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ADS8317: Can device used in Vref=2.5V when Vdd=4.75?

Hung Ching Hsu
Expert 3086 points
Part Number: ADS8317
Other Parts Discussed in Thread: ADS8863

Hi Team, 

My customer is considering ADS8317 and he has one question. 

He wants to use it with Vdd = 5.0V±5% and Vref = 2.5V. 

Datasheet says recommended Vref range is 0.5V~Vdd/2 when VDD is 5V. 

Can Vref=2.5V be used when VDD is 4.75V(5.0V-5%)? Since Vdd/2 will be 2.375V, which is smaller than 2.5V. 

Or when VDD is 4.75V, the Vref(max) will just be 2.375V rather than 2.5V?

Thanks.

Regards,

Jo

Thanks.

Regards,

Jo

  • 0
    TI__Mastermind 46200 points

    Hello Jo,

    The ADC should still operate correctly, but many of the ADC specifications may not be met, such as gain, INL, DNL, and so forth.  For this reason, I recommend that you keep the reference voltage Vref=<Vdd/2.  You have two options.

    1.  Reduce Vref to less than 2.375V, such as Vref=2.048V.  You will then have a full scale input range in single-ended mode of 0V to 4.096V.

    2.  Use a slightly higher voltage for Vdd with a better tolerance.  As an example, set Vdd=5.2V and choose a regulator with 3% or better accuracy such as TPS78401.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Expert 3086 points

    Hi Nicholas, 

    Thank you for the reply! Customer wants to confirm another. 

    Now customer uses ADS8317 with single-ended input with following condition. 

    Vdd: +5V

    Vref: +2.5V

    +IN: +0.5V ～ +3.0V

    -IN: +0.5V (Common Voltage)

    From figure 38. below, it shows common voltage range is +2.2V~ +2.8V when Vref=2.5V.

    Customer wonders if they can use as single-ended input with above condition?

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 46200 points

    Hello Jo,

    As you point out, this operating condition is outside the limits in the data sheet.   We cannot recommend using the device with these input voltages.  

    Please take a look at the ADS8863.  This ADC has true differential inputs, and is specified for operation with any valid input voltage on both the positive and negative inputs.  
     Regards,

    Keith