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'[OT] Sound card input electrical specifications'
1999\06\24@113800 by Adam Bryant

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Recently there have been discussions about using a PC soundcard as a
cheapo oscilloscope.  I have found a number of excellent looking sofware
applications for this purpose, but not wanting to take a chance at
blowing up my sound card, I was wondering what the typical specifications
for a soundcard microphone input were.

I was also starting to put together ideas for a circuit I could put
between the tested circuit and the sound card to protect it from any
nasties generated by the tested circuit.  The idea I had was as follows:

                                       +5v
                                       |
                                       |
                                     Diode
                                       |
                                       |
Input signal -------Res-----Res---+---------OpAmp--------Sound Card
                                       |
                                       |
                                     Diode
                                       |
                                       |
                                       Gnd

The OpAmp is wired as a unity gain (voltage follower).  An EE friend of
mine told me that would provide very high input impedance.  This is the
same basic circuit I was planning on using in my own DSO (whenever I get
the time to build it).

Thoughts?  Suggestions?  Is this a reasonable way to go about this?  Is
there a better way?

Thanks in advance,
Adam


Adam Bryant (age 0x23)
spam_OUTabryantTakeThisOuTspampeaktech.com (work)
.....adamdbKILLspamspam@spam@juno.com (home)
Parker, CO, USA
Robotics, RC Airplanes, anything using a PIC

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1999\06\24@120254 by Craig Lee

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Since you are limiting your range to between +5V and GND, you aren't going
to
get too decent a scope.  I would at least generate a -5V supply so the
signal
is centered around zero as audio is.

I would use the parallel port and a resistor divider network to ensure the
input signal stays well within your opamp rails.  Your mic input already has
a high impedance circuit, so you could probably just get away with the
resistor
divider network anyway.

Craig


> {Original Message removed}

1999\06\24@134621 by Eric Aos

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You can get a cheap sound card for under $20... probably less than the
time and money it would take to protect it. IMHO

>I was also starting to put together ideas for a circuit >I could put
>between the tested circuit and the sound card to >protect it from any
>nasties generated by the tested circuit.  The idea I >had was as
follows:

1999\06\24@162958 by Joseph A. Zammit

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Adam Bryant wrote:

{Quote hidden}

It is important that you clamp the input. I do not know exactly the input
range of the mic and if it accepts negative voltages.
The diode clamps are imporant BUT they must be connected to the maximum
voltage of the mic input.

If you are really interested, send me the complete specifications of the mic
and I will try to design a good buffer input. What I need to know is the:

-- Maximum allowable voltage on the mic
-- Wether the mic accepts -ve signals

Joseph A. Zammit

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