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differential gain amplifier for ADS1015

Anand Kumar Rai
Intellectual 720 points
Other Parts Discussed in Thread: ADS1015, INA333, LMP90100

Hi,

This is about ADS1015, I am exciting my four wire RTD using 1mA current source, and to feed my RTD differential output to ADS1015,I need a differential input and differential output gain amplifier. Please suggest a low cost gain amplifier that is compatible with ADS1015 in differential mode.

Thanks and regards

-Anand 

  • 0
    TI__Guru 63905 points

    Hello Anand,

    The ADS1015 has differential inputs and an integrated PGA already.  Does the internal PGA not have enough gain or were you having trouble with its performance?

    Could you share a basic schematic or diagram of how you're connecting the RTD to the ADS1015 including the current source?

     Thanks and Regards,
    Collin Wells
    Precision Linear Applications

  • 0 in reply to Collin Wells
    Intellectual 720 points

    Hi Collin,

    On excitation of 4-wire RTD with accurate 1mA current source for my temperature range, the voltages are mentioned below:

     

    Temp: -50 to 120 degree = 170 degree x 0.1K = 1700 step 

    Resistance at -50 degree      =        80.31   R       corresponding voltage          80mV

    Resistance at 0 degree         =        100.00 R        corresponding voltage         100mV

    Resistance at 120 degree     =        146.07 R        corresponding voltage         146mV

    and precision is 0.1K, considering FULL SCALE voltage of +/- 4.3821, I need to have gain of 30 times.

    Because of this I need to use external gain amplifier and internally ADS1015 has gain of X16 only.

    I am attaching pdf file of schematic for more clear reference.1362.PT00.pdf

    Thanks and regards

    -Anand

  • 0 in reply to Anand Kumar Rai
    TI__Guru 63905 points

    Hello Anand,

    Thank you for sharing more information about your design.

    With the design requirements you're trying to achieve, you are correct that the ADS1015 does not have enough gain in the internal PGA to meet the full-scale input.  So you are correct that you need to and an external gain block to gain the signal further.  However, the implementation shown in the schematic is not appropriate.  To properly amplify the differential signal formed across the 4-wire RTD, you need a difference amplifier or instrumentation amplifier to amplify only the differential signal.  You have currently implemented a single-ended amplification stage which will not achieve the results you're trying to achieve.  Also, You can create a difference amplifier out of a standard op-amp and 4 precision resistors, but with the accuracy you're trying to achieve I would strongly suggest purchasing an integrated difference amplifier or instrumentation amplifier such as the INA333 or similar.

    Best Regards,
    Collin Wells
    Precision Linear Applications

     

  • 0 in reply to Collin Wells
    TI__Guru 63905 points

    Hi Anand,

    I also forgot to mention that you've connected your op-amp inappropriately and have implemented positive feedback where I believe you meant to implement negative feedback.

    Regards,
    Collin Wells

  • 0 in reply to Collin Wells
    Intellectual 720 points

    Hi Collin,

    Thanks for your feedback, and reviewing the design, We will carry forward your suggestions and we have also come across  sensor AFE LMP90100 designs, which we are evaluating now.

    Thanks and regards

    -Anand