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TMS320F280049: If internal ADC reference is used, how do we connect VREFHIx?

Wayne Huang
Genius 17205 points

Part Number: TMS320F280049

Dear Champs,

If internal ADC reference is used, should we connect VREFHIx to VDDA directly?

From F280049 control card schematic doc, it says

"S8 chooses whether the internal or external voltage reference will be used
a) The ADC's respective switch in S8 is on
b) The respective ADC's switch in S8 is off
- ADC should be configured to disable the internal reference and
- ADC should use the internal voltage reference
instead a reference voltage should be attached to HSEC pin 45"

When S8 is off, doesn't it mean the VREFHIx are connected to VSSA?

However, in F280049 datasheet (sprs945c.pdf), Table 4-10 Connections of Unused Pin shows the unused VREFHIx should be connected to VDDA directly.

We are confused.

Wayne

  • 0
    TI__Mastermind 44190 points

    Hi Wayne,

    In internal VREF reference mode, VREFHIx pins should not be connected to anything except to the bulk capacitor as it is needed by the internal reference circuit (refer to table 4-2 in sprs945c.pdf regarding connection of VREHIx to an external filter capacitor regardless if internal/external VREFHI mode is used).  In other words, keep this pin floating in internal VREF mode. 

    We will put more clarity in how to deal with Connections for Unused Pins in Table 4-10, specifically on VREF pins on the next datasheet revision.  The new text for VREF pins will be as follows:

    VREFHIx Tie to VDDA (applies only if ADC or DAC are not used in the application)
    VREFLOx Tie to VSSA (applies only if ADC or DAC are not used in the application)

    Hope this clarifies connections to VREF pins during internal VREF mode.  Let me know if you have additional questions.

    Thanks and regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Genius 17205 points

    Joseph,

    Let me confirm again.

    So in internal reference mode, VREFHIx is connected to VREFLOx, where a 2.2-uF is put in between VREFHIx and VREFLOx.

    But VREFLOx still needs to be connected to VSSA.

    Is my understand right?

    Wayne

  • 0 in reply to Wayne Huang
    TI__Mastermind 44190 points

    Wayne,

    In internal VREF mode, there is a 2.2uF cap between VREFHIx and VREFLOx and keep VREFHIx pin floating.  You need to connect VREFLOx to VSSA.

    Regards,

    Joseph