The importance of physiological vital signs as an indicator of human health has long been understood by medical professionals, but the current covid-19 pandemic has also raised public awareness of its importance. Unfortunately, most people who find themselves undergoing continuous vital signs monitoring may already be in a clinical environment where they are treated for acute diseases. The future model of health care is not to use vital signs as an indicator of the effectiveness of disease treatment and patient rehabilitation, but to use continuous and remote vital signs monitoring as a tool to identify potential indicators of disease onset, allowing clinicians to intervene in the earliest opportunity before the development of serious diseases in the following ways.

Although many health and fitness wearable devices include vital sign measurement functions, the integrity of their readings may be questioned for a variety of reasons, including the quality of the sensors used (most are not clinical), their installation location, and the quality of the sensors. Body contact when wearing. Although these devices are sufficient to meet the desire of non health professionals to use a convenient and comfortable wearable device for random self-observation, they do not meet the requirements of trained medical professionals to correctly evaluate personal health and make wise diagnosis. On the other hand, the current devices used to provide clinical vital signs observation at long intervals may be bulky and uncomfortable, and have different degrees of portability. 2) heart rate (HR), electrocardiogram (ECG) and respiratory rate (RR) – and consider the best sensor type to provide clinical level readings of each sensor. Before the final introduction of highly integrated healthcare sensor AFE, we discussed the shortcomings of current measurement solutions. The sensor combines the functions of three independent biosensors into one package, and it is possible to bring the concept of wearable, disposable vital signs health patches closer

Blood oxygen saturation

The blood oxygen saturation level of healthy individuals is usually about 95-100%. However, a SpO 2 level of 93% or less may indicate that an individual is experiencing respiratory distress – a common symptom in patients with covid-19, for example – making it an important vital sign regularly monitored by medical professionals. Photoelectric plethysmography (PPG) is an optical measurement technology, which uses multiple LED transmitters to illuminate the blood vessels under the skin surface, and uses photodiode receivers to detect the reflected light signal, so that SpO 2 can be calculated. Although it has become a common feature of many wrist wearable devices, PPG optical signals are vulnerable to motion artifacts and transient changes in ambient lighting, which may lead to false readings, which means that these devices do not provide clinical level measurements. In a clinical setting, SpO 2 is measured using a pulse oximeter (Fig. 2) worn on the fingers, usually continuously connected to the fingers of stationary patients. Although there are battery powered portable versions, they are only suitable for intermittent measurements.

Heart rate and electrocardiogram

Healthy heart rate (HR) is generally considered to be in the range of 60-100 beats per minute, but the time interval between individual heartbeats is not constant. It is often called heart rate variability (HRV), which means that the heart rate is the average value measured over several heartbeat cycles. In healthy individuals, the heart rate and pulse rate are almost the same, because with each contraction of the myocardium, blood is pumped throughout the body. However, some serious heart diseases may lead to different heart rate and pulse rate. For example, in the case of arrhythmias such as atrial fibrillation (AFib), not every muscle contraction in the heart will pump blood to the whole body – on the contrary, blood will accumulate in the chamber of the heart itself, which may be life-threatening. Atrial fibrillation may be difficult to detect because it sometimes occurs intermittently and only at short intervals. According to the data of the World Health Organization, a quarter of strokes in people over the age of 40 are caused by AFib, which proves the importance of being able to detect and treat this disease. Since PPG sensors perform optical measurements under the assumption that HR and pulse rate are the same, they cannot be relied on to detect AFib. This requires continuous recording of the electrical activity of the heart over a long time interval – the graphical representation of cardiac electrical signals is called electrocardiogram (ECG). Holter monitor (Figure 3) is the most common clinical portable device for this purpose. Although these electrodes are used less than static ECG monitors used in clinical settings,

Respiratory rate

12-20 breaths per minute is the RR expected by most healthy people. The RR rate of more than 30 breaths per minute may be an indicator of respiratory distress caused by fever or other causes. Although some wearable device solutions use accelerometers or PPG technology to infer RR, the clinical level RR measurement is performed using the information contained in the ECG signal or using a biological impedance (BioZ) sensor, which uses two sensors to characterize the electrical impedance of the skin. One or more electrodes connected to the patient’s body. Although the ECG function approved by FDA can be used in some high-end health and fitness wearable devices, bio impedance sensing is a function that is usually not provided, because it needs to include a separate BioZ sensor IC. In addition to RR, the BioZ sensor also supports bioelectrical impedance analysis (BIA) and bioelectrical impedance spectroscopy (BIS), both of which are used to measure the composition levels of muscle, fat and water in the body. The BioZ sensor also supports impedance electrocardiogram (ICG) and is used to measure skin electrical response (GSR), which may be a useful indicator of stress.

Three in one sensor solution

Figure 4 shows the functional block diagram of AFE IC for clinical vital signs, which integrates the functions of three independent sensors (PPG, ECG and BioZ) into one package.

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Figure 4: max86178 ultra low power, three in one clinical vital signs AFE (source: analog devices)

Its dual channel PPG optical data acquisition system supports up to 6 LEDs and 4 photodiode inputs, which can be programmed through two high current 8-bit LED drivers. The receiving path has two low-noise, high-resolution readout channels, each of which includes an independent 20 bit ADC and ambient light cancellation circuit, providing more than 90dB of environmental suppression at 120Hz. The SNR of PPG channel is up to 113dB, supporting SP0 2 measurement of only 16 µ a.

The ECG channel is a complete signal chain that provides all the key functions required to collect high-quality ECG data, such as flexible gain, critical filtering, low noise, high input impedance, and multiple lead offset options. Other functions, such as rapid recovery, AC and DC lead detection, ultra-low power lead detection and right leg drive, can achieve robust operation in demanding applications, such as wrist mounted devices with dry electrodes. The analog signal chain drives an 18 Bit sigma delta ADC with a wide range of user selected output sampling rates.

The BioZ receiving channel has EMI filtering and extensive calibration functions. The BioZ receive channel also has high input impedance, low noise, programmable gain, low-pass and high pass filter options, and high-resolution ADC. There are several modes of generating input stimulation: balanced square wave source / perfusion current, sine wave current, and sine wave and square wave voltage stimulation. There are a variety of stimulation amplitude and frequency to choose from. It also supports BIA, BIS, ICG, and GSR applications.

FIFO timing data allows synchronization of all three sensor channels. This AFE IC uses a 7 x 749 bump wafer level package (WLP) with a package size of only 2.6mm x 2.8mm, which is very suitable for designing a clinical wearable Chest Patch (Figure 5).

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Figure 5: Chest Patch with two wet electrodes supporting BIA and continuous rr/icg, ECG, SpO2 AFE (source: analog devices)

Figure 6 illustrates how this AFE is designed as a wrist wearable device to provide on-demand BIA and ECG with continuous HR, SpO 2, and eda/gsr.

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Figure 6: wrist device with four dry electrodes, supporting BIA and ECG, with continuous HR, SpO2 and GSR AFE (source: analog devices)


SPO 2, HR, ECG and RR are important vital sign measurements used by healthcare professionals for diagnostic purposes. Continuous vital sign monitoring using wearable devices will be a key component of the future health care model, which can predict disease onset before symptoms appear. Many currently available vital signs monitors produce measurement results that cannot be used by healthcare professionals because the sensors they use are not clinical grade, while others simply do not have the ability to accurately measure RR because they do not include BioZ sensors. In this design solution, we show an IC that integrates three clinical level sensors (PPG, ECG and BioZ) into one package, and show how to design it into chest and wrist wearable devices to measure SpO 2, HR, ECG and RR. At the same time, we also provide other useful health-related functions, including BIA, BIS, GSR and ICG. In addition to being used in clinical wearable devices, the IC is also very suitable for integration into smart clothing to provide the type of information required by high-performance athletes.

Reviewed and edited by Huang Haoyu

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