# adapter to use a soundcard oscilloscope

I would like to use my soundcard as osciloscope to measure low voltage analog signal (between 0 and 5v).

I would like to insert a circuit between the sound card input and what is measured

The goal of this circuit is to provide: 1) high impedance input to not interfere on the measured circuit 2) be able to change the ratio: 1x, 0,5x and 1/10x 3) limit the value that goes to the sound card to less than 1V

Here is the solution I would like to know if it's correct 1) 1 opamp in folowing mode 2) a resitor bridge divisor + 1 opamp in folowing mode 3) a diode a 0,6-0,8v is fine

If I only mesure positive current do I need the AC coupling part descibed in the folowing diagram link http://xoscope.sourceforge.net/hardware/buff.gif

Is it necessary to have also 1M R1 in the input to have 1M input impedance instead of an infinite one ?

Thanks Franck

What is the goal of capacitor that can be seen on the input of probes as

simulate this circuit – Schematic created using CircuitLab

• Are you aware that (1) your soundcard is likely to have a decoupling (series) capacitor on the input so that you won't be able to measure DC? (2) D1 will provide a short-circuit to ground if the opamp output goes above 0.5 V. (3) That you'll be using the laptop / computer ground and that if you have an earth fault the fault current will go through the soundcard jack. Jun 11, 2016 at 17:09
• If you have a good idea of what you plan to measure and know it will be AC in a reasonable frequency range you could capacitively isolate the ground and signal pins or use a 600:600 Ohm isolation transformer after your buffer circuit to protect and make safer your installation. Jun 11, 2016 at 19:50
• It was for general purpose low frequency measurement, (AC and DC). I will use another solution based on stm32 or arduino .. Jun 12, 2016 at 18:40

I would like to use my soundcard as osciloscope to measure low voltage analog signal (between 0 and 5v).

Be aware that soundcards typically have limited frequency response. At the low end are AC coupled so you can't measure DC and can't really measure near-dc signals. At the high end don't expect to get much about 20 KHz even with the modern high-sample rate cards.

If I only mesure positive current do I need the AC coupling part descibed in the folowing diagram link

AC coupling at the input of a circuitry can be useful in a few cases.

Firstly when you are trying to measure an AC signal that is "on top of" a DC signal. Most real scopes have a switch for AC or DC coupled inputs.

Secondly when you are trying to build an AC circuit that runs on a single supply rail. In such cases you would want to bias the system at somewhere other than ground.

Thirdly when you have high gains AC coupling can be useful to get rid of DC offsets resulting from imperfect components before they are amplified.

Is it necessary to have also 1M R1 in the input to have 1M input impedance instead of an infinite one ?

If you want a 1 Megohm input impedance at DC then you need a 1 megohom resistor to ground.

Otherwise your input impedance will be very high and not especially predicatable (with an ideal op-amp it would be infinite but we are dealing with real components here).

At high frequencies stray capacitance will bring your input impedance down.

What is the goal of capacitor that can be seen on the input of probes

Generally the goal of capacitors in probe circuits is to balance out stray capacitance and hence maintain the divider ratio over a wider range of frequencies.

Given the low frequency limits of of sound cards it's pretty irrelevent here.

3) a diode a 0,6-0,8v is fine

Your diode on the output will limit positive outputs from the output amplifier but will not have any effect on negative outputs from the amplifier.