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ADS1282: Asymmetric bipolar analog supply for ADC

Pedro Querejeta Simbeni
Prodigy 10 points
Part Number: ADS1282

The ADS1282's datasheet states that this device can be used with unipolar (0V, 5V) or bipolar (-2.5V, 2.5) analog voltage supplies.

I would like to know if it is possible to use it with an asymmetric (w.r.t. 0V) bipolar configuration: e.g. (-1V, 4V) or (-2V, 3V)). If so, how the output data shall be interpreted?

As far as I understand, the output code will have an offset. This is, for an input voltage of 0V the output code will not be 00000000h but the equivalent of a negative value. Is this correct? 

Thanks in advance!

Best regards.

  • 0
    TI__Mastermind 46990 points

    Hello Pedro,

    Welcome to the TI E2E community!

    Yes, you can use asymmetric supplies with the ADS1282.  The limits are shown in the datasheet specifications:

    AVSS is allowed to be from 0V (with respect to DGND) down to -2.6V.  The AVDD supply should always be typical 5V above AVSS, with a range of AVSS+4.75 to AVSS+5.25 volts.

    The inputs are differential and always measure the difference between the inputs.   If the input is 0V (AINP=AINN), then the output code will still be 00000000h.  There will not be any offset.

    As an example, with AVSS=-2V and AVDD=+3V:

    AINP1=1V

    AINN1=-1V

    Vin-diff=(AINP1-AINN1)=(1 - (-1))=2V. 

    With AINP1=-1V

    AINN1=1V

    Vin-diff=(AINP1-AINN1)=(-1-1)=-2V

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points
    Hello Nicholas,
    Thank you for your prompt response!

    I still have doubts regarding the use of asymmetric reference voltages VREFP/VREFN in conjunction with AVDD/AVSS. If AVSS = VREFN = -1V and AVDD = VREFP = +4V, then with a differential input of 0V (AINP = AINN), will the output code still be 00000000h?

    In another question on the TI forum (https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/642126/ads1282-data-format-for-unipolar-power-in-avdd-and-vref-for-ads1282?tisearch=e2e-sitesearch&keymatch=ads1282%252525252520asymemetrical%252525252520Vref#), an answer suggests: "When you go from a bipolar setup using +/-2.5 V to a uniploar setup using 0-5 V, you are essentially level shifting everything by adding an offset to your input signal" From the responses, I understand that with a +/-2.5V configuration, the offset is 0V, and with a 0V/5V configuration, the offset is 2.5V. Therefore, I expected the same behavior in my case (level-shifting) with an offset ranging between 0V and 2.5V.
    Regards.
  • +1 in reply to Pedro Querejeta Simbeni
    TI__Mastermind 46990 points

    Hi Pedro,

    Yes, with a differential input of 0V, the output code will be ideally 00000000h.  The actual code will be different due to noise and offset.

    Yes, in order to meet the input common mode range of the internal PGA, you should keep the input common mode level to mid supply.  In the case of single 5V analog supply, this will be 2.5V.  In the case of bipolar +/-2.5V supply, this will be 0V.  If using asymmetric bipolar supplies, for example, -1V and +4V, the input common mode should be set to 1.5V.  (AVDD1+AVSS)/2

    Also to be clear, when I say input common mode, this is the average of the differential voltages on the inputs, with respect to DGND.

    Vcm=(AINP+AINN)/2

    Vdiff=(AINP-AINN)

    Regards,
    Keith