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xoscope buffer hardware questions

Discussion in 'Electronic Basics' started by Ben, Sep 18, 2007.

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  1. Ben

    Ben Guest


    I was looking at the buffer circuit designed to go with xoscope.



    He has an explanation of the circuit, but I was wondering if someone
    could explain how it works a bit more to me.

    For example
    Does it need to be AC coupled? Could I just remove C1 to measure DC?
    How does the impedance coupling work? The input protection?

    I hope these aren't too broad or too simple of questions. It vaguely
    makes sense to me, but I couldn't explain it to anyone, so I
    definitely don't understand. :)

    Thanks for any and all help!
  2. Phil Allison

    Phil Allison Guest


    ** Yes. But be aware that small ( sub 1 uA) leakage currents in the
    protection diodes may produce a small DC offset at the input and hence
    output of the op-amp.

    Low leakage type diode are preferred here.

    ** The 47kohm limits current flow in the 1N914 diodes to a safe value,
    even with very large input voltages.

    The parallel 100pF cap eliminates any high frequency attenuation due to
    op-amp input, diode & stray capacitances coming after the 47 kohm.

    ....... Phil
  3. Bob Masta

    Bob Masta Guest

    This amp doesn't *need* to be AC coupled, but the sound card that it's
    driving is almost certainly AC coupled... so removing C1 won't help.
    C1 and R1 form a high-pass filter with a corner frequency of
    about 16 Hz. Many sound cards will go quite a bit lower than
    this, so you might want to increase C1 to 0.1 or even larger.

    I'm not sure what they have in mind here. I'd be inclined to
    leave out R3 and D3 and connect the bottom of D2 right to
    -12. That will clamp the inputs to +/-12V max.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
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