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ADS130E08: Single-ended input range

Mark Guastaferro
Genius 11355 points
Part Number: ADS130E08

I'm a little confused by the stated analog input range and diagram when ADS130E08 is configured for single-ended input. Figure 18 shows the input range as CM voltage (INN) +/- Vref/2.

Ignoring CM range concerns and assuming Gain = 1, why is this limited to Vref/2? The ADC code range goes from -Vref to +Vref so it seems the input range can be larger than Vref/2. 

The reason for this question is if the INN inputs are tied to a voltage closer to a one of the supply rails rather than mid-range, it will obviously limit the ADC range in one direction as the input gets approaches the closer power rail, but what about the other direction where there is more margin to the power supply,  is the input really limited to Vref/2 in this direction?

Taking this to an extreme, if INN is tied to AVDD,  the positive half of the ADC range is not available as this is above AVDD,  Is the lower half of the ADC range reach all the way to -Vref  or Vref/2?  I understand that inputs within 0.2V of AVDD are not valid as this violates the PGA CM range. 

  • 0
    TI__Expert 5810 points

    Hello,

    The diagram here assumes that you are using a common-mode voltage that is located midrange, you could use a common mode voltage closer to one of the supply rails if you desire.

    The limitation of using single-ended mode is that you can only ever use half of the code range, and where the usable codes are is determined by the value of the INN. Setting INN close to AVSS would allow you to measure the voltages as positive, and setting INN close to midrange will cause you to measure the voltages as centered around the midrange codes.

    Let me know if you have any more questions,

    Levi DeVries

  • 0 in reply to Levi DV
    TI__Genius 11355 points

    Hi Levi,

    Basically this makes sense, but here's an example I wondering about:

    AVDD = 5.0V, AVSS = 0V, Vref  = 2.4V.  INN is tied to mid-supply = 2.5V. What is the max input range?  It seems I should be able to convert much more than half the code range. Since AVDD = 5.0V, wouldn't an input voltage of say 4.5V be converted? 

    Or is the assumption that with AVDD = 5V, Vref would be set to 4V? 

  • 0 in reply to Mark Guastaferro
    TI__Expert 5810 points

    Hi Mark,

    Mark Guastaferro said:
    Or is the assumption that with AVDD = 5V, Vref would be set to 4V? 

    Yes, I was assuming that Vref was close to your AVDD-AVSS, so the full scale was close to ±AVDD.

    Mark Guastaferro said:
    AVDD = 5.0V, AVSS = 0V, Vref  = 2.4V.  INN is tied to mid-supply = 2.5V. What is the max input range?  It seems I should be able to convert much more than half the code range. Since AVDD = 5.0V, wouldn't an input voltage of say 4.5V be converted? 

    Yes, you are correct, you could utilize the whole dynamic resolution of the part this way. My statements above only apply when Vref is close to (AVDD - AVSS).

    Let me know if you have any more questions,

    Levi DeVries