DC input on op amp integrator












4












$begingroup$


I am trying to integrate a DC tension with a classic op amp integrator. The problem is : when I simulate my circuit on LT Spice, the output will always be equal to the VEE tension of the op amp regardless of the 1/RC coefficient. If anybody can help me on this problem, I put the circuit and the graphics below.
enter image description hereenter image description here






operational-amplifier amplifier dc integrator







share|improve this question







New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$








  • 1




    $begingroup$
    You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
    $endgroup$
    – Phil G
    14 hours ago






  • 1




    $begingroup$
    Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
    $endgroup$
    – Sunnyskyguy EE75
    14 hours ago


















4












$begingroup$


I am trying to integrate a DC tension with a classic op amp integrator. The problem is : when I simulate my circuit on LT Spice, the output will always be equal to the VEE tension of the op amp regardless of the 1/RC coefficient. If anybody can help me on this problem, I put the circuit and the graphics below.
enter image description hereenter image description here






operational-amplifier amplifier dc integrator







share|improve this question







New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$








  • 1




    $begingroup$
    You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
    $endgroup$
    – Phil G
    14 hours ago






  • 1




    $begingroup$
    Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
    $endgroup$
    – Sunnyskyguy EE75
    14 hours ago
















4












4








4





$begingroup$


I am trying to integrate a DC tension with a classic op amp integrator. The problem is : when I simulate my circuit on LT Spice, the output will always be equal to the VEE tension of the op amp regardless of the 1/RC coefficient. If anybody can help me on this problem, I put the circuit and the graphics below.
enter image description hereenter image description here






operational-amplifier amplifier dc integrator







share|improve this question







New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$




I am trying to integrate a DC tension with a classic op amp integrator. The problem is : when I simulate my circuit on LT Spice, the output will always be equal to the VEE tension of the op amp regardless of the 1/RC coefficient. If anybody can help me on this problem, I put the circuit and the graphics below.
enter image description hereenter image description here






operational-amplifier amplifier dc integrator




operational-amplifier amplifier dc integrator






share|improve this question







New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question







New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question






New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









asked 14 hours ago









Nicolas RoriveNicolas Rorive

211




211




New contributor




Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Nicolas Rorive is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.








  • 1




    $begingroup$
    You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
    $endgroup$
    – Phil G
    14 hours ago






  • 1




    $begingroup$
    Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
    $endgroup$
    – Sunnyskyguy EE75
    14 hours ago
















  • 1




    $begingroup$
    You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
    $endgroup$
    – Phil G
    14 hours ago






  • 1




    $begingroup$
    Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
    $endgroup$
    – Sunnyskyguy EE75
    14 hours ago










1




1




$begingroup$
You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
$endgroup$
– Phil G
14 hours ago




$begingroup$
You have a DC gain of 1000, and 2V applied to the input. Why wouldn't you expect the output to settle at the negative rail?
$endgroup$
– Phil G
14 hours ago




1




1




$begingroup$
Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
$endgroup$
– Sunnyskyguy EE75
14 hours ago






$begingroup$
Integration time is 1V/s and decay time is 1000s neglecting leakage current in cap and input bias offset. If steady state says the output is saturated, then look at the startup and initial condition. Null offsets are sometimes used in Integrators with low input Vio. You do not want to apply an offset, do you? if so why?
$endgroup$
– Sunnyskyguy EE75
14 hours ago












2 Answers
2






active

oldest

votes


















4












$begingroup$

The problem is with the solver, it's finding the wrong DC operating point. The solver first tries to find the point that the circuit is operating at, then runs the simulation. In the case of this integrator, it's wrong. So you either need to set initial conditions on the capacitor OR force the solver to recognize that the voltage is zero. One way to do this is with a PWL supply, starting it at zero, then at 1us or shortly after the simulation starts, then move the supply to 2V.



enter image description here






share|improve this answer









$endgroup$













  • $begingroup$
    This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
    $endgroup$
    – Spehro Pefhany
    8 hours ago






  • 1




    $begingroup$
    The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
    $endgroup$
    – laptop2d
    3 hours ago



















1












$begingroup$

You have a two volt signal applied to the input so it will always start saturated at the negative rail unless you set the capacitor initial conditions to something like 0V, or at least within the operating range of the op-amp.



You can try Ctrl-Right Click on the capacitor. Change as below to add IC=0 for zero volts at t=0, or select "skip initial operating point solution" in the simulation (which may have other side effects).



enter image description here






share|improve this answer









$endgroup$













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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

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    active

    oldest

    votes









    4












    $begingroup$

    The problem is with the solver, it's finding the wrong DC operating point. The solver first tries to find the point that the circuit is operating at, then runs the simulation. In the case of this integrator, it's wrong. So you either need to set initial conditions on the capacitor OR force the solver to recognize that the voltage is zero. One way to do this is with a PWL supply, starting it at zero, then at 1us or shortly after the simulation starts, then move the supply to 2V.



    enter image description here






    share|improve this answer









    $endgroup$













    • $begingroup$
      This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
      $endgroup$
      – Spehro Pefhany
      8 hours ago






    • 1




      $begingroup$
      The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
      $endgroup$
      – laptop2d
      3 hours ago
















    4












    $begingroup$

    The problem is with the solver, it's finding the wrong DC operating point. The solver first tries to find the point that the circuit is operating at, then runs the simulation. In the case of this integrator, it's wrong. So you either need to set initial conditions on the capacitor OR force the solver to recognize that the voltage is zero. One way to do this is with a PWL supply, starting it at zero, then at 1us or shortly after the simulation starts, then move the supply to 2V.



    enter image description here






    share|improve this answer









    $endgroup$













    • $begingroup$
      This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
      $endgroup$
      – Spehro Pefhany
      8 hours ago






    • 1




      $begingroup$
      The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
      $endgroup$
      – laptop2d
      3 hours ago














    4












    4








    4





    $begingroup$

    The problem is with the solver, it's finding the wrong DC operating point. The solver first tries to find the point that the circuit is operating at, then runs the simulation. In the case of this integrator, it's wrong. So you either need to set initial conditions on the capacitor OR force the solver to recognize that the voltage is zero. One way to do this is with a PWL supply, starting it at zero, then at 1us or shortly after the simulation starts, then move the supply to 2V.



    enter image description here






    share|improve this answer









    $endgroup$



    The problem is with the solver, it's finding the wrong DC operating point. The solver first tries to find the point that the circuit is operating at, then runs the simulation. In the case of this integrator, it's wrong. So you either need to set initial conditions on the capacitor OR force the solver to recognize that the voltage is zero. One way to do this is with a PWL supply, starting it at zero, then at 1us or shortly after the simulation starts, then move the supply to 2V.



    enter image description here







    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered 13 hours ago









    laptop2dlaptop2d

    26k123381




    26k123381












    • $begingroup$
      This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
      $endgroup$
      – Spehro Pefhany
      8 hours ago






    • 1




      $begingroup$
      The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
      $endgroup$
      – laptop2d
      3 hours ago


















    • $begingroup$
      This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
      $endgroup$
      – Spehro Pefhany
      8 hours ago






    • 1




      $begingroup$
      The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
      $endgroup$
      – laptop2d
      3 hours ago
















    $begingroup$
    This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
    $endgroup$
    – Spehro Pefhany
    8 hours ago




    $begingroup$
    This is a valid method, however note that the starting output voltage will be the Vos the op-amp model has multiplied by -1000, which may or may not be close to zero.
    $endgroup$
    – Spehro Pefhany
    8 hours ago




    1




    1




    $begingroup$
    The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
    $endgroup$
    – laptop2d
    3 hours ago




    $begingroup$
    The Vos for the TL084 is nominally 3-5mV, so it will still be in range. Not a great amp to use for a high gain application
    $endgroup$
    – laptop2d
    3 hours ago













    1












    $begingroup$

    You have a two volt signal applied to the input so it will always start saturated at the negative rail unless you set the capacitor initial conditions to something like 0V, or at least within the operating range of the op-amp.



    You can try Ctrl-Right Click on the capacitor. Change as below to add IC=0 for zero volts at t=0, or select "skip initial operating point solution" in the simulation (which may have other side effects).



    enter image description here






    share|improve this answer









    $endgroup$


















      1












      $begingroup$

      You have a two volt signal applied to the input so it will always start saturated at the negative rail unless you set the capacitor initial conditions to something like 0V, or at least within the operating range of the op-amp.



      You can try Ctrl-Right Click on the capacitor. Change as below to add IC=0 for zero volts at t=0, or select "skip initial operating point solution" in the simulation (which may have other side effects).



      enter image description here






      share|improve this answer









      $endgroup$
















        1












        1








        1





        $begingroup$

        You have a two volt signal applied to the input so it will always start saturated at the negative rail unless you set the capacitor initial conditions to something like 0V, or at least within the operating range of the op-amp.



        You can try Ctrl-Right Click on the capacitor. Change as below to add IC=0 for zero volts at t=0, or select "skip initial operating point solution" in the simulation (which may have other side effects).



        enter image description here






        share|improve this answer









        $endgroup$



        You have a two volt signal applied to the input so it will always start saturated at the negative rail unless you set the capacitor initial conditions to something like 0V, or at least within the operating range of the op-amp.



        You can try Ctrl-Right Click on the capacitor. Change as below to add IC=0 for zero volts at t=0, or select "skip initial operating point solution" in the simulation (which may have other side effects).



        enter image description here







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 13 hours ago









        Spehro PefhanySpehro Pefhany

        209k5160422




        209k5160422






















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