How can i make the output of the multiplier follow the output of the filter in the below circuit?

Discussion in 'General Electronics Chat' started by Nanaiah, Mar 16, 2017.

  1. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    Hello,
    I am supposed to design a circuit where i can determine the change in Variable capacitance Cx by a change in DC output voltage at the output by keeping the reference capacitance constant. I tried using a lot of different filters but the output of the filter does not follow the output of the multiplier I am using. I am posting my circuit file (created in LTSpice) along with the paper i am referring. Any help is much appreciated.

    Thank you.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    I do not see a similar circuit in that referenced paper.

    Post a screen-dump picture of the schematic.
     
  3. radiohead

    Senior Member

    May 28, 2009
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    It appears, from the paper, that the sensor is based on sand flow through a PVC pipe (maybe for use in a cement manufacturing plant). The sand flow will generate a static electricity on the PVC pipe. If the flow of sand is interrupted, the capacitance on the sensor will change. It would be used to measure how much (gravity-fed) sand is being delivered. The author has done the math to determine a two inch pipe can allow only so much sand to flow per second.
    I cannot see your circuit as I do not have LTSpice on my computer. Can you post a PDF version?
    What kind of sensor are you using? Are you trying to filter voltage or frequency?
     
  4. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    The green waveform represents the filter output and the blue waveform represents the multiplier AD633 output. It is not following each other and also if the Cx is increased or decreased more from the Cref then i am having issues with the time of simulation as well.
    Screenshot (90).png Screenshot (91).png
     
  5. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    I am trying to convert AC to DC basically by varying Cx as you can see from the circuit picture. I also maintain an inverted version of the input wave and feed it to the comparator as i need a constant waveform to compare the variation. This is then multiplied by the multiplier and demodulated by the low pass filter. So I am supposed to get a sine wave and square wave at the input of the multiplier.
     
  6. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    I have not used a transformer. I have a used a normal input and inverted the input signal in order to avoid using the transformer. That is the only change i have done to the circuit on the paper.
     
  7. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
    25
    0
    I am trying to convert AC to DC basically by varying Cx as you can see from the circuit picture. I also maintain an inverted version of the input wave and feed it to the comparator as i need a constant waveform to compare the variation. This is then multiplied by the multiplier and demodulated by the low pass filter. So I am supposed to get a sine wave and square wave at the input of the multiplier.
     
  8. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
    25
    0
    The green waveform represents the filter output and the blue waveform represents the multiplier AD633 output. It is not following each other and also if the Cx is increased or decreased more from the Cref then i am having issues with the time of simulation as well.
    [​IMG][​IMG]
     
  9. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
    25
    0
    I have not used a transformer. I have a used a normal input and inverted the input signal in order to avoid using the transformer. That is the only change i have done to the circuit on the paper. Screenshot (90).png Screenshot (91).png
     
  10. crutschow

    Expert

    Mar 14, 2008
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    I missed the multiplier function in the reference schematic. :oops:

    Here's my simulation of your circuit.
    My only change was to rearrange the multiplier connections to correspond to the model symbol I had, and eliminate all the duplicate power supplies (node labels can really help simplify a schematic).
    The filter output signal looks okay to me.

    upload_2017-3-16_23-5-54.png

    Don't understand why yours doesn't.
    Where exactly where you looking at its output?
    Your filter appears to be not functioning and seems to have a misconnection of some sort.
     
    Last edited: Mar 17, 2017
  11. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    The circuit works fine now. Thank you but there is still a little time delay. How can i eliminate it?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    So what was the problem?

    You need to be more specific with your questions.
    Where are you seeing this time delay?
     
  13. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    Sorry. I meant the output of the multiplier must follow the output of the filter. So when the waveform of the multiplier is high, the waveform of the filter should also be high. But here i am observing that it is the opposite and i need to rectify it. I tried a lot of trial and error methods but nothing really helped. Screenshot (92).png
     
  14. crutschow

    Expert

    Mar 14, 2008
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    You are observing the normal delay of the filter.
    Why do you need the filter output high when the multiplier output is high? :confused:
    Aren't you just interested in the average DC value?
     
  15. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    That is exactly what i told my professor but he is adamant on having the filter output follow the multiplier output.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    Then your professor doesn't understand how filters operate.
    What is his training?

    If you lower the output filter frequency, or add additional filtering then the output will be a constant DC with no significant ripple and he won't be able to have that complaint. (see attached).

    upload_2017-3-17_13-26-40.png
     
  17. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    Thank you. I tried using AD734 multiplier as it has an error percentage of 0.1%. The multiplier connections are done as given in the data sheet but it seems that it is taking forever for simulation of the results. Any ways to speed up the process?
     
  18. crutschow

    Expert

    Mar 14, 2008
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    If you have a Measurement Timestep specified in the Transient Analysis options, remove it.

    You could try going to the Tools/Control Panel/Spice and changing the Integration Method to one of the options as see if that helps.

    You can also try reducing the precision of the various parameters (Gmin, etc.) by a factor of 10 to see if that makes a difference.
     
  19. Nanaiah

    Thread Starter New Member

    Mar 16, 2017
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    Nothing seems to be working. The best i could get is this. Screenshot (93).png Screenshot (94).png
     
  20. crutschow

    Expert

    Mar 14, 2008
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    Post your .sub or .lib, and .asy files for the AD734.
     
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