ECG sensor vs PPG sensor | Difference between ECG sensor and PPG sensor

This page compares ECG sensor vs PPG sensor and mentions difference between ECG sensor and PPG sensor used for heart rate monitoring. ECG stands Electrocardiogram where as PPG stands for photoplethysmogram.

Introduction: Due to recent outbreak of COVID-19 and other viruses, chances of heart attacks have been increased. People are becoming more vulnerable to cardiovascular and heart stroke risks. Due to this, it has become very essential to have self diagnosis including heart related tests when person feels mild symptoms such as fever, fatigue, cough, cold etc. There are two methods to measure heart rate parameters for diagnosis purpose viz. ECG and PPG.

What is ECG sensor ?

ECG technique measures bio-potential produced by electrical signals using electrodes attached to the skin of the patients. Small metal electrode patches can be attached to person's chest, arms and legs. It represents expansion and contraction of the heart chambers.

ECG test is carried out to detect and record electrical activity of the heart such as rate of heart beating its rhythm etc. ECG sensors measure wide range of heart metrics as they capture HR (heart rate) and HRV (heart rate variability) parameters.

AD8232 ECG sensor

The example of ECG sensor is AD8232 IC module. It consists of three electrodes which are attached to right arm (RA), left arm (LA) and right leg (RL). It generates output analog signal which represent heart beat. The output can be read by any microcontroller board or Arduino Uno board or can be displayed on oscilloscope. As shown AD8232 is powered using 3.3V LDO Voltage Regulator. It can also be powered using Arduino Uno board.

ECG curve

Complete ECG can be obtained using 10 electrodes capturing 12 leads (i.e. signals) to get complete picture of heart. Though 12 leads are needed for accurate diagnosis; however one lead is sufficient for initial assessment of the patient. The figure depicts ECG curve. As shown, it consists of five states viz. P, Q, R, S and T which corresponds to different phases of heart activities. Abnormal heart rates such as very fast, very slow or irregular can be detected by analyzing changes in ECG curve patterns.

What is PPG sensor ?

PPG technique uses light based non-invasive technology. It measures volumetric variation in blood circulation during heart pumping action using light source and photodetector attached on skin. LED and photodetector are used to sense blood flow rate to derive heart rate.

PPG sensor

The figure depicts attachment of PPG sensor on the finger and its working principle. As shown, PPG sensor consists of LED attached on one side of tissue and photodetector on the other side. PPG sensor measures changes in blood volume based on obstruction and absorption of incident light. When LED and photodetector are places adjacent to each other as shown, all the incident lights are reflected off the tissue surface.

PPG curve

The figure depicts PPG curve. This PPG signal waveform can be used to represent many things about the person under diagnosis. The systolic amplitude reflects AC variation in the blood volume around the measurement site. PPI interval (i.e. interval between two systolic peaks) in PPG curve represents cardiac beat to beat interval. To derive further information about the patient, first derivative and second derivative of the original PPG signal can be estimated. The second derivative of PPG signal indicates acceleration of blood.

Tabular difference between ECG sensor and PPG sensor

Following table mentions difference between ECG sensor and PPG sensor.

Parameters ECG sensor PPG sensor
Principle Electrodes are used to directly tap electrical signals produced by heart activity. Electrical signal is derived from reflected light due to changes in blood flow during heart activity.
Accuracy More accurate Less accurate
Settling time for reading Short Long
Size (PCB footprint) Small Large
HR measurement ECG sensor offers on chip HR measurement without the need of any external equipments. It can be measured accurately on beat by beat basis. PPG sensor requires external microcontroller for HR estimation. It uses ECG signal as reference for HR comparison. It is only suitable for average HR measurement.
HRV measurement It can be derived from ECG data as peak intervals can be extracted with ms accuracy. Accurate HRV can be obtained with short duration measurement. Peak interval accuracy is limited by usable sampling rate due to high power consumption of LEDs. Pulse rate variation correlates with HRV for longer periods of measurement (approx. > 5 minutes) but not for short duration measurement.
Operating power Lower (~ 2.5 mWatt) Higher (~ 30 mWatt)
Crystal Oscillator It consists of built in oscillator and hence no external clock is needed. External crystal is recommended for accurate timing control.
Software development It requires SDK (iOS or Android) for easy application development and integration. It requires Microcontroller firmware development and integration in addition to host platform app development/integration.

Also refer advantages and disadvantages of PPG sensor >> and ECG sensor >>.

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