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Centering output with a cap,then to comparator ..???

Discussion in 'General Electronics Discussion' started by wannabegeek, Jun 16, 2013.

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  1. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    hi all....

    I 'm stuck with something that seems pretty basic...

    I have a triangle wave coming out of an active filter that's all above zero, Vpp = 0 - 2 V.
    I want to connect that output to a decoupling cap to center the signal again, so that
    Vpp = -1 - 1 V.

    Looks great on the scope...Next, I want a comparator to get make a square pulse train that swings both pos and negative.

    This is where I fail, as the input impedance on the op amp comparator
    is huge and the cap just charges up.

    How is this done...?

    TIA,
    wbg
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    You need to provide a DC path from the input of the op-amp to ground.
     
  3. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    Yes, that sounds like a basic ...thanks Steve...that makes total sense...
    Did I mention that my electronics background comes from one crash course
    in a physics program...?

    If I get my project to do something interesting I'll start a thread in the projects section to
    "show it off"...

    Cheers !
    wbg
     
  4. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    Trying out the "fix"...

    I tried putting the DC path at the input of the OP AMP...no luck.

    But I did stick a 1M Ohm resistor to ground and that worked. I'm
    using a 0.1 micro F cap.

    In fact, this solution makes more sense to me...blocking caps in general never seem to work for me as initially expected.
     
  5. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    It appears that I'm stuck with this again...

    I've attached a schematic...

    The input is actually a photo-diode with op-amp for gain. I'm sending a PWM
    20kHz carrier via 650 nm laser diode. The photo-diode receives this pulse, which has a lot of chatter in it, and gets sent to the filters.

    To keep the schematic simple, I just put an AC voltage source in.

    I'd like to not use a negative voltage source. I'm using an OP07 op amp, and wondering if I should be using uni-polar op-amps....

    I've got the circuit working when using a negative supple and no voltage divider.
    I don't see why I need a negative supply though.

    [​IMG]

    Thanks again,
    wbg
     
    Last edited: Jun 28, 2013
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    Yep, that's what I meant. The input of the op-amp requires a DC path to ground (or to a virtual ground).
     
  7. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
  8. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    I totally forgot about setting all the f_3dB points...I was taught how to bias a transistor in school...the application note I posted covers it.

    I got the circuit to work with a single positive supply....and it wasn't worth it...
    too many more parts and a total PITA...two batteries is not such a big deal....

    However, I did try pushing more current through my laser module, 30mA, and it now
    has a small instability...or the Rx has an issue...

    Not sure but I broke my circuit...while making it better...:confused:

    Now I've got something to do all weekend...

    cheers,
    wbg
     
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