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Voltage Divider for the Soundcard Osc...

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 27 June, 2012 » Voltage Divider for the Soundcard Oscilloscope. « Previous Next »

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tmetford
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Post Number: 1
Registered: 06-2012

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Posted on Monday, 04 June, 2012 - 03:52 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I want to use the Souncard Oscilloscope suggested by the EPE Magazine http://www.zeitnitz.de/Christian/scope_en?mid=1022 and I need help in making a suitable voltage divider, together with some additional protection diodes, in to prevent damage to my PC. Have you a circuit diagram/component list that I could use to make this device please?
Iím a novice hobbyist with regard to electronics though Iím keen to learn. Your help will be much appreciated!
Thank you,
Tim
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vlf
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Post Number: 327
Registered: 10-2005


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Posted on Monday, 04 June, 2012 - 06:52 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Well this might not be as easy to do... it would be ideal to have an opto coupling from input to computer, but this would require a pre-amp stage first, to provide unity gain and high impedance too.

My thoughts would be to use a simple 1Meg-ohm variable pot as the high Z input, to give a simple input attenuator, feed into op-amp or a FET amplifier; then equalized the overall gain to unity through loss in the opto coupler.

If you look around the net, some sites provide circuits that could be applied with an opto device, but some real design work needs to be carried out, to get it to work across the 22Khz spectrum.

Here is one circuit that could be used as a start point:

http://www.circuitlab.org/2011/08/circuit-two-channel-audio-mixer.html#axzz1wqT4YZD5

Also here:

http://www.circuitlab.org/2010/07/schematic-pc-scope-probe.html#axzz1wqT4YZD5

This site provides a design for your software:

http://www.instructables.com/id/PC-SOUND-CARD-SCOPE-INTERFACE-FACILITATES-DC-RESTO/


Regards, Brian L.
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tmetford
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Posted on Tuesday, 05 June, 2012 - 08:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ah! There's far more to it than I thought (which qualifies me for true numptie status)!
I would love to be able to do this as a project some time, but it's probably something for me to "cut my teeth on" once I've had more experience.
So, for now, I'll keep the links on file and may I thank you very much for your really helpful advice and I look forward to the day when I can attempt this.
Kind regards
Tim
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terrym
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Posted on Wednesday, 06 June, 2012 - 12:42 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Could I make the suggestion that you save the information in those links now, rather than save just the links.

It is my experience, things will disappear just when you need them later. Better safe than sorry.

TM
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rob_guyer
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Post Number: 52
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Posted on Saturday, 09 June, 2012 - 01:49 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Welcome to EPE. The design and construction of a resistive/capacitive voltage divider can be easily realised if you get the oscilloscope input resistance and capacitance from the data sheet. A Google search will throw up the formula for a resistance voltage divider, eg for a 1 Megohm input, the probe should connect a 9 megohm resistor in series with the oscilloscope input, For a flat reponse up to higher frequencies, parallel the 9 meg with a capacitor having 9 times the reactance of the oscilloscope input.
Typically this be a non-ferrous screw threaded parallel to and touching the 9 meg. Aim for about 3-4pF. Fit the two into a discarded fibre-tipped pen case. Calibrate the probe using a 1KHz square wave and adjust for minimal under and over shoot. The empirical formula BW=350/risetime will give the upper 3dB passband.
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rob_guyer
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Posted on Wednesday, 13 June, 2012 - 02:51 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I should add that the capacitance of the shielded lead between the probe and soundcard oscilloscope input should be factored into reactance calculations. And, of course, the lead should have low piezo-electric properties.
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tmetford
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Posted on Wednesday, 20 June, 2012 - 06:43 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've begun assembling the following circuit: http://www.circuitlab.org/2010/07/schematic-pc-scope-probe.html#axzz1wqT4YZD5
However I don't understand what the unattached lead from C4 should connect to?
probe scope circuit diagram
Thank you for your help.
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joe
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Post Number: 1307
Registered: 05-2005

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Posted on Wednesday, 20 June, 2012 - 06:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It's just a poor circuit drawing I suspect.

C4 is basically connected across the battery.
The little triangle symbol means signal ground or negative in this case.

Joe

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