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LM324 buffering doesn't work

Discussion in 'General Electronics Discussion' started by osterchrisi, Mar 8, 2011.

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  1. osterchrisi

    osterchrisi

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    Mar 8, 2011
    Hi there!

    I have a question concerning buffering with an GND to +9V supplied LM324. I have a 30kHz pulsed IR signal with DC offset. The pulsed signal is my actual signal, the DC offset is sunlight - this circuit should measure objects in front of the transmitter and produce voltage according to amount of reflected light (i.e. proximity of object).

    I buffer this signal in the first section of the opamp then take it to the non-inverting input of the second section where I apply a certain amount of gain. The signal then goes into a passive highpass filter (get rid of vagabonding IR light and daylight). After the highpass filter (the signal is now swinging around GND), to get rid of negative voltage swings a diode is connected 'facing' from ground to the junction of the CR chain (DC 're-establishing') and after that another diode in direction of the signal to get rid of the last -0.6V DC offset (produced by the first diode).

    I hope you could follow until here.
    After that I want to buffer the signal again but strange things happen: When I put this signal into the non-inverting third section of the opamp (buffer setup - inverting input connected to output), the output is clamped to positive supply rail. I can see the changing signal at the input, but the output is always at the upper rail.

    What did I miss here that this is not working?
    Thankkkkks for any suggestions!!!

    //edit
    Ah yeah: The CR network consists of a 1nF cap in series followed by a 5K6 resistor to ground...
     
    Last edited: Mar 8, 2011
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    have you got a cct schematic you could show us ?
    it would help clarify things :)

    Dave
     
  3. osterchrisi

    osterchrisi

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    Mar 8, 2011
    I donno what cct means but for sure I know what a pencil is :) I drew that earlier this day...
    Here you go!

    [​IMG]
     
  4. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Turn the last diode around & add a pull-up resistor (or remove it, or add a pull-down resistor) and you'll be fine.
     
  5. davenn

    davenn Moderator

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    cct = circuit, tech talk abbreviation :)

    D
     
  6. osterchrisi

    osterchrisi

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    Mar 8, 2011
    As it happens to all of us many times (I guess), today the magik circuit simply works since the first second... :confused: I guess there's some kind of temperature variables I need to consider. But many thanks to all the helpers here!
     
  7. duke37

    duke37

    5,299
    739
    Jan 9, 2011
    The diode direction looks OK to me. I would shift the 5k6 resistor to the output and add a capacitor to ground there.
    I suggest that the problem is too much DC giving permanent offset. Try a capacitor (10nF?) between the photo transistor and the first op-amp and add a 1M resistor to give DC bias. Where this resistor should go depends on the type of op-amp you have, midway between the power supply would be reasonable.
     
  8. osterchrisi

    osterchrisi

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    0
    Mar 8, 2011
    Not sure, if I get you wrong but the capacitor in series followed by the resistor to ground is actually there to get rid of the permanent DC offset (highpass filter at around 28kHz). If I would turn them around they would make a lowpass filter which is exactly what I don't need.

    The actual problem in this circuit is that it's single supplied and the LM324 doesn't accept voltages below -0.3V so I really have to get every signal above GND.
    I also take 2 IR LEDs now to get a greater amplitude of my pulsed signal - that already helps a lot...
     
  9. Resqueline

    Resqueline

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    Jul 31, 2009
    If the diode after the 5.6k resistor is connected to the non-inverting input of an op-amp and nothing else then there's nothing preventing that input from floating up to Vcc.
     
  10. duke37

    duke37

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    Jan 9, 2011
    I have had another think, it takes time these days. You should check that the phototransistor is not saturated in ambient light, use a high impedance voltmeter to measure across the 1M resistor. The first op-amp can then be directly connected as you have it already. If the transistor is saturated, reduce the resistor value (to 100k or 10k?) The gain of the second op-amp may then be increased to compensate.
    The capacitor at the output of the second op-amp passes the AC and the two diodes act as a diode pump, putting charge into the output. This should be smoothed with a capacitor and loaded with a resistor - shift your 5k6 reistor to the output.
     
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