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Measuring unipolar and bipolar signals with single ADS1120

Denis Kotlar
Intellectual 790 points
Other Parts Discussed in Thread: REF5025, ADS1120, LM7705, LM337L

Sorry in advance for this question with possibly obvious answer. I have two input signals that I'd like to measure with single ADS1120. First is strictly unipolar in the range of 0 to 2.5V and another bipolar (range: -0.2V to 2.5V). I'd like to use external reference REF5025 and don't need any amplification therefore PGA will be disabled with gain set to 1.

Please let me know is it possible to do that with single device? In that case I presume that AVSS should be below zero (i.e. -2 to -2.5V to be within allowed limitations). I didn't find a single reference in datasheet about measurement with AVSS bellow zero. I'd like to use +5V for DVDD.


Many thanks in advance.

  • 0
    TI__Guru** 113765 points

    Hi Denis,

    The input voltage range for the ADS1120 when the PGA is disabled is shown on page 5 of the datasheet.  The measurement range is from AVSS - 0.1V to AVDD + 0.1V when the PGA is disabled.  This allows measurement slightly above and below the analog supply rails.

    The ADS1120 will always measure differentially, that is AINP relative to AINN.  If AINP is more positve than AINN you result is positve.  If AINP is more negative than AINN the result is negative.  With single-ended mesasurements you lose the negative range when connecting the AINN input to analog ground (assuming that the input on AINP never goes below analog ground). If your input goes below ground more than 0.1V, you will see a degradation in linearity as you approach the absolute maximum input voltage (AVSS - 0,3V).  If you want to measure accurately below ground by more than 0.1V, you will need a negative supply on AVSS.  A simple way to do this is to add slightly negative bias to AVSS using the LM7705.  This device will generate a negative voltage of 0.232 V which can be connected to AVSS.  You can supply the ADS1120 AVDD with the 5V used for the DVDD.  The 5V can also supply the LM7705.  Using this method you have extended the input range of the analog inputs. 

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 790 points

    Many thanks Bob. LM7705 looks nice, but I already have -15V on board so I hope that LM337L can be also used. Huge drop (~13V) shouldn't be a problem if power consumption on AVSS is tiny. Please confirm.

  • 0 in reply to Denis Kotlar
    TI__Guru** 113765 points

    Hi Denis,

    The LM337L should work for you.  Just make sure that AVDD - AVSS stays within the correct operating range for the ADS1120 when you adjust the regulator output.

    Best regards,

    Bob B