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

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

• 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

• 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

• 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