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ADS8688: External VREF

Paolo Beldenti
Prodigy 50 points
Part Number: ADS8688


Hi,

On ADS8688, when  the external reference is selected by setting high the REFSEL input pin, does the external reference MUST be a 4.096V or can it be a different voltage?

My aim is to connect an external 4,000V to get the industry standard input range of ± 10, ± 5 etc., instead of ± 10.24, 5.12V etc.

Thank you

Paolo Beldenti

  • 0
    TI__Guru 64125 points

    Hello Paolo,

    We had a meeting with Moog about two weeks ago where we showed this device.  I wonder if you would confirm what type of application it will be used for, and/or what types of inputs will be connected to it.

    The external reference voltage ranges are shown in the "External Reference Input" section of the spec table.  Unfortunately 4V is below the minimum acceptable VREF voltage of 4.046 as shown in the datasheet snippet below:

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Thanks Collin,
    I had already read the external range of vref on the datasheet but I didn't understand why it was so limited.
    Why make an external vref available only if its value is identical to the internal vref value?

    External Vref more accurate? or to couple more than one ADS with the same external Vref? and why not a 2.5v or 4v vref ?, so i thought about an error on the datasheet.

    From your answer I understand that the datasheet is correct and it is not possible to connect an external 4V Vref

    I can solve this issue in the digital mode or adding a gain in the analog chain. My application (for legacy) require ±10V, ±5V (+10, +5V) of full scale.

    Thank you

  • +1 in reply to Paolo Beldenti
    TI__Guru 64125 points

    Hi Paolo,

    Thank you for the good feedback on the requirements for the application.  The option for the external VREF input is to allow for a higher initial accuracy and/or lower-drift external reference which can be required if the performance of the internal reference is not sufficient.

    The ADS8688 has a higher level of integration than most standard SAR converters and as such is meant to be operated in a slightly more specific way.  The slightly increases ranges of +/-10.24 were requested by many customers to allow for some slight over-range.  Understood this does not directly fit your +/-10V = full-scale range requirements.  Other customers have achieved the desired transfer function for this by correcting in digital or in the up-front circuitry as you suggested.