help needed with opamp integrator :)

Discussion in 'The Projects Forum' started by Chapa3524, Jun 16, 2012.

  1. Chapa3524

    Thread Starter New Member

    Feb 19, 2012
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    0
    hi,
    i'm on a project and it needs a opamp integrator cct in it. i'v used lm358 opamp and used a 100uF electrolytic capacitor with a preset resistor. the problem is that usually the output of the integrator should be negative, for a positive input voltage signal, bt it always gets + saturated, even for a small input signal.
    i would be much obliged if anyone can figure this out and tell me what to do to get the correct performance out of it!

    thankyou!!! :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Firstly, post a circuit schematic to show what you are doing.

    Secondly, You need to calculate the time constant of your RC integrator.

    Thirdly, all integrators regardless of the size of the capacitor, will saturate eventually.
    You need to have a way to discharge the capacitor and this can be simply a resistor in parallel with the capacitor.
    You can also use a reset transistor to discharge the capacitor.
     
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  3. Chapa3524

    Thread Starter New Member

    Feb 19, 2012
    11
    0
    thankyou for the responce,
    actually other parts of the circuit is some what ok. the only problem is with this part.
    the needed time constant is abt 1.5 and i tried several RC combinations in project board but for all, it is the same problem even the input is a small + voltage or small - voltage, that is the output is suddenly saturated and doesn't change even the input is changed. :(
    the discharge mechanism is manually done by a pushbutton across a resistor. (but it doesn't matter as in the main circuit it uses a mux to change the input from a + voltage to - voltage. and it stops when it discharges.)
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
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    As MrChips asked, post your circuit schematic.
     
  5. Chapa3524

    Thread Starter New Member

    Feb 19, 2012
    11
    0
    here is the full circuit.
    there is also a problem with the U3 pin no 5. eventhough it simulates properly it doesn't work after soldering. is it due to the fan out? i havn't used a 4050 in the solderd circuit, is it affective to do so? i tried an opamp buffer bt it doesn't also work. what could i do ??
     
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  6. MrChips

    Moderator

    Oct 2, 2009
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    Here is the integrator section of the circuit:

    [​IMG]
     
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  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    Your opamps do not have supply voltages.
    Why is the output +88.8V?
     
  8. Chapa3524

    Thread Starter New Member

    Feb 19, 2012
    11
    0
    ahh nono, it's because it's a image export of protues. :) suppy is given as +Vcc= +12 and -Vcc = -12 v.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,446
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    The drawing I have shown has been cut from a larger drawing just to show the integrator portion. The opamp is powered by +12V and -12V.

    The +88.8V is just the image of the virtual instrument showing the 3-digit plus sign display. The simulation is not running.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Check for a missing ground connection on pin 3 of the op amp. That will cause an LM358 to saturate positive.
     
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