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ADS124S06: Differential input problem

Anna
Intellectual 940 points
Part Number: ADS124S06

Hi Dear,

ADS1224S06 Supports single ended input and differential input. When the input signal is differential, the AINP and AINN are 0.2V and -0.1V respectively.

The signal detected by ADC is the difference between AINP and AINN. Is this difference the absolute value of the two signals subtracted to obtain 0.1V? Or AINP-AINN= 0.3V?

May I ask which understanding is correct？

  • 0
    TI__Guru** 113765 points

    Hi Anna,

    The ADS124S06 always measures differentially regardless if the input connection is single-ended.  That is to say that AINP is always measured relative to AINN.  In the single-ended case AINP will always be greater than AINN as AINN is fixed to ground.  This means only the positive codes are available for the conversion.

    In the differential input case the codes can be either positive or negative as long as the input signal is within the input range of the ADC input.  So in theory you are correct in that the difference is 0.3V for the use case, but the input is outside of the measurement range if you use a unipolar supply (for example if AVSS is tied to AGND).  If the PGA is enabled the input is restricted to equation 5 on page 34 of the datasheet.  If the PGA is disabled and bypassed, the input restrictions are more relaxed.  However the input measurement range is restricted to the range as specified in equation 6.  So -0.1V is outside the measurement range and the result will be non-linear.

    With that said, if you use bipolar supplies for AVDD/AVSS (+/-2.5V for example) the measurement would be within the proper range and the outcome would be as expected.

    Best regards,

    Bob B