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RC filter

Discussion in 'General Electronics Discussion' started by MRF, Jul 3, 2015.

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  1. MRF

    MRF

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    Jul 3, 2015
    Hi my friends
    i have a sensor with pwm output(38khz). i want convert it to voltage.
    i used rc filter but it does not have a real time response!!!
    what should i do?????:confused:
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I think you need to elaborate a bit because I can't decipher what you've posted. :confused:

    Chris
     
  3. Laplace

    Laplace

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    Apr 4, 2010
    What is the corner frequency of the RC filter? What is the source impedance of the pulse driver in the ON state and OFF state? How fast is "real time" response?
     
  4. MRF

    MRF

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    Jul 3, 2015
    Sorry my friend i cant type english well

    I want to khow that what is the best of the value of the capacitor and resistor for RC filter in output of TSOP1138 sensor
     
    Last edited by a moderator: Jul 4, 2015
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This is not easy to answer conclusively because there are two contradicting requirements:
    • select a short RC timeconstant.
      - advantage: fast response
      - disadvantage: high ripple at the output
    • select a long RC timeconstant.
      - advantage: low ripple at the output
      - disadvantage: slow response
    You'll have to weigh the advantages vs. the disadvantages. The main question, based on your original post, seems to be: "what is real-time for you"? You will have to accept a delay, that is inherent to filtering a PWM signal, your only choice is the degree of filtering.
    Here is an extensive discussion on this topic.
     
    MRF likes this.
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  7. MRF

    MRF

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    Jul 3, 2015
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    The block diagram on page 1 indicates that the output stage is a common collector amplifier with a very high collector resistor of 80kΩ. With a collector resistor this high you won't need an RC filter. Just "C" alone and it will be very low value.
    Reading this again has me wondering if we're misinterpreting you. Do you want to filter the signal or obtain a DC level from it?

    Chris
     
  9. Arouse1973

    Arouse1973 Adam

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    I think he want's a D.C voltage. That's how I take it. Let me see if I can come up with something.
    Adam
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    If he wants a DC voltage I think it's going to require more than a Diode and Cap because of that very high value collector resistor. Especially if the output signal is going to be split in two. One being the PWM signal and the second.. DC. He's probably going to need a high Z input buffer driving a rectifier diode and filter cap. Not doing so, by hanging a diode and filter cap directly on that 80KΩ output, will certainly swamp out the PWM signal.

    Chris
     
  11. Arouse1973

    Arouse1973 Adam

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    I'm thinking a little op-amp circuit personally.
    Adam
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yeah, I think a common single ended LM324 in voltage follower config should suffice. On the other hand we're assuming we understand what he wants. I've been down that rocky road to many times to admit how many! :D

    Chris
     
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  13. Arouse1973

    Arouse1973 Adam

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    Yeah I agree Chris.
     
  14. Arouse1973

    Arouse1973 Adam

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    Here is a very crude circuit which will produce a voltage from a PWM signal. 1 us pulse produces approx. 300 mV and 25 us produces approx. 4.8 Volts. If that's what the OP wants.
    Adam

    PWM-V.PNG
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Adam, I'm a bit confused with this circuit. I didn't sim it but it would seem to me that R1 * C1 would more likely transform an (above zero) square or rectangular wave to something that resembles an above zero sinusoidal-(esc) wave. Am I missing something?

    EDIT: More like a Triangle wave.

    If the OP just wants to detect a PWM (go - no go) signal he might be better of using a 555 pulse stretcher. I hope he posts again soon so we can determine what he's desirous of doing.

    Chris
     
    Last edited: Jul 4, 2015
  16. Arouse1973

    Arouse1973 Adam

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    LOL, I thought he wanted a control voltage derived from PWM. See your right we both had different things on our minds. Yes you will get a triangle ish wave, your right. But if you make the RC at least 100 times the period it will be so small. You could add an LC filter to the output if you wanted to.

    I have simulated it now and here is the output at 25 us pulse.
    Cheers PWM-V-SIM.PNG
    Adam
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I see where I'm going astray here. A PWM wave is not a square wave but I was viewing RC as it would react to one. Which would be a near zero V output with a triangular ripple... Duh!

    Chris
     
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  18. Arouse1973

    Arouse1973 Adam

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    But what happens at 50% duty cycle :) When it is a square wave?
    Adam
     
  19. Harald Kapp

    Harald Kapp Moderator Moderator

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    Isn't that the same when it comes to PWM?
     
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  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Well perhaps if the high time is 100%. :)

    Chris
     
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