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ADC current/voltage bias unit

Jonathan Lock
Intellectual 775 points
Other Parts Discussed in Thread: TLV2781

Hi all,

Does anyone here know what unit the ADC bias offsets used in instaspin are expressed in? (IE. the values returned by DRV_getBias(...)). After performing an offset calculation I'm getting voltage offset values of around 0.023 and current offset values of around 0.052 (as returned by the DRV_getBias(...) function). My scaling parameters in user.h are IQ_FS_V = 80, ADC_FS_V = 68, IQ_FS_C = 700,  ADC_FS_C = 330.

The function description is not completely clear as to what the expressed value is reported as, but I had expected either a value close to 1.65V or a value < 30-50mV when scaled. None of the combinations of multiplication or division by V_SF and I_SF and/or other reasonable scale factors (such as the the ADC full-scale value (2^12-1) give what I values in the range I had expected or vary with changing IQ_FS values... Some help would be very appreciated!

Thanks,
//Jonathan

  • 0
    TI__Guru** 118000 points

    Jonathan,

    It's a per unit (meaning divided by the IQ_V or IQ_I from your user.h) offset (the delta required to zero the expected from the conversion) 

    Notice if you change your IQ_V or IQ_I the values rescale.

     

  • 0 in reply to ChrisClearman
    Intellectual 775 points

    Hi Chris,

    Um, so would that imply that the offset voltages are on the order of 0.052/700 = 74µV for the current sensors and 0.023/80 = 287µV for the voltage sensors?

    That is far lower than I had anticipated; I'm using a TLV2781 op-amp for the current sensors, and just that component has a typical offset voltage of 250µV. On top of that I'd expect the ADC offset to be significant, making the 74µV offset seem unreasonably small... I'm sorry, but I'm having a hard time following what you mean...

    (Just as a clarification, the values I've posted are what I've directly been returned by DRV_getBias(drvHandle,DRV_SensorType_XXX,n) after an offset calibration.)

    //Jonathan

  • 0 in reply to Jonathan Lock
    TI__Guru** 118000 points

    Jonathan,

    See section 6.5.2 of the User's Guide (SPRUHJ1)