Accurate Vital Signs

How to monitoring the state of health with accurate vital signs?

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 pervasive in areas such as telemedicine, sports, fitness and health, workplace safety, and more. While these extensions are implemented, high quality standards are still maintained because these applications are highly health-related.

Accurate vital signs

Visual 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 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 nutrition levels, an individual’s clinical status can be clearly displayed. Finally, measuring movement and posture can provide useful information about the subject’s activity.

Technologies 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:

Optical measurement

Biopotential measurement

Impedance measurement

Measurements with MEMS Sensors

Optical measurement

Optical measurements go beyond standard semiconductor technology. To perform this type of measurement, an optical measurement toolbox is required. 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.

It also requires the use of a series of silicon or germanium sensors (photodiodes) 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.

Signal chain for optical measurements to accurate vital signs

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 components, including LEDs, photodiodes, analog and digital processing chips.

Finally, a coating technique that creates an optical filter that selects the part of the spectrum needed for an application and removes unwanted signals is often required. 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 the LED, photodiode, and front end into one device, such as the ADPD1081.

Biopotential and Bioimpedance Measurements to accurate vital signs

Biopotential is an electrical signal caused by the effects of electrochemical activity in our body. Examples of biopotential measurements include electrocardiogram (ECG) and electroencephalogram (EEG). 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 Analog Devices offers several components to perform this task, including the AD8233, and ADAS1000 chip family.

The AD8233 is low power and suitable for portable devices and 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 is scalable (that is, the noise level can be reduced proportionally as power consumption increases), making it an excellent integrated solution for ECG systems.

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, a single-ended AD5940 can be used to perform all bioimpedance and impedance measurements. 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 measurements 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.

Complete Bioelectricity and Bioimpedance Measurement System

Motion Measurement Using MEMS Sensors to accurate vital signs

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 a subject while it is 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 make corrections. 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.

ADPD4000: Universal Analog Front End

Currently available wearable devices on the market, such as smart bracelets and smart watches, provide 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, 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, whether 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.

The ADPD4000 is used to implement optoelectronic, biopotential, bioimpedance, and temperature measurements.

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.

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