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Problems with analog interfacing ADS1247

Alexandra Castellon
Prodigy 150 points
Other Parts Discussed in Thread: ADS1247

Hello,

I'd like to convert a range of +-0,5 V with my ADS124 with unipolar 3,3-0 V voltage reference and power. To avoid putting a negative input to the device I thougt to measure the 0-5V using the voltage reference in the ADS, so my true input comes in range 2,5-2V aprox.

I studied the PGA and the ADC and I find this problem: if I use the AINp to measure, should I use the voltage reference in the AINN sol the input to the ADC is +-0,5V? In other words... Can I try to measure a bipolar range in ADC although I put a unipolar voltage analog input at the ADS?

I attach an image with my current idea and an alternative (although I don't really like it)

Which config should I use?

Thanks in advance

  • 0
    TI__Guru 55466 points

    Hello Alexandra,

    The ADS1247 device can be used to convert positive and negative differential signals; however, both the Positive (AINP) and negative (AINN) inputs are required to be more than 0.1V away from the analog supply rails (see common mode voltage range explanation below).  The internal reference of the device is 2.048V.  If you are planning to use an external voltage reference, the external reference has to be 1V smaller than the AVDD-AVSS analog supply voltage:

    VREF= VREFP-VREFN < (AVDD-AVSS)-1V

     The common mode voltage range requirement depends on the supplies, PGA gain and differential voltage to be applied:

    AVSS+0.1+{(PGAGAIN)(|VINdifferential|)/2} < VCM < AVDD-0.1-{(PGAGAIN)(|VINdifferential|)/2}

    where common-mode voltage of the device is the average voltage of the inputs  VCM= (AINP+AINN)/2.  The differential voltage is VINdifferential=  AINP-AINN

    Below is a link to a design note with more detailed information.

    http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/input-voltage-range-requirements-for-the-ads1248-and-ads1148-families.aspx?Redirected=true

    For example, If you were to use the device with unipolar supply of AVDD=3V, AVSS=0V, you could use an external reference of 1.5V and attach the AINN to the external reference, this set up will bias the inputs close to the middle of the AVSS/AVDD analog supplies and allow you to convert the required differential signal of +/-0.5V across the inputs

    Can you please post the schematic or diagram of the circuit you are planning to use?  The image does not appear to be attached in the post above.

    Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 150 points

    Hi Luis,

    Thanks for your answer and sorry for the inconvenience of moving my post. I read our links and this are my calculus:

    VINdifferential =  AINP – AINN = +-0.5V

    Full-scale input voltage = +/-VREF/PGAGAIN = +-3.3V         (G=1)

    VCM = (AINP+AINN)/2 => 1.75V < VCM < 2,25V

    AVSS + 0.1 + [(PGAGAIN)(|VINdifferential|)/2]  <   VCM   <   AVDD - 0.1 - [(PGAGAIN)(|VINdifferential|)/2] => 0.35<VCM<2,95

    Do these calculus say my design is correct?

    Here is the schematic, I hope it's ok now.

    The idea is just add an offset to the measure. I'd like to use the VREFOUT in order to avoid using another external reference (final design price issues)

  • 0 in reply to Alexandra Castellon
    TI__Guru 55466 points

    Hi Alexandra,

    Your calculations are correct and connecting the 2.048V internal reference  (VREFOUT) to AIN0 will work well with PGA=1; setting the device with the correct common-mode voltage.  The only issue is that per the datasheet,  the external reference input range needs to be 1V less than the analog supply voltage; if AVDD=3.3V and AVSS=0V; the maximum external reference allowed is 2.3V.  A possible option may be to use the internal 2.048V for conversion; since your expected input voltage range is +/-0.5V and you plan to use PGA=1.

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 150 points

    Thanks so much for your answers and advice! I hope it helps others the same as me. It helped me understanding how the ADS1247 works.

    Best Regards,

    Alexandra