sample & hold circuit....urgent

Discussion in 'The Projects Forum' started by onlyvinod56, Oct 30, 2008.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
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    I wan2 design a circuit that measures the time difference between two pulses in terms of voltage. I need a period 2 voltage converter.
    I hav a concept. When the first pulse appear, a ramp signal starts generating. When the second pulse initiates, a sample & hold circuit samples the ramp voltage. So ican use this voltage for further analysis.
    Can any body help me in this topic? Plz send me a link.
    I hav searched on the net. I need the sample & hold ckt using general purpose opamps like 741 or 328 ....etc.
     
  2. Bernard

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    Aug 7, 2008
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    Are the pulses on the same line ? What resets the ramp generator ? What is the approximate time periods between pulses, micro seconds or minutes? A short pulse ,a small capacitor and a CD4066 makes a simple sample and hold.
     
  3. Ron H

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    An enhancement is to stop the ramp by turning off the current source before you sample it. This avoids the ramp showing up on your output during sample time.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    What is the period for the further analysis? If it's more than a microsecond or so, you will need an op amp with a much higher input impedance than a 741. Here is a link that will give you information on the subject - www.intersil.com/data/an/an517.pdf.
     
  5. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
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    thanx for all.
    The period range is (0.3sec ~ 6 sec). Can i use HA-2420/2425? How much time it can hold the sampled value?
    And u people asked me about the reset of the circuit. I didnt get it.
    Can u make this a little bit clear plz.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you read up on sample and hold amps you find the concept of droop. The charge on the sample capacitor pushes current through the input resistance of the op amp buffering the cap. No matter what you do, the sampled voltage will decrease over time. Using larger capacitors will increase the time, but there are practical limits to how fast the charging can take place. If the capacitor does not have enough time to fully charge during the sample period, then your results are inaccurate to begin with.

    Converting the voltage to a digital value immediately after the sample time is about the only practical means of being able to work with the voltage value after some time has passed.
     
  7. Ron H

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    Another problem with sample and holds is hold capacitor dielectric absorption (Google). The best dielectrics are air (low capacitance), teflon (expensive), polystyrene (low melting point - difficult but not impossible to solder), NPO ceramic, and polypropylene.
    CMOS op amps generally have the lowest input bias currents.
    Ramp reset (to zero volts) needs to occur before you start a cycle. If your first pulse is long enough, that can do it.

    BTW, if this is so urgent (see thread title), why did you wait 5 days to come back?
     
    Last edited: Nov 4, 2008
  8. KL7AJ

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    Nov 4, 2008
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    At such a low sampling rate, dielectric absorption may not be a problem. But you are correct....the choice of capacitor is worth consideration
     
  9. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
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    Actually, the pulses are generated from a speed sensor where the gap between two successive pulses gives one revolution. I want to compare this gap with a preset value to form a closed loop to run an induction motor at constant required speed. I dont want to usen any micro controller here. Im using a comparator. As the comparator compares voltages, I need this sample and hold circuit. The speed of the motor is 200 rpm maximum. So the time gap is 0.3sec. If i set the preset to 0.6sec, the motor must run at 100rpm.
    So By calibration, i'll use a pot & i'll set the preset(in voltage - reference to comparator). I need a voltage to compare with the preset value. That volatage must be in proportion with the running rpm. From comparator, i'll connect a pulse generator for triac.
     
  10. onlyvinod56

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    Oct 14, 2008
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    Hey RON, sorry yar,
    I went to my home town in an emergency case where i dont have any network for my laptop. Anyways, I got the permission for a week days from my college.
     
  11. Ron H

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    I didn't mean to sound like I was picking on you.:( Just curious.
     
  12. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
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    did u get my idea? Now, how can i simply construct my circuit?
     
  13. Ron H

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    I know I'm correct. I discovered dielectric absorption the hard way about 40 years ago, while designing a sample and hold.:D
     
  14. Bernard

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    Welcome back. Sorry for more questions: Does the code wheel have just one gap ? Does it have more than one track and more than one senaor ? Is it acceptable to use alternate revolutions for measurements ? What is the line frequency ?
     
  15. Ron H

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    With your method, the voltage will be inversely proportional to RPM. Is that OK? It seems like it would be.
     
  16. onlyvinod56

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    Oct 14, 2008
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    The wheel is having only one slot. It generates one pulse for one revolution. The line frequency is 50Hz. I had another idea. If i keep a small DC motor made 2 work like a generator, I can fed the terminal voltage of this small dc genrtr to comparator.

    But can u help with this speed sensor(1 pulse / 1 revolution)
     
  17. onlyvinod56

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    Oct 14, 2008
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    Ok whatever the proportionality,,,, i need the voltage. i can build the later part
     
  18. Ron H

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    I'm guessing your speed sensor pulse width is a percentage of the period. Do you know how wide it is when the period is 300ms?
     
  19. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
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    it is very less. on period will be in milli sec. I dont know exactly. I didnt bother about the ON period pulse width. I want to convert the OFF period value. Ofcourse considering ON period will give the accurate result. Let me get success with this. later we can go for the accurate analysis.
     
  20. Bernard

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    Try this one. The output is updated every other rotation. IT USES CMOS ICs 4027 [1/2], 4528[2/2], un specified NAND, 4066[2/4]; constant current generator LM334, OP amps of your choice. All values approx.
     
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