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ADS131M08: ADS131M08 Offset and calibration

Amir Eshraghi
Expert 1240 points
Part Number: ADS131M08

Hi

I have couple of questions:

1. To Calculate output voltage you use Equation 10:

Which you take FSR from Table 8-1  (Is FSR = 75mV for gain 16 or is 150mV becuase +-)

Ex: in below equation for Gain 16 the LSB = 0.075/2^23  =  8.9407E-09

2. Calculate offset: 

  - Connect both AINXP and AINXN to AGND and measure offset and insert it to  CHX_OCAL_LSB and CHX_XCAL_MSB:
  - Ex: the offset is 8uV how do you convert this to bin with two complements  Do you divide this by LSB

         8uv/8.9407E-09 = 894.7848533   which will take int 894 and convert it to hex 0x37E, then I write 3 on MSB and 7E on LSB

3 - Gain calibration:

 - I use equation 13 

- How do I convert the Gain to bin. 

Ex: 0.98 is gain Error how do I find actual bin in two's complement. (Will be same as Offset calculation)


Please let me know if you have any questions.

  • +1
    TI__Mastermind 38265 points

    Hi Amir,

    I can not see the images in you post, however I checked the equations from the ADC's datasheet. Please see the answers below:

    1. Yes, the full-scale range(FSR) for gain=16 is 150mV (2*75mV). The LSB should be calculated: (2.4V/16)/2^24 or 150mV/2^24.
    2. When you short the input and read the code from the ADC, the code is already in two's complement format. You do not need to convert the code to a voltage and then convert it back, you only need to load this code into the offset calibration registers.
    3. The contents of the GCALn[23:0] bits are 24-bit unsigned values, not two's complement format. You can calculate the value from the gain calibration factor in the following table:    0.98/(1.19x10^-7)=8235294, convert it to a hexadecimal number, 7DA91E, then you can load this number into the gain calibration registers.

                

    Regards,

    Dale