Unexpected op amp behavior

Discussion in 'General Electronics Chat' started by kutalinelucas, Jun 17, 2013.

  1. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
    98
    0
    Hey guys.

    I build a circuit a few weeks ago and thought I cracked it with little problems, now I've come back to calibrate it and its not behaving as I'd expect it...

    I have a couple of op amps which are giving unexpected results, and was hoping somebody could see whats happening.

    The sensor part of the circuit consists of two op-amps each with a separate matched cold-junction compensation IC (http://www.linear.com/product/LTK001) to read temps from two K-type thermocouples, powered by a regulated split 12v (-6v to +6v) The result is fed into the ADC of a PIC18F4550.

    And here's the problem...when I heat the aluminium with the thermocouples fixed inside, the voltage measured across the thermocouple terminals increases with temperature as expected, but the output, measured against ground sits at about 0.3v until the thermocouple output measures about 2.5mA at 70 degrees. Then the voltage rapidly increases to ~5v, settles and continues to rise fairly linearly with temperature.

    Here's the schematic for the sensor board...

    http://postimg.org/image/amz6z7xiz/

    Any ideas would be welcomed, and thanks for looking

    [Edit]...just noticed, the 1025 isn't rail to rail, but I'm powering it off the +6v without any common ground, is this what could be causing my problems?
     
    Last edited: Jun 17, 2013
  2. wayneh

    Expert

    Sep 9, 2010
    12,124
    3,048
    Yup, the op-amp cannot sense to the ground rail and doesn't do anything (or worse, is unpredictable) when the input is not within the common mode voltage range. I keep some LM358 dual op-amps on hand for when I need the ground rail. It's old and slow but works for lots of things.
     
  3. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
    98
    0
    My edit above was wrong and it is powered rail-to-rail as well as sharing a common ground with all other peripheral circuits, so I assumed the 1025 IC would provide the reference ground for the op amp...although I am pretty rubbish when it comes to analog theory...

    You mention the common-mode threshold, could the outputs described in my post below be because both matched ICs are only powered by -6v to +6v? would it help if I ramped the supply voltage up to say -12v-+12v? reading the data sheet, the common mode input voltage range would be -5.25v to 5v, where I am only inputting between 0 to 5mA so I just don't know why I get no output until the thermocouple reads 2.5mA

    Thanks
     
    Last edited: Jun 19, 2013
  4. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
    98
    0
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