The type of amplifiers required for electrocardiogram (ECG) signals are called differential amplifiers because they do not just amplify one signal or another, but they amplify the difference between two signals. In the case of the ECG, they amplify the difference between the signals collected by the positive and the negative electrodes in one bipolar lead. This notch filter aims at blocking any power line interference signal (at 50 or 60 Hz) that remains after the common-mode rejection by the instrumentation amplifier. The configuration chosen is called the twin-T notch filter, as its circuital schematic resembles two Ts mirroring each other. The Instrumentation Amplifier will be built on a breadboard together with three types of signal filters (low-pass, high-pass, and a 60 Hz notch filter) that will eliminate the noise blurring the electrophysiological signal of interest – in this chapter an ECG – collected by the electrodes on the human subject.