CN0287 accuracy analysis

Hi,

One of our new designs, requires us to implement RTD & Thermocouple measurement circuit.

We need to get a accuracy of better than "0.2% of FSR" over the operating temperature range of "-20°C to 60°C".

We have looked at CN0287 board & planning to use the same circuit in our board if the required accuracy is met.

The CN0287 specification says that accuracy of 0.5°C. Is this accuracy guaranteed over a specific temperature range?

For our accuracy analysis , we have identified the error introducing components as below :

RTD measurement :

  • Resistor Rref  - Tolerance & Temperature coefficient
  • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift

Due to ratio-metric cancellation,  change in excitation current & ADC reference voltage cancel out each other. Thermo-couple measurement :

  • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift
  • ADC Reference ADR3425 - Initial Accuracy, Line regulation , Load regulation, Output Noise
  • Cool Junction Compensation ADT7310 - Accuracy ( 0.5°C )

Please let us know if we have missed out any parameter in our analysis.

Also, it will be helpful if you can share any document related to accuracy analysis done on CN0287.

Looking forward to your response.

Regards,

Suresha N S

  • Hello Willie,


    I have attached accuracy analysis document for our circuit based on CN0287. Only major change with respect to CN0287 is that we have changed our reference voltage to 2.5V (We are planning to use ADR3425 instead of ADR3440). This change is to fall in line with the "Absolute analog input voltage range" mentioned in the AD7193 datasheet when the RTD resistance goes up to 5KΩ.

    It would really help us if you can review our calculations.

    Please let us know, if you need any clarifications.

    Regards,

    Suresha


    RTD_Analysis.xls
  • 0
    •  Analog Employees 
    on Dec 16, 2013 1:49 PM

    Hello Suresha NS

    You did a really good job and I learned a lot from your accuracy analysis document.

    I made some comments in the attached document for your reference.

    With best regards,

    Willie.

    RTD_Analysis.xls
  • Hello Willie,

    Thanks a lot from your comments.

    Regarding your comments ,


    • For RTD, as per your suggestion we will configure ADC for unipolar input. This will result in better accuracy.
    • In thermocouple voltage reference (ADR3425), I had taken the parameters from ADR3412. Thanks for pointing this out.
    • We are planning to power ADT7310 using a 3.3V supply. I missed out to mention this in our document. Thus I have considered accuracy to be 0.5°C.


    Since we have a good margin with respect to accuracy in our calculation, we will go ahead with the design.

    We may need further help from you, during design & testing phase. We will keep you updated.


    Thanks & Regards,

    Suresha N S