Monitor your vital signs? Let the professionals do it.

Vital sign monitoring involves measuring a range of physiological parameters that can indicate an individual's health status. One of the most common parameters is heart rate, which can be detected through an electrocardiogram, which measures the frequency of heartbeats and, most importantly, the variability of heartbeats. Heart rate changes are often caused by activity. The rhythm is slower during sleep or rest, but 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). Respiration is another key vital sign. The oxygenation level of the blood can be measured using a technology called photoplethysmography (PPG). Hypoxia may be associated with an illness or disorder that affects the respiratory system. Other vital sign measurements that can reflect an 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, which in turn is directly involved in mediating emotional behavior. Measuring skin conductivity can reflect a patient's stress, fatigue, mental state, and emotional responses. In addition, measuring body composition, the percentage of lean body mass and fat body mass, as well as hydration and nutrition levels can provide a clear picture of an individual's clinical status. Finally, measuring movement and posture can provide useful information about the subject's activities.

To monitor vital signs such as heart rate, respiration, blood pressure and temperature, skin conductivity and body composition, a variety of sensors are needed and the solution must be compact, energy-efficient and reliable. Vital sign monitoring includes:

• Optical Measurement

• Biopotential measurement

• Impedance measurement

• Measurements using MEMS sensors

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 optical signal, which may consist of different wavelengths. Combining several wavelengths can achieve higher measurement accuracy. A series of silicon or germanium sensors (photodiodes) are also required to convert the optical signal into an electrical signal, also called photocurrent. The photodiode must have sufficient sensitivity and linearity when responding to the wavelength of the light source. The photocurrent must then be amplified and converted, so a high-performance, energy-efficient, multi-channel analog front end is required to control the LED, amplify and filter the analog signal, and perform the analog-to-digital conversion with the required resolution and accuracy.

The optical system package also plays an important role. The package is not only a container, but also a system containing one or more optical windows that can filter the outgoing and incoming light without excessive attenuation or reflection that would compromise the integrity of the signal. 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 technology that can create optical filters is usually required to select the part of the spectrum required for the application and eliminate unwanted signals. The application must function properly even in sunlight. Without optical filters, the size of the signal would saturate the analog chain and prevent the electronics from functioning properly.

ADI offers a range of photodiodes and various analog front ends capable of processing the signals received from the photodiodes and controlling the LEDs. Complete optical systems are also available that integrate the LED, photodiode, and front end into one device, such as the ADPD1081.

Biopotential is an electrical signal caused by the effects of electrochemical activity in our body. Examples of biopotential measurements include electrocardiogram (ECG) and electroencephalogram. They detect very low amplitude signals in frequency bands where there are many interferences. Therefore, before the signal can be processed, it must be amplified and filtered. ECG biopotential measurement is widely used for vital signs monitoring, and ADI offers several components to perform this task, including the AD8233, and ADAS1000 chip series.

The AD8233 has low power consumption and is suitable for portable devices. It can be combined with the ADuCM3029 (a Cortex®-M3 based system-on-chip (SoC)) to create a complete system. In addition, the ADAS1000 family is designed for high-end applications, with high performance features, power and noise scalability (that is, the noise level can be reduced proportionally with the increase in power consumption), making it an excellent integrated solution for ECG systems.

Bioimpedance is another measurement method that can provide useful information about the body's state. Impedance measurements provide information about electrochemical activity, body composition, and hydration status. Measuring each parameter requires a different measurement technique. The number of electrodes required for each measurement technique, as well as the point in time at which the technique is applied, varies 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 often used. Similarly, to measure hydration and correctly assess the amount of fluid inside and outside the cells, different frequencies are used.

Although the technology may be different, all bioimpedance and impedance measurements can be implemented using a single-ended AD5940. This device provides the excitation signal and the complete impedance measurement chain, generating different frequencies to meet a variety of measurement requirements. In addition, the AD5940 is designed to be used in conjunction with the AD8233 to create a comprehensive bioimpedance and biopotential reading system, as shown in Figure 2. Other devices used for impedance measurement include the ADuCM35x family of SoC solutions. In addition to a dedicated analog front end, this solution also provides a Cortex-M3 microcontroller, memory, hardware accelerators, and communication peripherals for electrochemical sensors and biosensors.

Because MEMS sensors can detect gravitational acceleration, they can be used to detect activity and anomalies such as unstable gait, falls or concussions, or even monitor the posture of a subject while they are resting. MEMS sensors can also complement optical sensors, which are susceptible to motion artifacts; when this occurs, the information provided by the accelerometer can be used to correct for it. ADXL362 is one of the popular devices in the medical field and is the lowest power triaxial accelerometer on the market today. It has a programmable measurement range of 2 g to 8 g and a digital output.

Wearable devices currently available on the market, such as smart bracelets and smart watches, all offer a variety of vital sign monitoring functions. The most common of these are heart rate monitors, pedometers, and calorie counters. In addition, blood pressure and body temperature, skin electrode activity, changes in blood volume (via photoplethysmography), and other indicators are often measured. As the number of monitoring options increases, the demand for highly integrated electronic components continues to increase. The ADPD4000 has an extremely flexible architecture designed to help designers meet this demand. In addition to providing biopotential and bioimpedance readings, it can also manage the photoelectric measurement front end, guide LEDs, and read photodiodes. The ADPD4000 is equipped with a temperature sensor for compensation and a switch matrix that can guide the required output and obtain signals, whether single-ended or differential voltage signals. The output can be selected, either single-ended or differential, depending on the input requirements of the ADC to which the ADPD4000 is to be connected. 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 common in all walks of life and in our daily lives. Whether used for treatment or prevention, such health-related solutions require reliable and effective technology. Those designing vital sign monitoring systems will be able to find a range of solutions to their design challenges in ADI's extensive product portfolio dedicated to implementing signal processing.