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TMS320F280049: ADC input's parasitic capacitance tolerance

Expert 3730 points
Part Number: TMS320F280049

Hi,

I have 2 questions regarding the on-chip ADC.

Q1: Datasheet, Table 5-45 shows the parasitic capacitance of each ADC input. I'd like to ask if we have any tolerance for these typical values. Can I use +/-20% for the worst case analysis?

Q2: How can I calculate the total conversion errors? Is this the Total Unadjusted Error (TUE) in this documentation?

  • Hi Kien,

    Q1: Yes, you can use +/-20% for worse case calculations for parasitics. This is consistent with what we measure on bench.

    Q2: Total conversion errors is not spec'd for the F280049 but you can somehow derive this from offset, gain and DC performance (DNL and INL) errors as indicated in the link you sent. Yes, this is the same as TUE.

    Regards,
    Joseph
  • Hi Joseph,
    Thanks for your answer.
    The Gain error is high (+/-45LSB) when using the internal reference, leading to the high TUE. If I just use the internal reference, is there any way to reduce the error?
  • Hi Kien,

    TUE would end up high because of gain error on the internal VREF mode unfortunately as we do not have control on the internal VREF source, however if we can predict what the gain and offset errors are, we can compensate for these at the input side and errors will be minimized. This would be total error with correction. Not sure what the intended application is but this may be an option you can take.

    For offset error compensation, the ADC in the F280049 device has a PPB (post processing block) that can take out a fixed error (in this case the offset) for every conversion without CPU overhead. For the gain error, you can first run a test to determine what input voltage would produce a fullscale conversion, translate that error in LSBs and put in PPB for correction and then you will have an offset and gain corrected conversion. That is one possibility I think.

    Regards,
    Joseph
  • Hi Joseph,
    Understood.
    Thanks for your great support!