Anand Udupa
With smartwatches and fitness bands widely used as personal health monitoring devices, the accurate acquisition of electrocardiogram (ECG) signals on these wearable devices has become a field of great interest. An ECG records activity in the heart through the acquisition of electrical signals. Conventional ECG systems such as patient monitors involve the connection of multiple electrodes to the patient. The signal acquired between a pair of electrodes is representative of the activity of the heart along the vector connecting the pair of electrodes, and gives a unique view of the heart. Figure 1 shows a multi-lead ECG signal acquisition system.
ECG signal acquisition on a wearable device like a smartwatch can be realized with electrodes that make contact to the left wrist and right hand. An electrode at the bottom of the wearable makes continuous contact with the left wrist, where the watch is worn. Then, to record the ECG signal, the user touches an electrode with their right-hand finger on the top or side of the device. While the right- and left-hand electrodes sense the ECG signal, a third electrode (for example, one in contact with the wrist) can drive the DC potential of the body and bias the electrodes to a voltage that is optimum to the ECG signal chain. Referred to as the right-leg electrode even though it is in contact with the wrist, it derives its name from its positioning on the right leg in clinical ECG systems.
There are some unique challenges and requirements for the electronics used for battery-powered ECG acquisition on wearable devices. Apart from the requirements of small size and low power, the analog front end (AFE) used for ECG signal acquisition also needs to overcome challenges of signal quality degradation resulting from small, dry electrodes. ECG signal acquisition using such electrodes suffers from high electrode-skin contact impedance, leading to potential signal loss and noise degradation.
Figure 2 shows an overview of an AFE in which an instrumentation amplifier (INA) senses the ECG signal between the left- and right-hand electrodes, while a right-leg drive amplifier drives the potential of the body through the right-leg electrode. The section labeled as “contact impedance” represents a resistor-capacitor model of the contact impedance in series between each electrode and the AFE pins.
Five of the challenges that can arise from high contact impedance include:
The AFE4950 biosensing AFE is well suited for ECG signal acquisition on wearable devices. To limit noise, its 300-Hz low-pass filter between the INA and the ADC acts as an anti-aliasing filter. For signal attenuation, its DC input impedance is in the range of 10 giga-ohms, keeping ECG signal attenuation to a minimum even when using small electrodes. The AFE4950 also enables synchronized acquisition of ECG and PPG signals; the time difference between appropriate points in these two waveforms has a high correlation to blood pressure, making the AFE4950 a good fit for implementing a cuffless blood-pressure monitoring feature on a wearable device.
The block diagram of the AFE4950 is shown in Figure 3.
Finally, another common challenge in ECG wearables design is R&D time. An integrated solution like the AFE4950, paired with FDA-cleared HeartKey® ECG algorithms from B-Secur, can help designers make clinical-quality monitoring easier to implement by eliminating months of R&D. You can read more in the announcement of our companies’ solution, “B-Secur collaborates with Texas Instruments to provide powerful, medical-grade ECG/EKG monitoring for consumer wearable devices.”
Implementing health monitoring applications like ECG in wearable devices presents unique challenges. The AFE plays a vital role in surmounting some of these challenges and enables acquisition of clinical-quality ECG signals on a personal device like a smartwatch. Such a technology has immense potential in helping with health monitoring.
In the near future, we will witness a surge of smartwatches that sound an early alert to seek medical intervention for an impeding heart condition. These technologies will make it possible to measure vital parameters from the comfort of your home and share them with your doctor at the touch of a button. With our extensive biosensing portfolio that spans multiple applications like photoplethysmogram (PPG), ECG and bioimpedance measurements, we are helping unlock the full potential of health and fitness monitoring in wearable devices.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2023, Texas Instruments Incorporated