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ADC Calibration problem

toop
Prodigy 210 points

Other Parts Discussed in Thread: TMS320F28035

Hi,

I am working on 320F28027 microcontroller based USB control stick. In this I am using ADC. It's out put shows saturated (ADC gives the value  4095)  when the input reaches 2.8V. I have called the device_cal() at the beginning.

I have attached the readings and its graph for your reff. I understand it is a gain error. TI documentation says that all these errors are taken care by Device_cal() routine. It says "The calibration function Device_cal() writes a factory trim value to correct the ADC full scale gain error into the ADCREFTRIM register. This register should not be modified after the Device_cal() function is called."  Any idea about this problem. Is it possible to trim this error by writing some value to ADCREFTRIM register.

4174.ADC CALIBRATION.pdf

 

 

  • 0
    TI__Prodigy 725 points

    What values are in the ADCREFTRIM and ADCOFFTRIM registers on the suspect devices directly after you have taken this data? Make sure you are not writing to the AdcRegs.ADCCTL1.RESET bit after you call Device_Cal() as this will clear out the ADC trim registers.

    Thanks,

    Nicholas Smith

  • 0 in reply to Nicholas Smith12553
    Prodigy 210 points

    Hi Nicholas Smith,

    Thank you very much.

    The problem is solved. As you have mentioned it was due to writing to the AdcRegs.ADCCTL1.RESET bit after I  called Device_Cal().

    Thanks and refards

    Toop

     

  • 0 in reply to toop
    Prodigy 10 points

    Hi,

    I have the same problem with TMS320F28035.

    But I don't write to the AdcRegs.ADCCTL1.RESET bit after Device_Cal().

    ADCREFTRIM register contains 0x20AD and ADCOFFTRIM register contains 0x0019 which seems to be correct values of factory calibration.

    But still I measure 30 when ADC pin voltage is 0 V and 4071 when ADC pin is connected to Vdda.

    Does it mean that there is a problem with factory calibration? Because I have tried with 3 chips. The values are different but the problem still exists. 

     

    Thanks and regards

    Huseyin Bulut