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OPA4376: OPA4376 - resistance range for thermistor sensing circuit

Dan Ringger
Intellectual 1790 points
Part Number: OPA4376
Other Parts Discussed in Thread: OPA376, TINA-TI

I am looking into using OPA4376AIPW for a thermistor sensing circuit.  Thermistor voltage divide on the non-inverting input of the OpAmp. Output of the OpAmp connected into A/D input of the microcontroller.  I need the gain of approx. 2.5.  What resistance range for the gain resistors (hundreds of Ohms, tens of kOhms, etc.) should I use to keep noise and error as low as possible? I am trying balance it against current consumption.  Any specific performance characteristics of this part that I need to be aware of?

  • 0
    Guru 140200 points

    Hi Dan,

    this heavily depends on the nature of thermistor and the temperature range. What sort of thermistor do you want to use? And what is the temperature range?

    As thermistors are rather slow you can decrease the noise by the help of low pass filtering. Because of that noise isn't usually an issue with thermistors.

    Kai

  • 0
    TI__Guru 60610 points

    Dan,

    I presume you plan to use OPA4376 in gain of G=+2.5 and if so the circuit you're contamplating is the one shown below:

    An intrinsic noise of an op amp adds with input resistor, R1, thermal noise as a square-root of squares – similar to vector quantities.  Since a thermal noise of 1k resistor is 4nV/sq-rt Hz at 25C while broadband noise of OPA376 is 7.5nV/sq-rt Hz, two of the quantities summed together would result in: sq-rt(4^2+7.5^2)=8.5nV/rt-Hz input referred noise (or Vout_noise=21nV/rt-Hz) while using 100ohm and 150ohm results in Vinput_noise of 7.51nV/rt-Hz and Vout_Noise of 18.8nV/rt-Hz, respectively – see below.

    .

    If you intergrate the spectral noise across the entire bandwidth of OPA4376, it will result in Vout noise of 45uVrms for 1k/1.5k vs vs 33uVrms for 100ohm/150ohm, respectively.  However, be aware that using ten times smaller resistor does not come free and will result in ten times higher output current, which may not be acceptable in your application.  On the other hand, using ten time higher resistors (10k/15k) will result in the total noise at the output of OPA4376 being dominated by the thermal noise of the larger resistors.

  • 0 in reply to Marek Lis
    TI__Intellectual 1790 points

    Marek - Thank you. 

    Using 1k/1.5K would be acceptable for my circuit.

    Do you have a model for OPA376 that I can use in LTspice?

  • 0 in reply to Dan Ringger
    Guru 140200 points
    Hi Dan,

    what sort of thermistor do you want to use? And what is the temperature range?

    Kai
  • 0 in reply to kai klaas69
    TI__Intellectual 1790 points
    NTC thermistor, 10K@25C, external to the control. Full temperature range on thermistors: 15C ~60C. Only temperature range 20C~42C requires high precision. I have 10 thermistors in the system. Thx
  • 0 in reply to Dan Ringger
    TI__Guru 60610 points

    Dan,

    All macro-models are available in our product folder under Tools & software - see below.  Since Tina-TI is also PSpice compatible simulator, you may click in Tina on the op amp symbol and copy the entire text file into any other Pspice compatable simulator - I have done this for your convinience and attached it in this post.  Btw, you may rename .CIR to any other extension name like .TXT

    OPA376.CIR

  • 0 in reply to Marek Lis
    TI__Genius 11135 points
    Dan

    We haven't heard back from you so we assume this answered your questions. If you need more help just post another reply below.

    Thanks
    Dennis