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AM2634-Q1: AM263x External VREF Source Current

Joe Even
Prodigy 10 points

Part Number: AM2634-Q1

Hello,

I am looking to apply an external 1.8V Reference to the ADC and DAC Reference Pins (ADC_VREFHIX and DAC_VREFX) on the AM263x series of Sitara MCUs and do not see a specification for the current sourcing capability of the external reference. 

In the Control Card there is a unity gain amplifier feeding each of the references, and in the Launchpad there is an external connector, but I can not find a required source current in the datasheet.

Please let me know what current is required!

Thank you,

Joe

  • 0
    TI__Intellectual 2970 points

    Hi Joe,

    The AM263x Control Card design uses REF3318AIDBZT to generate the Reference voltage and uses OPA4350EA/250 as the unity gain amplifier. The details can be found in AM263x Control Card schematics. Below screenshot for quick reference.

    For the specifications, please do refer to the Section 6.8.2 Analog-to-Digital Converter (ADC) of AM263x SitaraTm Microcontrollers with Real-Time Control datasheet (Rev. E) for the VREFHI input current of the ADC Input reference.

    Please do note that the AM263x MCU also has internal ADC reference generated by REFBUF[2:0], which can also be used for ADC reference. More details can be found at Section 7.4.2.2.3.1 Internal ADC Voltage Reference Buffer Control of AM263x SitaraTm Microcontroller Technical Reference Manual (Rev. I). Please do ensure this internal reference buffer is turned off when external reference is used.

    Thanks,
    Tejas Kulakarni

  • 0 in reply to Tejas Kulakarni
    Prodigy 10 points

    Thank you, Tejas!

    Not sure how I missed the VREFHI Input current, but here we are! Is there any further justification / requirements that are driving the use of the unity gain amplifier?

    I am intending to use the LM4128BQ1MF1.8/NOPB as my reference for both the ADC and DAC 1.8V reference for improved accuracy over the internal reference. Does this seem like a viable option to directly drive the ADC_VREFHI pins and the DACVREF pins with only a single LM4128BQ1MF1.8/NOPB? Or will I need a buffer as seen in the Control Card.

    Thanks,

    Joe

  • 0 in reply to Joe Even
    TI__Intellectual 2970 points

    Hi Joe,

    Joe Even said:
    Not sure how I missed the VREFHI Input current, but here we are! Is there any further justification / requirements that are driving the use of the unity gain amplifier?

    No worries!!. With proper decoupling (external 4.7µF), the requirement of a unity gain amplifier would get reduced. But having this unity gain amplifier would isolate the noise of one ADC to another (the references will have their individual reference source and hence during synchronous operation of ADC, they would not get affected from each other).

    If you are looking for the best possible accuracy, I would suggest having this unity gain amplifier. Else I would just connect the reference directly with proper decoupling in between.

    Also, since you are looking for the best accuracy, I hope you got to have a look at the Section 7.4.2.18.2 Choosing an Acquisition Window Duration of the AM263x Technical Reference manual and the further linked app notes in it, for ADC signal conditioning circuit design and evaluation.

    Thanks,
    Tejas Kulakarni