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ADS1248: Configuration for external Vref and unipolar inputs

Andrew5821
Genius 4565 points

Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08

Hello,

I'm trying to create a design using the ADS1248.  My input signal levels are unipolar 0-3V.  The equations in the datasheet for the common-mode input range do not seem to compute correctly for me.  I'm wondering if I'm improperly using them.  Equation 3, I get values that don't make sense when I use a Vinmax of 3V.  In the end I want to know if I have this configured properly for a 0-3V unipolar input on the 7 channels.  We will also use a PGA gain of 1.  Also, I want someone to double check and make sure that I am satisfying the requirement in note (2) of the datasheet. "(2) For VREF > 2.7 V, the differential input voltage must not exceed 2.7 V / Gain."  I believe I am since I'm providing a midpoint reference on channel 7.  Rough schematic is given below.

  • 0
    TI__Guru** 100775 points
    Andrew,


    I think your question depends on how you make the measurement. I believe that you are selecting AIN0 as the positive input (AINP) and AIN7 as a common negative input (AINN). If you have AINP going from 0V to 3V, but AINN is stationary at 1.5V. then your differential input is ±1.5 and Vinmax is 1.5V.

    I would like to point out a couple of things though. The input of the ADC is limited even in a gain of 1 to AVSS+0.1V to AVDD-0.1V. If you have 0V at the input AIN0, this is just outside the range of operation. However, the op-amp buffer itself won't be able to drive directly to ground either, so you may not be counting on that input either. However, if you really have AVSS and the op-amp negative supply connected to -0.2V, then this may not be a problem for you.

    As for the "(2) For VREF > 2.7 V, the differential input voltage must not exceed 2.7 V / Gain" spec, you should be ok with your setup. VREF is 3.0V, but the differential voltage should be as high as +1.5V. If you use a gain of 1, this should be fine.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Genius 4565 points
    Thank You. I was considering the -0.2V supply for Avss. After some further research, I think I might go with ADS124S08 as it has a few more channels, auto chopping mode, and a PGA bypass which, I think, will make unipolar inputs a bit easier. We have a large variety of sensors and this might be our best bet.
  • 0 in reply to Andrew5821
    TI__Guru** 100775 points
    Andrew,


    I think the ADS124S08 would be a good choice then. The ADS124S08 is a newer device, and I'd consider it the next generation of the ADS1248. It has more channels and slightly better specifications. You can also bypass the PGA, allowing for measurements to the rails.

    Regardless, if you have further questions, post back.


    Joseph Wu