correct sound card isolation?

Discussion in 'The Projects Forum' started by fawcetteng, Dec 16, 2010.

  1. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
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    a question on the common sound card as a function generator circuit:

    If I use an op amp to boost the output to say +/- 5v, is the op amp also isolating the sound card should I accidentally pull too much current? As in I'll damage the op amp, not the sound card? If not, how should I correctly do that? I'm new at op-amps. I see they can handle 20mA or so on the output. What would give me a bit more capability?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    FET-/JFET-input opamps provide a very high impedance to input signals; in the tens or hundreds of megohms. You should still isolate the sound card from the opamp using a capacitor, just in case you miswire the opamp.

    A sound card line output has nominally 10k impedance. The headphone output is lower impedance.

    Why don't you post the circuit you're considering?
     
  3. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
    10
    0
    here is a schematic, using one channel of the head phone output on the sound card. Where and what size cap. would I use for protection?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Have a look at the attached schematic and simulation.

    C1 isolates the DC input level from the opamp's input; only the AC signal is coupled through.

    The TL07x and TL08x family of opamps has a common mode range (input range) of Vee+3v (or -V+3v) to Vcc-1.5v (or +V-1.5v), so R1 and R2 bias the input to roughly 1.75v. This also keeps the polarity across the capacitor correct, so you can use a polarized capacitor.

    R3 provides a 10k impedance for the input signal, which matches the impedance of the line-out of the sound card.

    U1a is wired as a voltage follower/buffer; this isolates the input from the output. The output from U1a is basically the same as the input, but with a voltage offset from the R1/R2/R3 divider. C2 keeps the reference voltage stable.

    U1b is wired as an inverting amplifier with variable gain. This allows from a few mV p-p out to +/-5v out. C3 isolates the DC level from the actual output. R6 keeps the average output voltage centered around 0v.
     
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