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ADS1282: ADS1282 ADC temperature sensing ratiometric measure

Ian
Prodigy 20 points

Part Number: ADS1282

Tool/software:

Dear TI team,

We are using the ADS1282HKV ADC for RTD temperature sensing application. The sensing circuit is based on a ratiometric measurement approach to eliminate errors caused by current source noise or instability. In fact, Texas Instruments provides a design guide using this same ADC for temperature sensing with RTDs via ratiometric measurements: "Series Resistance Temperature Detector (RTD) Sensing Reference Design" (www.ti.com/.../tiduf19.pdf).

The high level diagram of the system we are designing is the following one:

During initial testing with a prototype design, I noticed that when the ADC is operational, it exhibits a relatively high bias current at the reference inputs, around 6 µA coming out of Vref pins. In our application, this introduces a measurement error, since this 6 µA adds to the 100 µA provided by the current source, making the actual current through the temperature sensor unknown.

However, I could not find any mention of this reference input bias current in the datasheet. Is there any additional information or characterization data available regarding this bias current?

Thank you in advance for your support.

  • +1
    TI__Guru 58981 points

    Hi Ian,

    The reference impedance can approximate the amount of bias current you'd expect from the ref pins. See below from the datasheet where the reference impedance is 85kohm typical. This is approximately 10uA of current, or at least similar to what you are measuring in your circuit. You might consider buffering the ADS1282 to reduce the ref bias current to negligible levels. This will also eliminate any gain error issues by having the RREF resistance (maybe 1k?) in parallel with the ADC ref impedance (85k).

    Please also see this ref design using the ADS1282-SP for RTD measurement applications: https://www.ti.com/tool/TIDA-060042

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Understood we will evaluate adding a buffer to Vref, thank you very much Bryan.