DC Offset Issue

Discussion in 'The Projects Forum' started by rueffy, Sep 6, 2013.

  1. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Hello Again,

    For those who aren't familiar with my project this is a PWM LASER Comms system, specifically a transmitter circuit I am currently developing.

    I'm having issues getting rid of a DC offset on one of my circuit lines. As can be seen from the waveform diagram below, my 'pink' or oscillator output is offset by approx 200mV. I've tried a range of different capacitors between oscillator output and comparator input to try and block the DC but with no luck. Having the oscillator and 'blue' audio signal both with zero DC biasing is vital for my PWM comparison to be accurate ('green' output).

    Any feedback appreciated. Thanks.

    [​IMG]

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,343
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    Pin 3 of the LM311 has no resistor to allow the input bias current to escape to ground.
     
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  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Pin 3 of the 741 op amp also must have a resistor to ground to allow bias current since you have a cap on it's lead.

    I think the 311 is OK without one because it's current flows back into the output of the 741.
     
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  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    An other issue is that the opamps have + and - 9 Volts and the comparator only + 9 Volts.
    Better put also an capacitor at the audio line and lift BOTH inputs of the comparator to about half the supply voltage of the comparator.

    Bertus
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Do you really want your carrier oscillator output to be sinusoidal? I would have thought a triangular wave would better produce a typical pwm result. LM741's may not be of much use at that (albeit relatively low) frequency.
     
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  6. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thanks for your help

    An improvement there for sure, still about 30mV DC offset there with a 22k resistor between LM311 Pin 3 and Earth. Not sure if this is significant enough to warrant any issues, perhaps if I increase my oscillator amplitude a bit there wont be as much risk of the the audio signal extending beyond the tracking range.

    [​IMG]
     
  7. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    The oscillator is a triangle wave oscillator, just with rounded peaks. Circuit has been constructed on a breadboard, I expect a more triangular waveform when stray capacitance removed from soldiering to a PCB.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think the rounded peaks have to do with the low slewrate of the 741.
    You could try a TL071 that has a better slewrate.

    Bertus
     
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  9. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Excellent thanks Bertus and t_n_k that I will do.
     
  10. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Also, I tried a resistor from Pin 3 of the 741 but it had no effect.

    After having a little play with the resistor size from Pin 3 of LM311 to GND I've got a perfectly averaged AC signal from both Mic and Oscillator. 10k was the magic number. Will pick up a couple of TL071's tomorrow and hopefully get the swing looking a little sharper.

    Your advice has been exceptional, thanks.

    [​IMG]
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here I have lifted BOTH inputs of the comparator to about 1/2 of its supply:

    [​IMG]

    You could try it and see how the out out behaves.

    Bertus
     
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  12. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thank you Bertus, I wasn't sure how to implement the solution but your diagram is very helpful and should be simple to do.

    My end requirement is that I have a square wave PWM output that varies from 0V and 3V as that's the LASER's turn on voltage. Just wanting to confirm this won't have any effect on those levels before I make the changes?

    Also what is the advantage of raising the inputs up to 1/2 VCC?

    Thanks.
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When raising the inputs to about 1/2 the powersupply, you can use input signals of almost the full powersupply range.

    I also have an idea for an alternative output to give the 3 V for the laser:

    [​IMG]

    When the output transistor is "high" the output voltage is limited by the voltage divider.
    When the output is "low" the out put voltage will be close to 0 Volts.

    Bertus
     
    Last edited: Sep 6, 2013
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  14. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Excellent that makes sense, I will make those changes and let you know how it goes.

    Thanks again.
     
  15. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    I promised I'd get back to you, I've made a few changes including the raising of the inputs to 1/2 Vcc that you suggested. Finally got to test it this week, circuit is functioning beautifully.

    http://www.youtube.com/watch?v=eQGYkKTWqp8
     
  16. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thank you to all those that helped contribute to the development of my circuit. The project is now fully functional and I've made a video as part of my presentation to a class, the video shows the system in operation with a full description of signal flow etc.

    http://www.youtube.com/watch?v=CCAkpJ-jG6A

    rueffy.
     
    Last edited: Nov 25, 2013
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You could have filtered the audio at the input of the amplifier.
    A lowpass filter of about 20 KHz would work.
    This would prevent the 49 kHz to be amplified to a large amplitude wich might go radiate in the air.

    Bertus
     
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  18. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    I hadn't really given any thought to radiation but thanks for the advice.
     
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