Vital sign monitoring has moved beyond medical practice and into multiple areas of our daily lives. Initially, vital sign monitoring was performed in hospitals and clinics under strict medical supervision. Advances in microelectronics have lowered the cost of monitoring systems, making these technologies more pervasive and ubiquitous in areas such as telemedicine, sports, fitness and health, workplace safety, and the automotive market, which is increasingly focused on autonomous driving. Although these extensions are implemented, high quality standards are still maintained because these applications are highly related to health.
Vital signs monitoring involves measuring a range of physiological parameters that can indicate an individual’s health. Heart rate is one of the most common parameters that can be detected with an electrocardiogram, which measures the frequency of the heartbeat and, most importantly, changes in the heartbeat. Heart rate variability is often caused by activity. During sleep or rest, the rhythm is slower but tends to speed up with factors such as physical activity, emotional responses, stress or anxiety.
A heart rate outside the normal range may indicate a condition such as bradycardia (when the heart rate is too low) or tachycardia (when the heart rate is too high). Breathing is another key vital sign. The degree of oxygenation of the blood can be measured using a technique called photoplethysmography (PPG). Hypoxia may be associated with disease flares or disorders affecting the respiratory system. Other vital sign measurements that can reflect an individual’s physical condition include blood pressure, body temperature, and skin conductance response. The skin conductance response, also known as the galvanic skin response, is closely related to the sympathetic nervous system, which in turn is directly involved in mediating emotional sexual behavior. Measuring skin conductance can reflect a patient’s stress, fatigue, mental state, and emotional response. In addition, by measuring body composition, percentages of lean and fat body mass, as well as hydration and nutritional levels, an individual’s clinical status can be clearly displayed. Finally, measuring movement and posture can provide useful information about the subject’s activity.
Figure 1. Signal chain for optical measurements
Technology to measure vital signs
To monitor vital signs such as heart rate, respiration, blood pressure and temperature, skin conductivity and body composition, a variety of sensors are required, and the solution must be compact, energy efficient and reliable. Vital signs monitoring includes:
u Optical measurement
u Biopotential measurement
u Impedance measurement
u Measurements using MEMS sensors
Figure 2. A Complete Bioelectricity and Bioimpedance Measurement System
Optical measurements go beyond standard semiconductor technology. To perform this type of measurement, an optical measurement toolbox is required. Figure 1 shows a typical signal chain for optical measurements. A light source (usually an LED) is required to generate the light signal, which may consist of different wavelengths. Several wavelengths are combined to achieve higher measurement accuracy. A series of silicon or germanium sensors (photodiodes) are also required to convert the light signal into an electrical signal, also known as photocurrent. Photodiodes must have sufficient sensitivity and linearity in response to the wavelength of the light source. Afterwards, the photocurrent must be amplified and converted, so a high-performance, power-efficient, multi-channel analog front end is required to control the LEDs, amplify and filter the analog signal, and perform analog-to-digital conversion with the required resolution and precision.
Optical system packaging also plays an important role. A package is not just a container, but a system containing one or more optical windows that filter incoming and outgoing light without undue attenuation or reflection that would compromise signal integrity. To create a compact multi-chip system, the optical system package must also contain multiple devices, including LEDs, photodiodes, analog and digital processing chips. Finally, a coating technique capable of creating optical filters is also suggested to select the desired part of the spectrum for the application and to eliminate unwanted signals. The app must function properly even in sunlight. Without an optical filter, the magnitude of the signal would saturate the analog chain, preventing the electronics from functioning properly.
Analog Devices offers a range of photodiodes and various analog front ends capable of processing the signals received from the photodiodes and controlling the LEDs. A complete optical system is also available, which integrates LED, photodiode, and front-end into one device, such as the ADPD188.
Biopotential and Bioimpedance Measurements
Biopotential is an electrical signal caused by the effects of electrochemical activity in our body. Examples of biopotential measurements include electrocardiograms (ECGs) and electroencephalograms. They examine very low-amplitude signals in frequency bands where multiple interferences are present. Therefore, before the signal can be processed, it must be amplified and filtered. ECG biopotential measurements are widely used for vital signs monitoring, and ADI offers several components to perform this task, including the AD8233, ADAS1000 chip family.
Designed for wearable applications, the AD8233 can be combined with the ADuCM3029, a Cortex®-M3-based system-on-chip (SoC), to create a complete system. In addition, the ADAS1000 series is designed for high-end applications with low power consumption and high performance, especially for battery-operated portable devices, with scalable power and noise (i.e., noise levels can be reduced proportionally with increasing power consumption) , is an excellent integrated solution for clinical level applications.
Bioimpedance is another measurement that can provide useful information about the state of the body. Impedance measurements provide information on electrochemical activity, body composition, and hydration status. Measuring each parameter requires the use of different measurement techniques. The number of electrodes required for each measurement technique, and the point in time at which the technique is applied, varies depending on the frequency range used.
For example, when measuring skin impedance a low frequency (up to 200 Hz) is used, while when measuring body composition a fixed frequency of 50 kHz is usually used. Likewise, in order to measure hydration and properly assess intracellular and extracellular fluids, different frequencies are used.
Although techniques may vary, all bioimpedance and impedance measurements can be implemented using a single-ended AD5940. The device provides excitation signals and a complete impedance measurement chain to generate different frequencies to meet a variety of measurement requirements. In addition, the AD5940 is designed to be used with the AD8233 to create a comprehensive bioimpedance and biopotential reading system, as shown in Figure 2. Other devices for impedance measurement include the ADuCM35x family of SoC solutions. In addition to a dedicated analog front end, the solution provides a Cortex-M3 microcontroller, memory, hardware accelerators, and communication peripherals for electrochemical sensors and biosensors.
Motion Measurement Using MEMS Sensors
Because MEMS sensors can detect gravitational acceleration, they can be used to detect activity and abnormalities such as erratic gait, falls or concussions, and even monitor the posture of subjects while they are at rest. In addition, MEMS sensors can complement optical sensors, which are susceptible to motion artifacts; when this happens, the information provided by the accelerometer can be used to correct it. The ADXL362 is one of the hottest devices in the medical field and the lowest power triaxial accelerometer on the market today. It has a programmable measurement range from 2 g to 8 g and digital output.
Figure 3. ADPD4000 for implementing optoelectronic, biopotential, bioimpedance, and temperature measurements
ADPD4000: Universal Analog Front End
Currently available wearable devices on the market, such as smart bracelets and smart watches, provide a variety of vital signs monitoring functions. The most common of these are heart rate monitors, pedometers, and calorie counters. In addition, blood pressure and body temperature, galvanic skin activity, changes in blood volume (via photoplethysmography), and other indicators are often measured. As the number of monitoring options increases, so does the need for highly integrated Electronic components. The ADPD4000 features an extremely flexible architecture designed to help designers meet this need. In addition to providing biopotential and bioimpedance readings, it can manage optoelectronic measurement front ends, guide LEDs, and read photodiodes. The ADPD4000 features a temperature sensor for compensation and a switch matrix that directs the desired output and capture signal, either single-ended or differential voltage. The output can be selected, which can be single-ended output or differential output, which is determined by the input requirements of the ADC to be connected to the ADPD4000. The device can be programmed to use 12 different time bands, each dedicated to processing a specific sensor. Figure 3 summarizes the key features of the ADPD4000 in several typical applications.
As technology advances, vital sign monitoring is becoming more and more common in all walks of life, as well as in our daily lives. Whether for treatment or prevention, such health-related solutions require reliable and effective technology. Those designing vital signs monitoring systems will be able to find a range of solutions to their design challenges within ADI’s extensive product portfolio dedicated to implementing signal processing.
About the Author
Cosimo Carriero joined Analog Devices in 2006 as a Field Applications Engineer providing technical support to strategic and key customers. He holds a master’s degree in physics from the Università degli Studi in Milan, Italy. His past experience includes defining and developing experimental instruments for nuclear physics at the Italian National Institute of Nuclear Physics, working with small companies to develop sensors and systems for factory automation, and as a senior designer of satellite power management systems at Thales Alenia Aerospace engineer.
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