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ADS131E08S: Are the differential inputs on these devices bipolar?

Connor Connaughton
Expert 4460 points
Part Number: ADS131E08S

Hi Team,

I am working with your ADS131E08S and have some questions on the device below. 

1) Are the differential inputs on these devices bipolar? 

2) If so, can the device be configured to have the negative differential input grounded while the positive differential input swings between -2.5V and +2.5V?  

3) If #2 is possible, do the PGA and ADC have rail-to-rail performance where the device can properly resolve signals across this entire [-2.5V to +2.5V] span if provided with -2.5V and +2.5V analog supplies?

Thanks!

Connor

  • 0
    TI__Mastermind 18900 points

    Hi Connor,

    Connor Connaughton said:
    1) Are the differential inputs on these devices bipolar? 

    In short, yes. For clarity's sake, we are defining bipolar inputs as the act of the differential voltage between inputs ending up negative and the ADC properly interpreting that relationship. This is shown in the Recommend operating conditions.

    Connor Connaughton said:
    2) If so, can the device be configured to have the negative differential input grounded while the positive differential input swings between -2.5V and +2.5V?  
    Connor Connaughton said:
    3) If #2 is possible, do the PGA and ADC have rail-to-rail performance where the device can properly resolve signals across this entire [-2.5V to +2.5V] span if provided with -2.5V and +2.5V analog supplies?

    We can easily swing between -2.5V to 2.5V, but only with the correct supply voltages. But only with the correctly supply voltages as shown below:

    Essentially, we need bipolar supply voltages instead of unipolar supply voltages. In unipolar, a typical set up would be AVSS = 0V, AVDD = 5V, DGND = 0V, and DVDD = 3.3V. If we were to take bipolar supply voltages instead, we would do AVSS = -2.5V, AVDD = 2.5V, DGND = 0V, and DVDD = 3.3V. Because VREFN is referenced to AVSS, everything should work out if we check out math in the bipolar voltage supply condition:

    • AVDD - AVSS = (2.5) - (-2.5) = 2.5 + 2.5 = 5V < 5.25V (recommended) < 5.5V (abs max). This falls within AVDD to AVSS condition
    • G = 1, VREF = 4V, V_IN = V_AINP - V_AINN = (-2.5) - (0) = -2.5V
      • V_DiffMin = -VREF/G = -(4)/(1) = -4V (recommended) < -2.5V (V_IN) 

    So yeah, it should work fine.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    TI__Expert 4460 points

    Hi Cole,

    Thank you for the great answers above! The team did have some follow up clarifications below. 

    With regard to your answer, I believe part of my confusion stemmed from the Absolute Maximum Ratings section where “Vref input to AVSS”  is limited to “AVSS – 0.3 to AVDD + 0.3.”  The “Vref input to AVSS” part is clear but the limit conditions are not (at least to me) as I am unsure of what AVSS and AVDD are referenced to in this instance.  Based on your reply, it would seem AVSS is referenced to itself and AVDD is referenced to AVSS as well.  Is this correct?

     

    Moving to the External Reference portion of the Electrical Characteristics table; it seems, for the bipolar supply case of AVSS = -2.5V and AVDD = +2.5V (both with respect to the power supply’s ground) and with VREFN tied to AVSS and VREFP = AVSS + 2.5V, that VREFP would essentially be at the ground of the power supply providing AVSS and AVDD.  Does this seem like a proper understanding/application of this part?

    Thanks!

    Connor

  • 0 in reply to Connor Connaughton
    TI__Mastermind 18900 points

    Hi Connor,

    Connor Connaughton said:
    Based on your reply, it would seem AVSS is referenced to itself and AVDD is referenced to AVSS as well.  Is this correct?

    I'd say this interpretation is fine.

    I personally acknowledge that AVSS is the negative analog supply and DGND is the digital supply. Then I establish DGND to be true 0V, and then AVSS is referenced from GND (AVSS - GND = -3V through 0.2V, so AVSS is effectively -2.5V or 0V) and then it cascades from there. If we are doing AVSS = DGND = 0V, then it makes math easy, and if AVSS = -2.5V, I need to remember to do that in the equations like I illustrated above. Hope that answers it.

    Connor Connaughton said:
    with VREFN tied to AVSS and VREFP = AVSS + 2.5V, that VREFP would essentially be at the ground of the power supply providing AVSS and AVDD.  Does this seem like a proper understanding/application of this part?

    That is correct. Essentially, VREFN is connected to AVSS, which is the negative analog supply to AVDD.

    That being said, it is recommended keep the path from the external reference source to the VREFN as low inductance as possible, without the possibility of other current to travel across the same trace and cause the voltage to fluctuate. In other words, AVSS should be connected VREFN at voltage reference, not at the AVSS pin, if that makes sense. Its similar to the mind set of a Kelvin connection except the VREFN trace will carry current as well so the trace shouldn't be really skinny.

    Best,

    -Cole