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I cannot seem to properly ground this circuit.

  1. May 2, 2012 #1
    Hi All!
    I am trying to build a modular addition to a spectrophotometer (it needs to be mobile and easily removed) thus I have decided to base my design around an Arduino.

    My application requires that I power/control a High Power (1W) Infrared LED, and read off temperature measurements from an NTC thermistor.
    Currently I am primarily having difficulty with the Thermistor part of the circuit (left side for all schematics)

    My original design (See Image):
    OriginalPolyNIPAMCircuit.png


    Used a voltage divider to measure the resistance of the thermistor. This worked fine, however due to the limitations of my thermistor and the small range of temperatures I am interested in, the voltage changes in that setup were minimal (on the order of a few tenths of a volt maximum). Thus I decided to add an OP-AMP in differential arrangement to amplify (and give a desired DC offset) my signal.

    This led me to the following (current) design:
    PolyNIPAM-Circuit-ForHelp_schem copy.jpg

    The OP-AMP that I am using called for a dual polarity power source. As such I put together the arrangement at the top left of the above schematic. My understanding is it produces a virtual ground at 8V.

    The Issue I am having is tying this ground into the Arduino ground, so that the signal entering the Arduino at A0 (an analog input port) is between 0-5V

    I really am stumped as to what I should connect my "Virtual Ground" up to.

    Currently I keep reading non-trivial bias voltages between the input to A0 and the Arduino ground.

    I have been beating my head against this for a few days now I would most appreciate some help.

    Thank you so much,
    Justin
     
  2. jcsd
  3. May 3, 2012 #2

    jim hardy

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    Gold Member
    2016 Award

    that circuit should deliver positive voltage to Arduino.

    Look at it.
    Noninverting input is fixed at 4.123 volts by voltage divider R5-R9.
    Were thermistor not even present, in order for opamp to balance its inputs:
    again by voltage divider action:

    inverting input ##= 5 - \frac{2}{6.7} X (5-Vout) = 4.123##

    ## Vout = 2.062 ##

    which means output would have to be 2.062 volts or greater.
    Is that what you are seeing?

    As thermistor can only lower the voltage at junction of the two 1K resistors your opamp will have to make his output go up to restore balanced inputs.

    You might check out LM324 opamp. It uses single supply. But i think your trouble is not with the virtual ground.
     
    Last edited: May 3, 2012
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