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ADS131M08: Design an External current source for RTD

Sahil Nayak
Prodigy 110 points
Part Number: ADS131M08
Other Parts Discussed in Thread: LMP2011, LM4140

Hi TI Team,

We are planning to use the ADS131M08 ADC to read the data from the 2/3/4 wire PT1000 RTD sensor(s). Since the ADS131M08 ADC doesn't have an inbuilt IDAC, we are planning to deisgn an external precision current source.

Initially, to determine the minimum and maximum value of the IDAC, we made some calculations in an Excel sheet (attached below). 
The first sheet, "Calc as per ADC " helps us to determine the minimum IDAC current requirement.
The second sheet, "Calc as per Self-heating " helps us to determine the maximum value of the IDAC that will avoid self-heating, considering the self-heating coefficient and the desired max error.

Secondly, based on the above calculations, we studied the two ways to design an external current source:

  1. We studied the application note that used LM4140 and LMP2011 as per Figure 3.
  2. The adjustable current source.

Request you to kindly go through the attached Excel sheet and confirm the calculations are correct. Also, suggest to us which would be the right way to move forward with the external current source design from the mentioned above.

Let me know if any further details are required.

Thanks & Regards,
Sahil Nayak

RTD Current Source Calculation.xlsx 

  • 0
    TI__Mastermind 46055 points

    Hi Sahil Nayak,

    Welcome to TI E2E forum and thanks for your interest in our ADCs.

    I strongly suggest you to check the feedback from me and my colleague Joachim in the following link in the E2E forum:

    ADS131M08EVM: Using a RTD with the ADS131M08

    Let me know if you have any additional questions.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 110 points

    Hi Dale,

    Thank you for your quick response.

    I have gone through the mentioned thread.

    However, we are looking to use ADS131M08 itself with designing an external current source since it is a simultaneous sampling ADC. So, I request you to please go through my calculations regarding the current required and confirm our understanding.

  • 0 in reply to Sahil Nayak
    TI__Mastermind 46055 points

    Hi Sahil Nayak,

    You can directly use the noise and effective resolution table in ADS131M08 datasheet, not ENOB which is AC specification calculated from SNR. 

    The required current depends on the RTD resistance range(measured temperature), ADC's FSR(+/-1.2/Gain) and the value of your external reference resistor. If the maximum resistance of your RTD is 1479.5ohm, the maximum excitation current should be limited to 1.2V/1479.5ohm= 811uA, you can make the excitation current smaller for using the gain on the ADC.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 110 points

    Hi Dale,

    Thank you for the information.

    However, I am still unable to relate the mentioned noise table to the calculations you provided. Could you please correlate them and explain in some more detail?

    Also, do we have to consider the self-heating coefficient of the PT1000 in our calculations?

  • +1 in reply to Sahil Nayak
    TI__Mastermind 46055 points

    Hi Sahil Nayak,

    You can see the equation above the noise table 7-1 which is used to calculate effective resolution in table 7-2, you can directly use it.

    Self-heating should be considered. This self heating will cause an error in the measurement, but Self-heating coefficients will vary with RTD construction and the measurement medium (in air or in water, for example), you can consult the RTD manufacturer data sheet for sensor characteristics.

    The following design guide can be a good reference for you, it includes a section of 1.5.2 Set the Excitation Current Sources and Consider RTD Self Heating. 

    A Basic Guide to RTD Measurements

    BR,

    Dale