Need Asistance for Temp -20C to +200C with OP amplifier

Discussion in 'The Projects Forum' started by laco, Dec 12, 2011.

  1. laco

    Thread Starter New Member

    Aug 29, 2010
    8
    0
    I need asistance, how calculated OP Amplifier Resistors R1, R3, R4 & R5 for following:
    Vout(ATMEGA16 ADC or MCP3204)= 0 - 5V
    PT1000 (R2) Min Resistance= 822 Ohm (-20C) and Max. Resistance=1758 Ohm (+200C).
    Vcc= +10 V
    Vref= +5.0V
    R1 = ????
    R3 = ????
    R4 = ????
    R5 = ????
    IC1 = TLC271CP (TLC271CP, AD621, AD620, LM358, LM324, TL071, TL072 this IC's have in CROATIA.)

    See Attached schematic
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    You should not start another thread on the same topic.
     
  3. mcasale

    Member

    Jul 18, 2011
    210
    12
    Hi. Your circuit is ALMOST correct. You don't really need the R3 to 5V. I used this type of circuit in a real product design, but I cannot share the exact details. You don't need a bridge, unless you need to tweak out any resistive offsets.

    First, use the 5V reference to drive the PRT divider, or your signal will have noise.
    Next, choose the best op amp you can in terms of input offset error and noise. If you are using a 12-bit ADC, this shouldn't be too hard.
    Next, get rid of the zener on the output. It's another noise/load source. You're better off with a 5V single-supply (analog only) op amp (if you have one).
    Then, adjust the gain resistors to give you the maximum output signal at the highest temp.
    This should get you in the right ballpark.
    Mike
     
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