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ADS114S08: Question on 4~20mA signal detection application

Yuanchen Zhu
Intellectual 425 points
Other Parts Discussed in Thread: ADS114S08

Dear TI Expert,

  We have an application that requires to detect 4~20mA signal. We plan to use 10ohm resistor of high precision to transfer to 40mV~200mV voltage signal. And we are thinking of using PGA inside ADS114S08 to amplify the incoming signal, but we've seen minimum absolute input voltage of VAIN is AVSS+0.15+|VINMAX| (Gain-1)/2, which is way higher than 40mV (e.g. we plan to use PGA=8). 

  Using extra bias resistors or simply using a larger sampling resistor does not seem like a good try (we need high precision), any advice on how one should deal with this situation?

Thanks.

Yuanchen Zhu

  • 0
    TI__Guru** 113765 points

    Hi Yuanchen,

    If you want to use a small value of resistor and the PGA, you need to make sure that the voltage is within the input range of the PGA as you have stated.  One way to do this is to use the reference output to bias the measurement resistor.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 425 points

    Using REFOUT to bias, does it mean that the only choice is to bias above the level of 2.5V, and thus one has to use much higher voltage supply for data conversion? (In our case, we plan to employ voltage source of 3.6V for AVDD, based on 0.2V(20mA X 10ohm), we have another limit of high side AVDD -0.15-|VINMAX|（Gain-1）/2. No much room left really.

    Any other way to suggest? Or should we just avoid using PGA inside in this case?

    Yuanchen

  • +1 in reply to Yuanchen Zhu
    TI__Guru** 113765 points

    Hi Yuanchen,

    The input range calculation can be difficult to follow and understand.  TI has available a downloadable calculator tool to help determine the input range.  Below is a screenshot from the application.

    So it is possible to use 3.6V AVDD, gain of 8 and REFOUT as the bias.  If you are still concerned about margin you could reduce the gain to 4 or use another voltage source to lower the common-mode.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 425 points

    Really thanks for your analysis, Bob.

    Yuanchen