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Turning soundcard into oscilloscope - advice

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MrNobody

New Member
Hi,
I am thinking of turning the computer's soundcard into oscilloscope..
I found this circuit using CA3140 opamp operating at 9V. Since I have some spare LM358 opamp, can I use that instead and operate it at 5V instead of 9V..? Anybody tried this thing before..?
By the way, for the input 'probe' can I use a simple wire instead of the normal oscilloscope probe..? I am only using it for 20kHz and below..
The oscilloscope program I hope to use is this one.
Please advice if possible..
Thanks..
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi,
I am thinking of turning the computer's soundcard into oscilloscope..
I found this circuit using CA3140 opamp operating at 9V. Since I have some spare LM358 opamp, can I use that instead and operate it at 5V instead of 9V..? Anybody tried this thing before..?
By the way, for the input 'probe' can I use a simple wire instead of the normal oscilloscope probe..? I am only using it for 20kHz and below..
The oscilloscope program I hope to use is this one.
Please advice if possible..
Thanks..
hi,
The LM358 has a lower spec than the CA3140, when feeding into a high resistance load the CA3140 pulls down close to 20mV or so, where the LM358 will not go much lower than about 300/400mV.

The input impedance of the 3140 is VERY high.

Both OPA's require a supply voltage of more than 9V in order to give an output swing upto 5V.
When powered from say 5V, the Vhigh will be about 3V.

For your low spec scope leads use TV aerial cable, thats about 50/75:eek:hm:

IIRC the the low freq end end of the PC scope card dosn't extend down to dc,
I would expect the low end freq to be about 20/50Hz.

Dont exceed the max input amplitudes of the sound card.

Lets know how it goes.:)
 
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MrNobody

New Member
Oh.. i see.. its much complicated than i thought..i thought its just similar to microphone as in plugging in the cables normal stereo cables and able to see the waveform.. i totally missed the impedance matching part..
Hey.. i thought impedance matching is only needed for high frequency like MHz or GHz and not really needed for few kHz..?
My current plan is just to use it for 4 kHz signal..

By the way, wats IIRC..?
 

ericgibbs

Well-Known Member
Most Helpful Member
Oh.. i see.. its much complicated than i thought..i thought its just similar to microphone as in plugging in the cables normal stereo cables and able to see the waveform.. i totally missed the impedance matching part..
Hey.. i thought impedance matching is only needed for high frequency like MHz or GHz and not really needed for few kHz..?
My current plan is just to use it for 4 kHz signal..

By the way, wats IIRC..?

hi,
I mentioned the high input resistance compared to the LM358 in case it made a difference in the circuits operation. I'll look at the circuit diagram

The IIRC is If I Recall Correctly... on the web there is a full glossary of these abbreviations.

Can you say where you are located.?

EDIT:
Looked over the circuit, it uses two, 2Mohm resistors to fix the centre operating voltage.
A LM358 is a dual OPA, you could build it using a LM358, you may not get the full output voltage swing.

If you are not happy with its performance with a LM358, a CA3240 [dual] is a direct swop.
You could make a stereo/dual chann scope..

Get the Audacity freeware off the web. it works with sound card, you can analyse waveforms that way.!
 
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MrNobody

New Member
I am located in Malaysia..
I did look up IIRC in google but i thought that couldn't be what u mean.. I thought it has comething to do with Resistor and Capacitor, maybe IIRC is 2 stage RC filter..
I dunno... :p

Thanks alot..
 

audioguru

Well-Known Member
Most Helpful Member
The lousy old LM358 has crossover distortion because the output transistors operate without enough bias current in class-B for low power. The distortion is eliminated if a load resistor to the positive supply or to the negative supply is added to the output so that one output transistor operates in class-A.
 

Blueteeth

Well-Known Member
Hmm,

I was planning on making a buffer for my crappy soundcard to use it for oscilloscope purposes. The whole piont was 'cheapness', just using what I got around. I guess the 358 isn't too bad considering I'm not after accuracy, mainly digital waveforms (sans DC component).

It'll do until I get my DSO built. I'm sure the 358 will run happily at 5v (you can get it off of the game port on the back of your PC) as I'm adding clamp diodes to keep the input to the card to +/- 1V ish.

Would a simple voltage divider at the input be sufficient for an input voltage of 0-5v? The input buffer will just be a voltage follower, simgle supply, biased with two 1Mohm resistors. That will be built inside of a probe housing to keep it as close as possibe to the working circuit. Any more tips guys?

Blueteeth
 

audioguru

Well-Known Member
Most Helpful Member
does your sound card have a line-level input, or just a microphone -level input?
1Vp-p would overload a microphone input. I don't know what signal level that will overload a line-level input.

The output of an LM358 that has a +5V supplt is from close to 0V to +3.8V.
 

Blueteeth

Well-Known Member
Oh yeah I'm using line in, same as I do for recording music. Only ever use mic input for voice, and apps that require it. Because lets face it, even on the expensive cards, the mic input is crap.

As for the LM358, 0-3.8 sounds fine. Centred at 2.5V (its virtual ground) with a +/-1v thats 1.5-3.5V on its output. Although I'm unsure if standard soundcard line-in input is 1v RMS or p-p. I guess I could reduce the output of the opamp to 1v p-p, meaning I'll only need two clamping diodes (+/-0.65v ish).

I know the soundcard can't do DC, and probably not wise to attempt to 'measure' anything below 50hz. But as I said, its digital waveforms, just for quick checking of timing, ans possibly slow envelopes. I know the limitations of uising a soundcard for an oscilloscope, hardly professional, but 10 minutes to knock up a circuit for it would be handy for debugging with my laptop.

So I'm tihnking the following specs, roughly:
Input impedence: 500K - not high enough but good enough for most things eh
Output impedence: hmm maybe 1-5k.
Max input voltage (before clipping by diodes/opamp) 5v - 10v. adj.
Gain? Would require an extra pot, worth it?
5v power supply, taken from PC's gameport, filtered with inductor.

Trying to keep the parts down and cost to an absolute minimum without completely ruining its usability. I'll be built on stripboard too ^^ Probe(s) will be simple old multimeter probes with shielded cable + croc clip ground.

Of course, all the above is app specific, but a bit of versatiliy won't hurt and my desktop only has a crappy SB32 on board. And my laptop...well we all know whats up with laptop soundcards. I'm modding the mic input for line in. Well, just switching out the amp/buffer.

Blueteeth
 

ericgibbs

Well-Known Member
Most Helpful Member
Thanks for that..

hi,
This is another input circuit, with some level of protection.
I would use a better quality OPA, a low cost choice would be a CA3240

C:\Documents and Settings\Administrator\My Documents\Dump1\Buffer Hardware for xoscope.htm
 
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