Create analog function with op amps

Discussion in 'Analog & Mixed-Signal Design' started by Nikos Mparoutis, Oct 17, 2016.

  1. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    Hello ,i am building a system and I have to create the analog signal V(t)=c1*t^2+c2*t - 1/t with op amps and analog components.I have to minimise the cost so i beleive i have to use only one multiplier ,so i want to know if i should use capacitors to store the signals and reuse the multiplier and do the final operations.Or to use buffers or suggest me an other way.
    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It could likely be done more easily using digital techniques.
    Why must it be done using analog components?
    What is the timeframe for "t"?
    What are c1 and c2?
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    1/t isn't practical unless non-zero time limits are defined. At time zero you would need an infinite voltage :eek:.
    Is this a Homework problem?
     
  4. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    I stated ,no homework.I finished with homework years before but i have not experience with op amps,it is my first attempt to build somethings big!
    I want to keep the cost to minimum but i realizes that it is imposible to use only one multiplier.So i will use 2 multiplier to do the x^2 and now my question is to do the C1*t^2 (c =constant).
    I believe to do this C1*T^2 is better with gain manipulation of an op amp than usage of a multiplier.
    Lets me think your thoughts.
     
  5. crutschow

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    Mar 14, 2008
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    You can do X² with just one multiplier by connecting X to both multiplier inputs (giving X * X).
    Then you run the multiplier output through an op amp with a gain of C1.
     
  6. #12

    Expert

    Nov 30, 2010
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    @crutschow
    Could you dumb this down for me?
    I think it's interesting, but I can't imagine how to use C1 (capacitance?) to multiply (time squared).
    If my question seems stupid, I confess. I am lost!
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Is C1 a cap's value?
     
  8. bertus

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    Apr 5, 2008
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  9. crutschow

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    In post #4 the OP said c was a constant so I interpreted C1 as a gain value
    But that assumption could very well be wrong. :confused:
     
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  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Using C and t sure confuses things. Can you please explain what exactly is that equation supposed to calculate, and what each symbol means?
     
  11. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    c (=constant) ,no capacitors.
    Also i had the same though...The gain of a op amp can be the constant
    If i decide to use ONE multiplier is it possible to do the function?
     
  12. ramancini8

    Member

    Jul 18, 2012
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    I believe that you need 2 multipliers; one to do t^2 and one to do 1/t. You probably can't use storage because of the time delays. I did implement an Observer set of equations, and timing is critical.
     
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  13. kubeek

    AAC Fanatic!

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    But is t really time, or is it x?
     
  14. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    Yhea Right!
    I done some work and now for a weird reason the Multisim does not compile the circuit and the result is wrong
    Here is the Multisim citcuit and the datasheet of AD633 multiplier.As you can see the connections are right,so iis this a problem with the mutlisim?
     
  15. atferrari

    AAC Fanatic!

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    Niko,

    Nice of you ignoring all but one who took the time to answer your OP!
     
  16. crutschow

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    Mar 14, 2008
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    What do you mean the result is wrong? What does the program do?
    Could be a problem with the model.
    I had no problem with my LTspice simulation (below).
    I used a different op amp since I didn't have the AD711, but that shouldn't make a significant difference in the simulation.

    upload_2016-10-20_13-14-11.png
     
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  17. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    Yes Right!
    I uploaded the wrong pic.It actually works with small error.
    Can i trim this error?
    Is there a way to trim the *10 scalling ?
    Thank you all
    (The t is time ,the V(t) is a voltage function with the variable of time)
     
  18. crutschow

    Expert

    Mar 14, 2008
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    We can better advise you on how to reduce the error if you tell us what the error is (what is "small").
     
  19. Nikos Mparoutis

    Thread Starter New Member

    Oct 8, 2016
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    In the pic the right result is 5 V but the output show is 5.12 V.
    I know that the low cost multiplier affects this but i like the perfectionism.
    The scalling *10 : i think i can put it inside the constant C =c1/10.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    It isn't whether you like "perfectionism" or not, it's what you need for your application that's important.

    It's hard to say where the error occurs (my simulation shows only 3mV of error).
    You'll have to go through the circuit, measuring all the nodes to determine where the error is occuring.
     
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