This portion of the circuit creates a voltage follower that supplies a stready 2.5 V to the rest of the circuit. This is an important component because it provides a reference voltage to compare the ECG signals to. The voltage follower is also important because it prevents attenuation of the input signal by having a high input impedence and a low output inpedence. The LM358 chip provides a 2.5 V singal with a gain of unity (1) by effectively creating a voltage divider with the two 6.8 kOhm resistors.

The AD623 chip is used as an instrumentation amplifier to amplify the voltage difference between the leads coming from the patient (positive lead to inverting input, negative lead to non-inverting input). This is helpful because for an ECG we want to focus on the difference in electrical activity between two points, not the electrical activity at one or two individual points. The difference between the two nodes is amplified with respect to the reference voltage and the signal proceeds to the bandpass filter.

The voltage coming from the AD623 intrumentation amplifier first passes through the high-pass filter (Ci and Ri). The capacitor allows high-frequency signals to pass through with a cutoff frequency of 21.28 Hz. The signal then passes through the low-pass filter (Cf and Rf in series). The capacitor Cf forces low frequency signals to pass through the resistor in series, with a cutoff frequency of 174.07 Hz. The combination of the low- and high-pass filters effectively creates a bandpass filter, which allows the OP2 LM358 chip to amplify the cardiac signals we care about. Without the bandpass filter, interferences from contracting muscles, environmental noise, and baseline signal drift would prevent us from analyzing the ECG data. After the bandpass filter, the OP2 LM358 chip amplifies the voltage.