An inverting adder works by converting multiple input voltages into currents and then combining those currents at a single point. The inverting input of the op-amp is held at a virtual ground, meaning its voltage stays very close to 0 V even though signals are applied. Because of this, each input signal sees a fixed resistor to ground and pushes a current proportional to its voltage. In this circuit, several sine waves of different frequencies are applied through equal-value resistors. Each resi