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TMS320F28P650DK: The capacitors used between VREFHI and VREFLO

Wayne Huang
Genius 16455 points
Part Number: TMS320F28P650DK
Other Parts Discussed in Thread: OPA350

Dear champs,

I am asking this for our customer.

It seems there are inconsistencies in the datasheet and in TI F28P65x control card/Launchpad schematic.

Questions:

1. What are the required minimum capacitors for below conditions? Is below table correct?

Please specifically confirm the cap. value for ADC 16-bit differential - 2.2uF or 22uF? From datasheet 6.13.2.2.8 ADC Characteristics 16-bit Differential, it's 2.2uF, but from Table 5-1. Pin Attributes, it says 22uF and in the F28P65 control card/Lauchpad, it uses 4.7 uF.

Internal reference External reference
ADC 12-bit Single-ended 

Connect VREFHIA/B/C externally

Cap. >= 2.2 uF

Use an external reference (like REF5030) + buffer (like OPA350)

Cap. >= 2.2 uF
ADC 12-bit Differential

Connect VREFHIA/B/C externally

Cap. >= 2.2 uF

Use an external reference (like REF5030) + buffer (like OPA350)

Cap. >= 2.2 uF
ADC 16-bit Single-ended

Connect VREFHIA/B/C externally

Cap. >= 22 uF

Use an external reference (like REF5030) + buffer (like OPA350)

Cap. >= 22 uF
ADC 16-bit Differential

Connect VREFHIA/B/C externally

Cap. >= 2.2 uF?

Use an external reference (like REF5030) + buffer (like OPA350)

Cap. >= 2.2 uF?

2. Some inconsistencies.

1) In datasheet, it says Table 5-1. Pin Attributes below.

Because it has internal reference mode, shouldn't we remove/correct "This voltage must be driven into the pin from external circuitry.".

From the descriptions, it seems >=22uF is required for both 16-bit single-ended and differential. If so, then the datasheet 6.13.2.2.8 ADC Characteristics 16-bit Differential and F28P659 control card schematic are wrong.

2) datasheet 6.13.2.2.8 ADC Characteristics 16-bit Differential

3) TI F28P65x control card

F28P65x Launchpad

F28P65x CC and LP use 4.7uF. Does that mean it cannot be used for 16-bit single-ended mode, which needs 22uF?

  • 0
    TI__Mastermind 38180 points

    Hello Wayne,

    Thank you for pointing this out.  Will sort this out and will have an answer for you by this week.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Genius 16455 points

    Dear Joseph,

    Any update?

  • +1 in reply to Wayne Huang
    TI__Mastermind 38180 points

    Hi Wayne,

    Wayne Huang said:

    1. What are the required minimum capacitors for below conditions? Is below table correct?

    Please specifically confirm the cap. value for ADC 16-bit differential - 2.2uF or 22uF? From datasheet 6.13.2.2.8 ADC Characteristics 16-bit Differential, it's 2.2uF, but from Table 5-1. Pin Attributes, it says 22uF and in the F28P65 control card/Lauchpad, it uses 4.7 uF.

    JC: For internal VREFHI, minimum cap requirement for 12-bit (single-ended and differential) is 2.2uF.  For 16-bit ((single-ended and differential), minimum cap requirement is 4.7uF.  This is because of the improved internal VREF buffer in the F28P65x.  For external VREFHI, cap requirement is based on the reference IC/op amp buffer output ability to maintain stable VREFHI.  On older devices, we typically have indicated minimum VREFHI cap size but there are reference ICs and op-amp buffers that cannot drive higher capacitances and will become unstable.  We're in discussion with design team to see how we can best indicate the minimum caps for external VREFHI for F28P65x and will update the datasheets accordingly.

    Wayne Huang said:

    1) In datasheet, it says Table 5-1. Pin Attributes below.

    Because it has internal reference mode, shouldn't we remove/correct "This voltage must be driven into the pin from external circuitry.".

    JC: Yes, we will modify the note in the datasheet as this is only applicable to external VREFHI mode of operation.

    Wayne Huang said:
    3) TI F28P65x control card
    Wayne Huang said:
    F28P65x CC and LP use 4.7uF. Does that mean it cannot be used for 16-bit single-ended mode, which needs 22uF?

    JC: Initial design of LP and CC have used 4.7uF as this was the minimum cap required for 16-bit for both single-ended and differential modes.  Silicon characterization proved 4.7uF is the minimum cap value, so the VREF caps in CC and LP would support both single-ended and differential modes.

    Best regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Prodigy 30 points

    HI JC

    For external VREFHI, the capacitance requirement is based on the reference IC/op amp buffer output capability to maintain stable VREFHI. On older units we usually indicate a minimum VREFHI capacitor size, but some reference ICs and op amp buffers cannot drive higher capacitances and become unstable. We are discussing with the design team how best to indicate a minimum upper limit for external VREFHI for the F28P65x and will update the datasheet accordingly.

    According to your reply, I currently need to use external external VREFHI as the reference for the ADC. Do I currently need to use 22uF or other?

    TKS

  • 0 in reply to Guan You Chen
    TI__Mastermind 38180 points

    Hi Guan,

    If your external VREFHI circuit can drive 22uF capacitance without any stability issues, then go ahead and use 22uF.

    Regards,

    Joseph