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TMS320F28379S: ADC buffer interface voltage

Mojo
Expert 1570 points

Part Number: TMS320F28379S
Other Parts Discussed in Thread: OPA320, REF6033, REF3433

Hi,

We're using the 28379S in motor control application.

we use Vrefhi @ 3V with REF5030AIDR (powered from 5V) followed by OPA320 (powered from 3.3V). output series cap and resistor is 560mOhm and 2.2uF.

each ADC Vrefhi has it's own OPA320.

All the analog signals go through a unity-gain buffer before connected to the ADC pins.

we're using the buffer TLV9064IDR. output capacitor between TLV9064IDR and ADC pin set to 22pF.

the question is what voltage should we supply the buffer?

we think the reasonable answer would be the 3V ref.

however in all the designs we saw the buffer is connected to the 3.3V. so maybe another consideration?

thanks a lot and appreciate also comments about the Vref circuit.

  • 0
    TI__Mastermind 41900 points

    Hi Ari,

    The TLV9064IDR op amp buffer is spec'd as rail-to-rail so, yes you can supply the buffer with 3V level, but make sure not to use the 3V output from the VREF to supply it.  The op amp buffer can be powered by 1.8V to 5.5V per specifications and seems like in your system, there are two options to choose to supply it.  You can either take the supply from the same 3.3V rail that powers the OPA320 or from the 5V rail where REF5030AIDR suppy is connected to.  Either supply should be fine.

    Best regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Expert 1570 points

    Hi Joseph,

    thanks for your explanation.

    is it possible to use an accurate REF to supply 3.3V to VREFHI?

    or even better to use the VDDA for VREFHI? this will save us a lot of circuitry...

    If we use 3V we basically lose 10% accuracy...

    that's our VDDA:

    thanks

    Ari

  • 0 in reply to Mojo
    TI__Mastermind 41900 points

    Hi Ari,

    That really depends on how accurate you want the conversion results to be.  Using VDDA as reference to VREFHI is possible but conversion results will not degrade.  VDDA supply powers the analog blocks inside the chip and part of the analog block are the clock circuits and if clocks are running, the VDDA supply line will be noisy and this will couple to VREFHI and this will impact conversion results.  The reason why we recommend using an independent, accurate and buffered VREFHI supply to the ADC is to ensure that conversion will not be affected by supply rail noises on the F28379 chip.

    REF50xx family does not have a 3.3V variant, I believe.  It may be worthwhile looking into REF6033 which is an accurate 3.3V VREF and with an integrated buffer (so you would not need to use OPA320) or look at REF3433 which is also a 3.3V VREF but requires buffering for VREFHI.

    Regards,

    Joseph