LM324 Func. Generator testing

Discussion in 'The Projects Forum' started by naitek, Apr 25, 2014.

  1. naitek

    Thread Starter Member

    May 31, 2008
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    hello everyone. I am building a circuit for the umpteenth time, but dont understand the v+/2 in this circuit. How will i get 1/2 the voltage or is it just stating that 1/2 the voltage will be here. Please see attached
     
    Last edited: Apr 25, 2014
  2. naitek

    Thread Starter Member

    May 31, 2008
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    Forgot the attachment..
     
  3. AnalogKid

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    Aug 1, 2013
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    The circuit assumes that you will provide V+ (or Vcc) divided by 2 and decoupled for low source impedance. So if the circuit power is +9V, you have to supply +4.5V to the reference pin of the hysteretic comparator that converts the integrator's triangle wave into a square wave.

    If you have a spare LM324 section, you can do this with a 2:1 voltage divider driving a LM324 as a voltage follower. If you don't have a spare section, replace with 51K coming from V+/2 with two 100K resistors, one to V+ and one to GND. Do *not* add a decoupling capacitor to the juction of the three 100K resistors and the + input. Just in case you don't know, this is called a Thevenin equivalent (actually a reverse-Thevenin).

    ak
     
    Last edited: Apr 28, 2014
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here is the schematic again:

    [​IMG]

    The point with V+/2 should be on half the powersupply voltage.

    In an other schematic from the same datasheet, you can see two resistors and a capacitor (R2, R3 and C1) to make the V+/2:

    [​IMG]

    Bertus
     
  5. naitek

    Thread Starter Member

    May 31, 2008
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    I am just starting on this, but the ac coupled invert. amp is assuming ac voltage or no?

    if so , then what app would i be using it for since i only want a 9v supply.

    thanks for such quick responses..

    -i
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    Not sure if the OP is aware of this, but in this circuit the opamp needs both a positive and and a negative power supply.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Why not the cap AK? I think I've seen a cap used all the time.

    What are the three resistors (10K)?

    Confused here.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    OOPs! A Rose Rosanadan moment ("Never mind"). I just simultaneously realized that the second stage gets VCC/2 for bias and clearly understood the original question.

    You can use one of Bill's excellent circuits or you can probably do very well by making a voltage divider with a pair of resistors, say 3.3k or so.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Because unlike the 2nd schematic in bertus' post, your right opamp is acting as a comparator, not a linear amplifier. The plus input is the trip point for the comparator to change states, and must move "instantly" when it does so to effect the hysteresis effect.

    The original schematic is designed for single-supply operation, as long as Vcc/2 comes from somewhere. I've always wondered why they didn't go with the two 100K resistors as in the AC coupled example.

    There were two typos in my post, both corrected now. I meant to say two 100K resistors (not 1000K), and later it was supposed to be three 100K resistors, the original feedback 100K and the two you add to form Vcc/2.

    ak
     
  11. atferrari

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    Gracias AK. Crystal clear now.
     
  12. AnalogKid

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    Don't think so. The thevenin equivalent resistance of the divider (in your case 1.65K) directly affects the amount of hysteresis and therefore the amplitude of the triangle wave.

    ak
     
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