“Vital sign monitoring has gone beyond the scope of medical practice and has entered many areas of our daily lives. Initially, vital signs monitoring was carried out in hospitals and clinics under strict medical supervision. Advances in microelectronics technology have reduced the cost of monitoring systems, making these technologies more popular and common in areas such as telemedicine, sports, fitness and health, and workplace safety, as well as in the automotive market that is increasingly focusing on autonomous driving. Although these extensions have been achieved, because these applications are highly related to health, they still maintain high quality standards.
Vital sign monitoring has gone beyond the scope of medical practice and has entered many areas of our daily lives. Initially, vital signs monitoring was carried out in hospitals and clinics under strict medical supervision. Advances in microelectronics technology have reduced the cost of monitoring systems, making these technologies more popular and common in areas such as telemedicine, sports, fitness and health, and workplace safety, as well as in the automotive market that is increasingly focusing on autonomous driving. Although these extensions have been achieved, because these applications are highly related to health, they still maintain high quality standards.
Vital sign monitoring includes measuring a series of physiological parameters that can show the health of the individual. Heart rate is one of the most common parameters, which can be detected by an electrocardiogram. The electrocardiogram can measure the frequency of the heartbeat, and most importantly, it can measure the changes in the heartbeat. Changes in heart rate are often caused by activity. During sleep or rest, the rhythm is slower, but it tends to speed up with factors such as physical activity, emotional reactions, stress or anxiety.
A heart rate outside the normal range may indicate conditions 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 blood oxygenation can be measured using a technique called photoplethysmography (PPG). Hypoxia may be related to the onset or disorder of the respiratory system. Other vital sign measurement factors that can reflect the individual’s physical condition include blood pressure, body temperature, and skin conductance response. Skin conductance response, also known as galvanic skin response, is closely related to the sympathetic nervous system, and in turn directly participates in mediating emotional behaviors. Measuring skin conductivity can reflect the patient’s stress, fatigue, mental state and emotional response. In addition, by measuring the percentage of body composition, lean body mass and fat body mass, as well as the degree of hydration and nutrition, the individual’s clinical status can be clearly shown. Finally, measuring movement and posture can provide useful information about the subject’s activity.
Figure 1. Signal chain for optical measurement
Techniques for measuring vital signs
In order to monitor vital signs such as heart rate, respiration, blood pressure and temperature, skin conductivity, and body composition, various sensors are required, and the solution must be compact, energy-efficient and reliable. Vital signs monitoring includes:
Measurements made with MEMS sensors
Figure 2. A complete bioelectricity and bioimpedance measurement system
Optical measurement goes beyond standard semiconductor technology. In order to perform this type of measurement, an optical measurement toolbox is required. Figure 1 shows a typical signal chain for optical measurement. A light source (usually an LED) needs to be used to generate a light signal, which may be composed of different wavelengths. Several wavelengths are combined to achieve higher measurement accuracy. It is also necessary to use a series of silicon or germanium sensors (photodiodes) to convert light signals into electrical signals, also known as photocurrents. The photodiode must have sufficient sensitivity and linearity when responding to the wavelength of the light source. After that, the photocurrent must be amplified and converted. Therefore, a high-performance, energy-saving, multi-channel analog front end is required to control the LED, amplify and filter the analog signal, and perform analog-to-digital conversion according to the required resolution and accuracy.
Optical system packaging also plays an important role. The package is not only a container, but also a system containing one or more optical windows, which can filter out and inject light, but will not produce excessive attenuation or reflection, thereby compromising the integrity of the signal. In order 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 that can create optical filters is also recommended for selecting the part of the spectrum required for the application and eliminating unwanted signals. Even in the sun, the application must be able to operate normally. If there is no optical filter, the size of the signal will saturate the analog chain and make the Electronic devices unable to work properly.
ADI provides a series of photodiodes and various analog front ends that can process the signals received from the photodiodes and control the LEDs. A complete optical system is also provided, which integrates LEDs, photodiodes and front-ends into one device, such as ADPD188.
Biopotential and bioimpedance measurement
Biopotential is an electrical signal caused by the effects of electrochemical activities in our body. Examples of biopotential measurement include electrocardiogram (ECG) and electroencephalogram. They check very low-amplitude signals in frequency bands with multiple interferences. Therefore, before processing the signal, it must be amplified and filtered. ECG biopotential measurement is widely used in vital signs monitoring, ADI provides a variety of components to perform this task, including AD8233, ADAS1000 chip series.
AD8233 is specially designed for wearable applications and can be combined with ADuCM3029 (Cortex®-M3 based system on chip (SoC)) to create a complete system. In addition, the ADAS1000 series is designed for high-end applications and has the advantages of low energy consumption and high performance. It is especially suitable for battery-powered portable devices with scalable power and noise (that is, the noise level can be reduced in proportion to the increase in power consumption) , Is an excellent integrated solution very suitable for clinical level applications.
Bioimpedance is another measurement method that can provide useful information about the state of the body. Impedance measurement provides information about 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 when the technique is applied vary depending on the frequency range used.
For example, when measuring skin impedance, low frequencies (up to 200 Hz) are used, while when measuring body composition, a fixed frequency of 50 kHz is usually used. Similarly, in order to measure hydration and correctly evaluate the fluid inside and outside the cell, different frequencies are used.
Although the technology may be different, a single-ended AD5940 can be used to implement all bioimpedance and impedance measurements. This device provides excitation signal and a complete impedance measurement chain, which can generate different frequencies to meet a variety of measurement requirements. In addition, AD5940 is dedicated to use with AD8233 to create a comprehensive bioimpedance and biopotential reading system, as shown in Figure 2. Other devices used for impedance measurement include the ADuCM35x series of SoC solutions. In addition to a dedicated analog front end, the solution also provides Cortex-M3 microcontrollers, memory, hardware accelerators and communication peripherals for electrochemical sensors and biosensors.
Motion measurement using MEMS sensors
Because MEMS sensors can detect the acceleration of gravity, they can be used to detect activities and abnormalities, such as unstable gait, falls or concussions, and even monitor the subject’s posture while resting. In addition, MEMS sensors can also be used as a supplement to optical sensors, as the latter are susceptible to movement artifacts; when this happens, the information provided by the accelerometer can be used for correction. ADXL362 is one of the popular devices in the medical field, and it is also the three-axis accelerometer with the lowest energy consumption on the market. It has a programmable measuring range from 2 g to 8 g and a digital output.
Figure 3. ADPD4000 is used to implement photoelectric, bioelectric potential, bioimpedance, and temperature measurement
ADPD4000: General purpose analog front end
Wearable devices currently available on the market (such as smart bracelets and smart watches) provide multiple vital sign monitoring functions. The most common of these are heart rate monitors, pedometers and calorie counters. In addition, blood pressure and body temperature, electrical skin activity, changes in blood volume (through photoplethysmography), and other indicators are often measured. As the number of monitoring options increases, so does the demand for highly integrated electronic components. ADPD4000 adopts extremely flexible architecture, designed to help designers meet this demand. In addition to providing biopotential and bioimpedance readings, it can also manage photoelectric measurement front ends, guide LEDs, and read photodiodes. ADPD4000 is equipped with a temperature sensor for compensation and a switch matrix, which can guide the required output and acquisition signals, whether it is a single-ended signal or a differential voltage signal. The output can be selected, it can be a single-ended output or a 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 zones, each dedicated to processing a specific sensor. Figure 3 summarizes the key features of the ADPD4000 in several typical applications.
With the advancement of science and technology, vital signs monitoring has become more and more common in all walks of life and in our daily lives. Whether for treatment or prevention, this health-related solution requires reliable and effective technology. Those who design vital signs monitoring systems will be able to find a series of solutions in ADI’s extensive product series dedicated to signal processing to meet the design challenges they face.