My purpose with this project is to let my gain adjustments easier with just a LED-diode or something instead of using my oscilloscope's LCD display
and watch for distorted sinus signal coming from source or/and after amplifier.
What is the magnitude of distortion? Down to 0.1 - 0.2 % THD would be easy to measure with arduino alone, less than that would require external ADC. Is it audio amplifier?
But it seems that it misses some features.. maybe someone can help him/me to complete it.
Maybe choose another Arduino hardware also instead of that Arduino Nano that is used.