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TMS320F28335: Digital or analog power for ADC

Yiftah Kowal
Prodigy 90 points

Part Number: TMS320F28335

Hi,

I'm designing a custom PCB with the F28335.

Should I connect "ADC Analog Power Pin" to the digital 3.3V that is connected to VDDIO and to digital 1.9V that is connected to VDD?

or should I connect it to the 3.3VANALOG that I use for my analog sensors that are connected to the ADC?

If it is the latter, I don't have 1.8VANALOG in my circuit - do I need to add it to avoid noise on ADC?

Same question for "ADC Analog Ground Pin": should I connect it to the digital ground or the analog ground?

Best regards,

Yiftah

  • 0
    TI__Guru 62975 points

    Yiftah,

    There are no strict requirements regarding the sharing of supplies. TI only recommends that the supply levels as seen by the device fall within the Recommended Operating Conditions table.

    A reasonable approach would be to evaluate the performance of a F2833x Experimenter Kit (reference design) to see if it meets your needs.

    -Tommy

  • 0 in reply to tlee
    Prodigy 90 points

    Hi Tommy,

    That's exactly what I did. In the reference design, the "ADC analog power pin" is connected to the digital voltage (VDDIO).

    This is what I did in the first batch of my PCB.

    But now I'm looking at ways to reduce the noise in the ADC measurements. So before wasting time and money on a second batch of the PCB, I was wondering if you guys at TI have any experience with the change I proposed: connecting "ADC Analog Power Pin" to 3.3VANALOG instead of VDDIO.

    Best regards,

    Yiftah

  • 0 in reply to Yiftah Kowal
    TI__Guru 62975 points

    Yiftah,

    Are you experiencing a specific issue with the ADC performance?  We (TI engineers) usually experience ADC noise with sub-optimal ADC configuration (like ACQ_PS) or poor PCB routing (like switching noise coupling to ADC trace).

    We have experimented with the power supply configurations to look for performance differences, but we typically revert to sharing a single 3.3V supply between VDDIO and VDDA for our own setups.  The ADC is able to reject supply noise well enough for our bench environment.  The supply inductors on the controlCARD are also very likely excessive.

    If sourcing 3.3V from the analog sensor supply, you will want to confirm that the Power Sequencing recommendations from the datasheet are met (keep VDDIO and VDDA within 0.3V of each other).  I would not expect as much improvement from an isolated 1.8V supply for VDDAnA18 -- this is powering the digital components of the ADC.

    -Tommy