basic DIY function generator and scope

Question

I'm currently traveling and only have access to basic parts (soldering iron, an arduino nano and a breadboard!) but I'm able to buy parts from local suppliers. for a project I need to be able to create some waveforms and verify the output of the circuit (I tend to only do it for about 10 to 100Hz, so nothing high frequency!).

I think I can use an instrumentation amplifier followed with a summing amplifier to adjust DC offsets (add or remove) and another inverting amplifier to get the original signal, which is finally fed into the MCU ADC input and sampled as fast as possible, then displayed on the PC using a serial monitor program.

I'm having some doubts about the audio output of my PC however. I'm not sure if it's bipolar (i.e goes below ground as well) or if it's unipolar with a certain amount of DC offset? I did some googling but couldn't find anything and I'm not sure how I can verify that.

The signals will be created using this website:

https://www.szynalski.com/tone-generator/

which gives quite a lot of options for the output waveform except anything for DC offset!

obviously the ADC on the arduino works only in the 0-5 range and is unipolar. I try to take a picture of my current schematic and upload it, in case that's relevant information.

Answer 1

Your PC's audio output is really neither bipolar nor DC-biased – it's pretty certainly AC coupled, i.e. there's a capacitor in series with it! (In effect, a headphone port would be centered around ground potential, due to the headphone being connected to ground on its other end.)

That means you can just AC couple it yourself (I recommend doing that to avoid any stray DC current, although the AC output coupling technically renders that redundant) to a biasing of any desired DC voltage, e.g.:

^{simulate this circuit – Schematic created using CircuitLab}

Although I called it optional, you pretty much always want to have an output voltage buffer in a signal generation setting, so add this block to the output of whatever you build – I've omitted it below for clarity's sake. Good thing: even local dealers will stock multi-unit opamps with at least two opamps per IC, so this isn't even any extra component effort.

none of these component values are really critical: A soundcard can drive much larger loads than a couple kΩ; choose the resistor or capacitor larger and get a lower lowest frequency you can pass through.

In a simple case, you could just use a voltage divider to bias your signal:

^{simulate this circuit}

If you need adjustable bias, that can be easily done with a microcontroller like your arduino. Use the PWM unit to generate a specific duty cylce:

^{simulate this circuit}