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ads1274 differential reference, VREFN = AGND+/-0.1V?

Tjarco Boerkoel
Expert 1725 points
Other Parts Discussed in Thread: ADS1274

Hi All,

I was reading the datasheet of the ads1274 data converter for a new design and I noticed that the voltage reference input is a differential input, but when I look to the maximum ratings it's stated that the input range for the reference input is:

VREFN = AGND -0.1V (MINIMUM) and VREFN = AGND +0.1V (MAXIMUM)

Shouldn't the maximum be AVDD+0.1V? If I take a look at the Figure 70 - Equivalent Reference Input Circuity, both reference inputs are clamped to AGND and AVDD which would indicate that the maximum rating is:

VREFN = AGND -0.1V (MINIMUM) and VREFN = AVDD +0.1V (MAXIMUM)

Could someone please verify this because I would like to use a 2.5V full differential buffered voltage reference, with 2.5V VCOM as offset.

Best regards,

Tjarco Boerkoel

the Netherlands

  • 0
    TI__Guru 57296 points

    Hello Tjarco, 

    Thanks for your question.

    I believe that, in theory, you are correct - each analog pin can tolerate -0.3V to AVDD + 0.3V (per the Absolute Maximum Ratings). The ESD protection diodes are in place to protect each pin in the event that there is a fault condition where the input voltage exceeds the supplies.

    However, as you noticed, the reference input is differential. If VREFN equaled 5.1V (AVDD + 0.1V), we would have to drive VREFP to 7.6V to achieve a 2.5V VREF. This would exceed the absolute max rating for VREFP and would not be an acceptable solution in practice.

    I recommend that you use VREFN = 0V, VREFP = 2.5V. VCOM can be set to 2.5V as well, which will set the common-mode level of your input signals. As long as the differential input (AINP - AINN) is less than +/-VREF, the ADC will be able to convert properly.

    Let me know if this answers your question.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Expert 1725 points

    Hi Ryan,

    Thank you for your explanation. I would like to apply a differential (balanced ) reference voltage to the ADS1274.

    You mentioned the limitations of the balanced inputs and explained the recommended range for VREFN and VREFP, and I'm aware of the limitations of the internal (electrical) limitations (with regard to AVDD 5V). But I'll be needing this.

    With a supply AVDD of 5V I would like to apply a differential reference voltage of 2.5V to VREFN - VREFP, with a common mode offset of 2.5V. This will result in an absolute voltage of 1.25V on VREFN and an absolute voltage of 3.75V on VREFP.

    I assume this is within operation limits and that the ADC will operate properly according the specifications???

    (AGND < VREFN < VREFP < AVDD ) && ( AGND < (VREFP-VREFN) < AVDD)


    Best regards,

    Tjarco Boerkoel

  • 0 in reply to Tjarco Boerkoel
    TI__Guru 57296 points

    Hi Tjarco, 

    I made sure with our team that the limits on VREFN in the Electrical Characteristics table are correct at AGND +/-0.1V.

    The Abs Max Ratings may be a little misleading. I should have explained in my first post that the Abs Max Ratings are there to ensure that the device is not operated in a condition where it may be damaged. This does not mean that if you exceed the Electrical Characteristics specs, but remain within the Abs Max Ratings, that the device will function properly.

    Therefore, as an analog pin, VREFN may be able to tolerate 2.5V, but the device will not function as it is intended. Basically, this spec is telling you to tie VREFN to AGND. 

    One important concept to keep in mind that your input signal and your reference do not need to operate around the same common-mode voltage. The ADC conversion takes a differential measurement at the input and compares it to the differential voltage at the reference. Shifting the common-mode of the input signal grants you more output swing and allows you to use the full resolution of the ADC. In the end, it is only the differential input (AINP - AINN) that is compared to VREF. 

    I hope this helps, let me know if anything is still unclear. 

    Best Regards,

  • 0 in reply to Ryan Andrews
    Expert 1725 points

    Hi Ryan,

    Thank you for your detailed answer. This clears up a lot for me. Well, I've got a buffered VCOM so it was easy to use them on all differential amplifiers (incl the VREF buffer).

    Best regards,


    Tjarco Boerkoel