration. Then readback the offset register, you will noticed that the value of the offset register will be changed from default. The new coefficient will be applied on the conversion automatically therefore the main conversion of codes to associated voltages should still be use in the final results. 

    Code = 2^ (N – 1) × [(0.1 x VIN/VREF) + 1]

    Thanks,

    Jellenie