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F28235: ADC gain error with external reference

yang dahua
Prodigy 70 points

The boot ROM automatically calls the ADC_cal() routine to initialize the ADCREFSEL and ADCOFFTRIM registers with device specific calibration data. ADCREFSEL bit [13-0] is for gain error correction and ADCOFFTRIM is for offset correction.

My question is that the calibration data stored in boot ROM is for the internal reference?

If yes, should I reset both registers to zero when external reference is used?

From the datasheet, the overall gain error with external reference is +-30LSB, does it include external reference error and the other parts of ADC error ?

  

  • 0
    TI__Guru 60865 points

    Hi Yang,

    All you need to do to use external reference mode is change the mode select bit.  No need to change the trim.  

    The gain error spec assumes an ideal external reference but includes all internal sources of error.  Any error in the external reference will proportionally add additional gain error. 

  • 0 in reply to Devin Cottier
    Prodigy 70 points

    Hi Devin,

    Thanks. But I still feel confused about it. 

    From data sheet, the overall gain errors with internal reference or external reference are the same. 

    Does the overall gain errors with internal reference also assumes an ideal internal reference?

    If yes, TI does not give the detailed spec of internal reference, how can I know the total error?

    If no, then there's no reason to use external reference. If fact, TI recommend to use external reference to improve spec.

  • 0 in reply to yang dahua
    TI__Guru 60865 points

    Hi yang,

    Yes, based on the spec, there is not much of a benefit to using external reference for this device.  I can again confirm that the internal reference spec. does include all movement of the internal reference, while the external spec. does not include movement in the external reference (see note 6 in the datasheet for gain error).    

    Can you point out where we recommend using the external reference?  If this is a general advise for C2000 microcontrollers,  then this may be correct, since Piccolo and Concerto family devices do see improved accuracy with external reference (not to mention that they can use the external reference to adjust the conversion range).  If this is for F2833x/F2823x devices, I will try to get the documentation updated to be clearer.        

  • 0 in reply to Devin Cottier
    Prodigy 70 points

    Hi Devin，

    You are right. I assume IC designer do some optimization design with internal reference. From spec and note 6, the spec will be worse with external reference.

    I misunderstood note 8 in data sheet.  There's no document to say improved accuracy with external reference.