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MSP430 ADC Calibration (SD24CAL = 1)

Weiren Tee
Expert 1100 points

Hi,

I would like to confirm that under the MSP430 ADC calibration mode(when SD24CAL = 1), negative values offset can be measured. The reason why I am asking this is, so far, no negative value offset data is collected and there are a few prototype boards with offset compensation error. These error and seems to be due to wrong signed offset value. Please see an example below for illustration :

 

Measured offset value = +10, //(SD24CAL = 1)

Raw measurement = 90

Actual measurement = Raw measurement - Measured offset value

Actual measurement = 90 - (+10) = 80

However, the expected value of the known sample should be 100 instead of the 80. Based on this, we are suspecting that the measured offset should be -10 instead of the +10. So we wish to confirm that the negative offset can be captured during calibration(SD24CAL = 1).

Thank you very much.

Regards,

Weiren

  • 0
    Guru 227245 points

    IIRC, the offset calibration is for internal offsets of the SD24 only. Any external offsets due to board hardware are valid input signals to the SD24 and can't be calibeated automatically.
    This makes internal calibration almost useless in many if not most real-world situations and is the reason why we manually calibrate all devices with a two-point calibration after building them. This eliminates all offsets and gain errors caused by ADC as well as by external component tolerances. After all, it’s the total result precision that counts, not (only) the precision of the very last step in the chain, the ADC.
    Internal calibration is only useful if the input directly goes to a calibrated, offset-free signal source with no further circuitry between.

  • 0 in reply to Jens-Michael Gross
    Expert 1100 points

    Hi Jens-Micheal,

    Thank you very much for your explanation. It is very help for me.

    I would also appreciate if TI could address my question on the on chip calibration so that we can better understand the MSP430 internal ADC

    Regards,

    Weiren

  • 0
    Expert 2480 points

    To answer your question: SD24 can spell out negative values, be it the offset from calibration or raw measurement of your signal,  if you set:

    SD24BCCTL0 |= SD24DF_1;

    This results in digital out in the format 2s complement mode, ranging from -FS to +FS. Please refer to user guide page 762.

    In other words, if your offset is actually a negative one but you don't set SD24DF_1 bit, that could cause trouble. This is what happened in my case. So I post it for your reference.

  • 0 in reply to Zhiyong Li (FT)
    Expert 1100 points

    Hi Zhiyong,

    Thank you so much for sharing your case and guidances. These are very useful for me.

     

    Best regards,

    Weiren

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