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Original question:

TMS320F2800157-Q1: TMS320F2800156-Q1 PWM Current Output

TMS320F2800157-Q1: VREFHI tied connected to VDDA when using ADC

Chris Willis
Intellectual 290 points
Part Number: TMS320F2800157-Q1


There is a not on page 45 table 5-10 of the datasheet that I have that says 

VREFHI  Tie to VDDA (applies only if ADC is not used in the application)

but 6.14.3.2.1 ADC Operating Condictions, shows VREFHI, for external reference, range to have a maximum of VDDA

Why does the datasheet say we cant use the same supply to power the chip as we use for our ADC external reference?

  • 0
    TI__Mastermind 36526 points

    Hi Chris,

    It is possible to use the same supply to power the chip and provide the ADC external reference, but that is not usually recommended. 

    When the ADC is using an internal reference (which will usually result in better performance than using VDDA as a reference), the device drives a different voltage onto the VREFHI pin, so it would not be good to externally drive 3.3V onto VREFHI. 

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Intellectual 290 points

    Hi Ben

    We plan to power the Micro with an externally regulated supply of approximately 3.3V and connect the same 3.3V supply and ground to a pair of resistances in series.

    The datasheet says that the reference can be configured to be internally connected to 3.3V – however it looks as though that is actually an internal 3.3V bandgap reference, not the VDDA supply.

    This is not what we want – although the 3.3V bandgap will give a more accurate measure of the absolute ADC VOLTAGE, what we care about is the voltage RATIO, which will not be accurate if the voltage supplying the pair of resistors is different to the voltage reference of the ADC.

  • +1 in reply to Chris Willis
    TI__Mastermind 36526 points

    Hi Chris,

    Just wanted to clarify that the internal 3.3V reference mode actually uses a 1.65V bandgap reference and divides the inputs to the ADC by 2, which is mentioned here in the TRM. 

    Also, it sounds like it makes sense to use VDDA as a reference in your case. There is no problem with doing this, except for the accuracy concerns that I mentioned above.

    Best Regards,

    Ben Collier