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need help with PT100

Discussion in 'General Electronics Discussion' started by 466576266, Mar 30, 2010.

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  1. 466576266

    466576266

    31
    0
    Mar 12, 2010
    hello,my friends.

    Recently, i am designing a circuit with PT100. what the requirements of the circuit are as follows:

    Temperature Range: 0-100°C.
    output range :1.0000V-1.9999V.
    resolution:0.01 °C. or 0.1°C

    i have tried a lot of circuits, but thay all donnt work.

    could you plz me? thanks for your suggestion.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Why don't you show us some of those circuits and tell us why they didn't work.

    With that sort of resolution, are you contemplating corrections for non-linearity of the sensor?

    What sort of sensor do you have?

    Do you have the specs for it?
     
  3. 466576266

    466576266

    31
    0
    Mar 12, 2010
    PT100 circuit design

    the circuit ia attached. there are two simulated current and voltage source. in the practical circuit design, they shou be replaced by concrete circuit. and i use the PT100 to simulate the RTD. froming this circuit ,when the PT100 change from 100 ohm to 138.5 ohm, the output can arrive to 1.0000 to 1.9999. but when i add the source, i cann't get the correct result.
    plz help me to look at it.
    appreciate with circuit you designed.
     

    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    have you tested both the voltage and current source?

    Can the voltage source sink 1mA without problems?

    Can the current source supply 1mA at whatever voltage is at the top end of the sensor?

    Does the amplifier have too low an input impedance?

    What supply rails are you using (I presume it's a double ended power supply).
     
  5. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    0.01C is an extremely high resolution. This is the type of resolution used by lab grade temperature sensors and they require a lot of expertise to design. A 100 Ohm Platinum RTD has a resistance change of 0.4 ohm/C. Do you realize you are looking to resolve 0.004 ohms, or 4 miliohm?

    Even 0.1C is not trivial. Even at that resolution, you can not just ignore the higher order terms of the Callender-VanDusen equations. Do you really want to be solving a 4'th power equation in analog hardware?

    If you really need the accuracy, why do you want to build one rather than just buy?

    Stepping back, what are you trying to achieve?

    ---55p
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    I thought that too, but since it's an analogue voltage output, the accuracy and resolution is pretty much dependant on noise, stability of his references, and the linearity of the sensor, and the dissipation within the sensor.

    I'm pretty sure that getting a resolution of 0.01 C would be far easier than getting an accuracy of 0.01 C (I'm pretty sure he can easily get a resolution of 0.01 C even if his accuracy is within a degree C or so) :)
     
  7. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Well, not really. In order to keep the self heating down, you are looking at keeping the drive current under 10mA. At this current 4mOhm will result in a voltage change of 40 microvolts. In order to resolve that, you are looking at keeping your noise to +/-10 microvolt. At that level passive component noise, IC noise, chopper noise from the chopper stabilized op-amp, environmental noise, noise from other parts of the design all become relevant. That is quite manageable for someone who knows what they are doing. But it is not something that a student, a hobbyist or even the average engineer is going to be able to handle.

    A second angle is that he is looking to resolve 1 part in 10,000. That requires a noise free 14 bit ADC. In real life, you will have a hard time getting a stable reading out of a 16 bit ADC. I know he is not looking to do an ADC, but this gives you an idea of the level of complexity involved.

    Then, as you pointed out, accuracy is a whole other thing. The first part is accurately measuring the resistance of the element (remember to exclude the resistance of the cable and connectors). The second part is solving the Callender-VanDusen equation, which does not have a closed form solution in that direction, so it needs to be iterated from the other direction.

    ---55p
     
  8. 466576266

    466576266

    31
    0
    Mar 12, 2010
    hey,55p. thanks for your analysis. i get what you mean. yes, you are right. the 0.01 resolution is hard to design. also, the 0.1 resolution is not easy to arrive. whatever, i have to finish this project. that's to say, i have to draw a circuit for the PT100, no matter the resolution is 0.1, or 0.01. according to your analysis, i changed my design. in other way, i want to set the resolution 1. if the resolution of the circuit is 1, do you have some suggestions, or, give me some ideas to design the circuit. thanks.
     
  9. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Now that I know that it is a school project, I have no more suggestions. You are supposed to be doing your own homework, not asking others on the internet to do it for you.

    My rule of thumb is, if you are not willing to print out a copy of the post and hand it in with your assignment, you are crossing the line and I want no part of it. Others here do have a different view and they may be happy to keep helping.

    I am out.

    ---55p
     
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