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PT100 input to ADC

Discussion in 'Microcontrollers' started by slepax, Jul 2, 2009.

  1. slepax

    slepax New Member

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    Hello all,

    I want to use a PT100 to read temperatures using an ADC. I understand the basics [and more] on both sides, the PT100 and the ADC, however joining both for optimum result seems a bit tough:
    • My ADC (ATmega8 to be exact) has 10bit input, which gives me 1024 sampling values.
    • The input for the whole circuit is 5v (Vin).
    • My temperature range is zero to 300deg, which means PT100 resistance of 100ohm at minimum and 217.25ohm at maximum.
    • Theoretically, with 1024 sampling values in this temperature range I should be able to sample at 1/3 deg.
    • Ideally, I would like to have 0v input when the temperature is 0deg and 5v input when the temperature is 300deg.
    My calculations showed it would be impossible to get the full range of 1024 sampling values, the maximum I could get is 2-3deg accuracy.

    I used a voltage divider to make my calculations but I am wondering if there is any better approach for this. I would love some feedbacks/opinions.

    Thanks,
    Ronen
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,:)
    Are you planning to use an amplifier for the PT100 RTD, so that you can span the 0 to 300C range and apply any required offset.?
    or
    just a resistive potential divider.?

    Do you have a type number or datasheet.?
     
    Last edited: Jul 2, 2009
  3. slepax

    slepax New Member

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    Hey Eric, thanks for the prompt reply!

    Not too sure about what you meant by amplifier and offset (I am kinda new to this), but the best results (only in calculations) I managed to get where using a voltage divider and 1200ohm resistor as R1 is:
    • Mininum - 0.453v
    • Maximum - 0.766v

    Then using a x6 voltage multiplier it will give me 2.7v to 4.6v

    And no, sorry, no datasheet.
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Looking at your figures show 0.453V at 0Cdeg and 0.766V at 300Cdeg.
    Which is a range of 0.313V for 300C change

    So for a 10bit ADC 1023, using a +5Vref, would give [0.453/5] *1023 =92 decimal.
    and [0.766/5] * 1023 = 156 decimal.

    Whats ideally needed is an amplifier with offset.
    The offset voltage of 0.453V nulled to 0V and the 'range' of 0.313V multiplied by the amp to give +5V, a gain of 15.9

    This would give the best resolution and span.

    Do you follow.?
     
  6. slepax

    slepax New Member

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    Loud and clear :)

    Just to summarise. Having an offset of 0.453v to 0, would make:

    0.453v -> 0v
    0.766v -> 0.313v

    That's great, because then I can multiply by 16 and get:
    Code (text):

    0.453v [offset 0.453v]   ->   0v [x 15]     -> 0v
    0.766v [offset 0.453v]   ->   0.313v [x 15] -> 4.7v
     
    This is great because it will almost give me the full range of 0v to 5v.

    I know how to make the voltage amplifications (LM386 and co.), but how do you make the offset?
     
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    As you dont have any data on the PT100, I have posted a page from a text book that may help guide you.:)

    I would suggest you consider driving the PT with a constant current in order to get the voltage output for the amplifier.
     

    Attached Files:

  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Multiplying in software will not give you the resolution you are expecting.

    The base value of the ADC will have a range of 64 decimal, so thats
    300/64 = 4.6, say 5C/bit.

    Is that resolution acceptable.?

    With amplification 300/1023 = 0.29C/bit

    EDIT:
    The LM386 is an audio amp..
     
    Last edited: Jul 3, 2009
  9. slepax

    slepax New Member

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    I am familiar with that. I have managed to calculate the various PT100 resistance values using the widely available linearisation equation:

    Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3)

    Also see the attached snapshot from my Excel file.

    The questions is how do I reduce the 0.453v of the voltage divider Vout to be 0v?
     

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  10. slepax

    slepax New Member

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    For the application I am using 5C/bit is acceptable, but I still want to reach the maximum resolution possible, merely for learning purposes.

    0.3C/bin is just superb! but the question is how to I reduce (offset) 0.453v to 0v?
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    The OPA that amplifies the signal also removes the offset voltage.

    Do you have any OPA's in your spares box.?
     
  12. slepax

    slepax New Member

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    Actually yes :)

    I have a couple of LM386 I played with to learn how to amplify (multiply) voltage. I started searching the net and it seems OP amps can also do voltage offset. Such handy little things they are... :)

    I forgot to thank you for your help so far - thank you so much!
     
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    As I said earlier, these are audio amps..:)

    EDIT:
    A circuit like this would do the job.
    Note the OPA type, the LM386 isnt suitable for your application.
     

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    Last edited: Jul 3, 2009
  14. slepax

    slepax New Member

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    Hey,

    Sorry for the late reply, but the circuit you posted threw me off a little bit. I searched some info about CA3140 on the net, but I still find it hard reading your circuit.

    My electronic knowledge is very basic but I am eager to learn. Could you please explain the circuit a little bit? What makes the offset? What makes the amplification?

    For the LM386 I played with, the ratio of two resistors defined the voltage amplification, it looks a bit different with your circuit.

    Thanks, Ronen.
     
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I'll post a schematic, that first circuit is from the simulation program.:)

    Look for a LM358 that should be close enough for your project.
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Look at this simplified drawing.

    Can you follow it OK.?:)
     

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  17. slepax

    slepax New Member

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    Yes, your comments really helped!

    So in a sense the amplification for this OP AMP is quite similar to how the LM386 works, it is set by the ratio of R5 and R1+SPAN. Am I right?

    And as for the offset, if I understand this right, it is a simple resistor (pot in this case) reducing the voltage by 0.453?

    Other than, I just have a few more questions (sorry for all the questions):

    • What is the purpose of R3?
    • What is the purpose of everything before R4? (the diode, capacitors, extra resistors)

    You said the LM386 is not suitable, but why? It seems to be doing the same thing as the CA3140.

    Many thank!
     
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hope this answers your questions.
     

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    Last edited: Jul 5, 2009

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