ECG Sensor

EKG (Electrocardiography) sensor is an electronic module designed to measure and monitor the electrical activity of the heart. These sensors detect bio-potential signals generated by the heart through electrodes placed on the body, amplify, filter, and convert these signals into a format readable by a microcontroller (e.g., Arduino). Commonly used in hobby electronics, biomedical projects, and educational applications, these sensors provide a practical tool for understanding the fundamental operating principles of professional medical devices. By measuring low-amplitude electrical potential differences that occur during each contraction and relaxation of the cardiac muscle, they enable the determination of heart rate and rhythm.

AD8232 EKG SENSOR (Generated by Artificial Intelligence)

Working Principle

The working principle of an EKG sensor can be examined in three main stages: signal detection, signal processing (amplification and filtering), and transmission of data to a microcontroller. The heart operates via an electrical stimulation system that originates in the sinoatrial (SA) node and spreads throughout the cardiac muscle. This electrical activity creates very low voltage potential differences on the body surface, in the millivolt to microvolt range. The primary function of the EKG sensor is to remove these weak signals from environmental noise and convert them into meaningful data.

Signal Detection via Electrodes

EKG signal measurement is performed through electrodes attached to specific points on the body. A typical configuration uses three electrodes: two for measurement and one as a reference (ground) electrode. Following the principle of Einthoven’s triangle, these electrodes are usually placed on both arms and one leg (typically the right leg). The measurement electrodes detect the potential difference on the body surface. The reference electrode is used in conjunction with a technique known as the Driven Right Leg (DRL) circuit to suppress common-mode noise, such as 50/60 Hz mains interference. This is a critical step that significantly improves signal quality.

Signal Processing: Amplification and Filtering

The raw EKG signal obtained from the body is very weak and highly susceptible to noise. Therefore, it must undergo a series of processing steps before it can be interpreted by a microcontroller.

Instrumentation Amplifier

The detected raw signal first enters an instrumentation amplifier (INA) stage. Instrumentation amplifiers are known for their high common-mode rejection ratio (CMRR). Thanks to this feature, they effectively suppress noise signals that are common to both measurement electrodes while amplifying the tiny potential difference between them (i.e., the EKG signal) by thousands of times (typically 1000x or more). Specialized integrated circuits are used for this purpose in popular EKG sensor modules such as the AD8232.

Filtering Stages

The amplified signal may still contain various noise components. To remove these, the sensor module incorporates several filter circuits:

• High-Pass Filter: This filter blocks low-frequency noise caused by electrode-skin contact and patient movement (typically below 0.5 Hz). This type of noise is known as baseline wander and causes a slow drift along the vertical axis of the EKG waveform.
• Low-Pass Filter: Used to eliminate high-frequency noise originating from muscle contractions (EMG noise) and external electronic devices. It is typically set with a cutoff frequency between 40 Hz and 150 Hz. This preserves the essential components of the EKG signal (P, QRS, and T waves) while filtering out unwanted noise.
• Notch Filter: A narrow-band filter specifically designed to suppress mains interference at 50 Hz or 60 Hz. This is one of the most common and disruptive sources of noise.