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TMS320F28388D: VREFHI below VDDA OR VREFHI at or below VDDA

Hrishikesh Nene
Prodigy 95 points

Part Number: TMS320F28388D

Hello C2000 gurus,

The datasheet is a bit unclear on allowable voltage on the VREFHI pins to achieve specified ADC/DAC performance parameters. I have always believed that the VREFHI pin can be driven by a 3.3V ref if the VDDA was also 3.3V. However, the highlighted part from the datasheet below suggests that VREFHI must be below VDDA. Is this correct? Should this instead say VREFHI pin must be at or below VDDA? If VREFHI must be below VDDA then by how much should it be below VDDA? Is the max VREFHI reference limited to 3V when VDDA is 3.3V, in order to achieve specified performance parameters?

Thanks,

Hrishi

  • +1
    TI__Mastermind 43150 points

    Hi Hrishi,

    What is driving this note is that there is a corner case in F28388 that if VREFHI exceeds VDDA, the linearity performance close to FSR degrades.  There is VDDA tolerance to account for as well so VREFHI should be kept below VDDA - tolerance to ensure linearity is met, otherwise if VREFHI is equal to VDDA, without accounting for the supply tolerance, linearity error for input signal levels close to VREFHI value will be higher.   Yes, I agree that we need to be more clear on VREFHI requirement with respect to VDDA.  I'll propose this to our team and will let you know the change in the notes.

    Best regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Prodigy 95 points

    Hi Joseph,

    Great to hear from you! Hope all is well. In most cases linearity near FSR might not matter too much but for some it might (depending on the application and usage).

    This helps a lot. Thanks!

    Hrishi

  • 0 in reply to Hrishikesh Nene
    Prodigy 95 points

    Hi Joseph,

    Can you please confirm that this is not the case with the F2837x devices? The TMS320F2837x datasheets suggest restricting VREFHI to below VDDA + 0.3V (see below). I should be okay using a 3.3V VREFHI with the F2837x devices so long as VDDA is also 3.3V and the supply tolerances are below 0.3V?

    Thanks,

    Hrishi

  • +1 in reply to Hrishikesh Nene
    TI__Mastermind 43150 points

    Hi Hrishi,

    Good to hear back from you and hope everything's good on your end!  On F2837x, you can use 3.3V VREF so long as VDDA is 3.3V and supply tolerances are below 0.3v.  Curious to what application is this for or what ADC is sensing.

    Best regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Prodigy 95 points

    Hi Joseph,

    Thank you for the quick reply. This is for a bi-directional converter, so the current feedback is restricted to half the total ADC scale in either direction and centered around half-scale. Scaling the ADC inputs to 3.3V instead of 3V improves overall SNR and system stability at low loads.

    Everything is good. Still amazed by how feature rich the C2000 Analog sub-system is. Such a perfect complement to the on-chip PWMs (also the CLBs, eCAPs, and eQEPs). Even after all these years I still come across features/tricks with these peripherals that I had not known before. Of course I also come across some errata and datasheet notes like this one (for the F2838x) that throw me off at times. But overall not much to complain.

    Thanks again!

    Hrishi