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    PT100 interfacing with controller

    Hi all.
    i have made a circuit to interface PT100 with my microcontroller Nuvoton N78E517A.
    Circuit diagram attached of Amplifier, my issue is with the software part.
    ADC Used is MCP3202 and amplifier used is LMV358.
    what my problem is how should i calibrate the PT100 sensor. for example, at 100 Ohms the temperature is 0 Degree. so to calibrate it i added a 100 ohms resistor at input and whatever value i get at ADC, i calibrated to 0 degree. but what the problem is when the resistance rises, say like if i add a 220 ohms or resistor , the voltage at ADC too rises, and the value temperature what i get is something different that the PT100 Chart.
    i am bit confused about the logic, how should i calibrate the PT100 to get appropriate reading.

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    Re: PT100 interfacing with controller

    220 Ohm is approx. 315 degrees C.
    You should try different approach for measuring with Pt-100. Connect it to 1mA constant current source and then measure/amplify voltage that dropped on Pt-100.
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    Re: PT100 interfacing with controller

    Do you realize that in your schematic R47 is only connected between supply and ground, and not doing anything?
    The rest of the circuit itself is similarly convoluted.

    You really require to build a proper Wheatstone bridge, whereas the PT100 is located in one arm. And then amplify the minute voltage difference, which is riding with a significant common mode voltage, with a proper instrumentation amplifier.
    Either roll your own instrumentation amp with 3 opamps plus very tightly matched resistors, or buy a ready made IC.

    https://www.analog.com/en/products/a...mplifiers.html
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    Re: PT100 interfacing with controller

    Another way to interface to a PT100 is with a dedicated interface chip like the Maxim MAX31865.



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    Re: PT100 interfacing with controller

    Hi,

    I have a couple of questions to ask you based on your schematic so I'll start from the left.

    1) Why did you leave the S+ terminal unconnected? Are you trying to implement a 3-wire measurement scheme?
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    Re: PT100 interfacing with controller

    Quote Originally Posted by Akanimo View Post
    Hi,

    I have a couple of questions to ask you based on your schematic so I'll start from the left.

    1) Why did you leave the S+ terminal unconnected? Are you trying to implement a 3-wire measurement scheme?
    yes, its a 3 wire RTD sensor.
    i did not use dedicated IC's because i want to use all type of sensor using 1 ADC, like j, K, T etc..
    the only thing is i have to select the sensor type which i am going to use, its the same concept which now a days temperature controller have.



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    Re: PT100 interfacing with controller

    Thermocouples are generating voltage, Pt-100 is temperature dependent resistor!
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    Re: PT100 interfacing with controller

    Hi,

    And thermocouple is relative temperature measurement to cold junction, while RTD is absolute temperature measurement.
    It makes no sense to combine all in one circuit.

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