The source of the signal was a photodiode current transformed in voltage by a TIA and process by a low pass, then a high pass filter (Sallen Key). The last stage of the processing was a non inverter additionner aop with a gain of 20

But I think the current from pin was not due to my circuit, since i have the same voltage on pin20 where nothing (literally nothing) was connected.

Pin 20 or pin 17? What you are seeing seems odd, there shouldn't be any output on those pins. Can you post code and schematics?

The pin16 is the one i used on my circuit (Sorry i make a mistake on the first post), the pin20 was just a try to see if i observe output here, since he was not used in my circuit and not connect to anything

Here is my code :

Serial pc(USBTX, USBRX);

Sensor sensor(p16);

LedP *selected(&sensor.getRed());


char buffer('n');
bool progress(true);




int main()
{
    pc.puts("debut du programme\r\n");
    while(progress) {
        buffer = pc.getc();
        pc.printf("Ordre : %c\r\n", buffer);
        if(buffer == '1') {
            selected = &sensor.getRed();
        } else if(buffer == '2') {
            selected = &sensor.getIRed();
        } else if(buffer == 'a') {
            (*selected).allumer();
        } else if(buffer == 'e') {
            (*selected).eteindre();
        } else if(buffer == 's') {
            if(sensor.getRed().getEstAllumer()) {
                pc.puts("Led rouge allumer\r\n");
            } else {
                pc.puts("Led rouge etainte\r\n");
            }
            if(sensor.getIRed().getEstAllumer()) {
                pc.puts("Led infrarouge allumer\r\n");
            } else {
                pc.puts("Led infrarouge etainte\r\n");
            }
        } else if(buffer =='m') {
            string stime;
            string sfrequency;
            char car('y');

            //pc.puts("Time(s) : ");
            pc.puts("\r\nFrequency(Hz) : ");
            do {
                car = pc.getc();
                if(car != ' ') {
                    pc.putc(car);
                    sfrequency.push_back(car);
                }
            } while(car != ' ');
            
            float frequency = atof(sfrequency.c_str());
            puts("\r\n");

            sensor.measure(frequency);

        } else if(buffer == '*') {
            progress = false;
        }
    }
    pc.puts("Fin de la boucle\r\n");
    return 0;
}

I observe the output when i'm in or out of the while and even if I don't give any order

Here is the constructor for Sensor and Led, the other methode was only call when I give an order, but the output was observable without this so they are not useful I think

//Methode

//Constructeur qui prend un PinName comme argument pour l'AnalogIn
Sensor::Sensor(PinName pin) : a_signal(pin),a_local("local"),a_red(p26),a_iRed(p25),a_pc(USBTX, USBRX)
{   
}

//Argument
AnalogIn a_signal();
LocalFileSystem a_local();
LedP a_red();
LedP a_iRed();
Serial a_pc();

//methode

//Constructeur qui prend un nom de pin comme argument
LedP::LedP(PinName pin): a_led(pin)
{
    a_led = 0;
}

//attribut
PwmOut a_led();
bool a_estAllumer(false);

And the schematic of my board :