The population is aging and more people need healthcare support, which is having a big impact on the overall cost of medical care. As a result, authorities and health insurance companies are putting more emphasis on prevention, health awareness, and lifestyle including more interest in monitoring certain vital body parameters. This is why companies in the smart watch and health watch business have seen their revenue grow over the past few years. Buying a health watch and monitoring certain body parameters over a period of time gets the user familiar with these numbers and uses them to adapt day-to-day life for improvement.
This article focuses on Analog Devices’ wearable VSM platform and the sensor technologies used (Figure 1). ADI is not a manufacturer of final products; however, this platform has been designed as a reference to help the electronic designer and system architect speed up the development process while designing smarter, more accurate wearable devices for the professional and medical market.
What Are We Measuring? How and Where?
A broad range of vital parameters can be measured with a wearable device. Depending on the overall objective, certain parameters are more important to measure than others. The location of the wearable device on the body has a big impact on what can be measured and what cannot. The most obvious location is the wrist. We are accustomed to wearing a device on our wrist, which is why so many smart watches and wrist-worn devices are on the market. Besides measuring on the wrist, the head is another good location for wearables; for example, headphones and earbuds are offered in different styles that contain embedded sensors to measure parameters such as heart rate, oxygen saturation, and temperature. The third location for wearables on the body is the chest. First-generation heart rate monitors were designed around a chest strap and this biopotential measurement method is still a very accurate technique. Today, we tend to prefer a chest patch, as the strap is not very comfortable to wear. Several manufacturers are involved in the design of smart patches to monitor vital parameters.
Depending on body location, we are not just faced with the choice of which parameters can be measured but what technology should be used. For heart rate measurement, biopotential measurement is one of the oldest technologies. Signals are strong and easy to retrieve from the body by utilizing two or more electrodes. For this approach, integration of the circuitry in a chest strap or headphones is perfect; however, measuring biopotential signals at a single point like the wrist is nearly impossible. You need to measure across the heart, where these electric signals are being generated. For single-spot measurement, optical technology is more appropriate. Light is sent into the tissue and its reflection, as a result of blood flow in the arteries, is captured and measured. From this optically received signal, beat-to-beat information can be retrieved. This technology sounds rather straightforward; however, there are several challenges and influencers that can make the design difficult, such as motion and ambient light.
Analog Devices’ GEN II wearable device reference platform has most of the previously described technologies onboard. The device is designed to be worn on the wrist but the soft belt can be removed and the device attached to the chest to use it as a smart patch. The device includes technology to support biopotential measurement, optical heart rate measurement, bioimpedance measurement, motion tracking, and temperature measurement — all integrated in a tiny, battery-operated device.
The goal for a system like this is to evaluate various sensing technologies and to measure, in an easy way, several vital parameters on the body. These measurements can be stored into flash memory or sent over a BLE wireless connection to a smart device. Since the measurements are done simultaneously, it also can help to find correlation among several parameters. Biomedical engineers, algorithm providers, and entrepreneurs continuously are looking for new technologies, applications, and use cases to detect diseases at an earlier stage in order to minimize negative effects or damage to the body that might occur at a later stage.
Sensors Make the Device
The device is designed around two PCBs that are stacked as a sandwich. The main board contains a low-power processor, a BLE radio, and the complete power management section including battery conditioning and charging. A second board supports all sensing technologies. The optical system for PPG (photoplethysmogram) measurement is built around the ADPD107, ADI’s second-generation optical analog front end. The block diagram is shown in Figure 2.
The analog front end operates as a complete transceiver, driving the LEDs in the system and measuring the return signal from the photodiode(s). The objective is to measure photocurrent that is as high as possible for a given amount of LED current spent (current transfer ratio). The input receive signal chain is designed around a configurable transimpedance amplifier where the gain can be programmed in four steps up to 200k. The second stage is responsible for ambient light rejection. Ambient light interferers are a big issue, especially when the light is modulated, as with solid-state lighting systems with LEDs or energy-saving lamps. The ambient light rejection block contains a bandpass filter followed by an integrator to support synchronous demodulation. This is a key function and rejects external light interferers very effectively. When the ambient light rejection stage is not needed, this block can be bypassed completely.
The optical system makes use of light pulses. There are three LED current sources that are fully programmable. The maximum LED currents are programmable and can be as high as 370 mA. Also, the pulse width is programmable and can be as narrow as 1 μs; however, for a good signal response, pulse width should be around 2 μs to 3 μs. Usually a series of LED pulses is given while the analog-to-digital converter is sampling the photodiode receive signals related to the pulsed LED transmit pulses. The digital engine is able to average multiple samples to increase the overall effective number of bits.
Along with the optical system, mechanical design also has a major impact on overall performance. In this GEN II device, the optical components have been selected as discrete devices. This provides flexibility on the photodiode selections and the LED wavelengths as well as mechanical constraints such as spacing between LEDs and photodiodes. The GEN II device supports two green LEDs, one red LED, and one infrared LED. For those without experience in designing optical systems, it might be easier to integrate a complete optical module.
There are different options in terms of the number of photodiodes, their sizes, and the selection of LED wavelengths. The latest modules have been developed in such a way that they show a great optical performance even when they are mounted behind a plastic window. The first generation required a split window to reject internal light pollution, which can be seen as optical crosstalk. A split window helped to reduce dc offsets from light coming directly from the LEDs without penetration into the body. Such a split window is not easy to integrate, nor is it attractive from a cost point of view. The latest families have been improved substantially and even with just one complete window, the ILP effects have been reduced to almost zero.
Biopotential measurement is supported by two individual AD8233 analog front ends. The AD8233 is a single-lead ECG front end with embedded right leg drive (RLD) capability, and has been designed to extract, amplify, and filter small biopotential signals in the presence of noisy environments. Focus applications for this component are wearable devices, portable home care systems, and exercise equipment. The AD8233 operates in a dc coupled configuration. The input stage is divided over two gain stages. The first stage, with limited gain, is followed by a second-order, high-pass filter and a second gain stage. The total gain of this input block is 100 V/V, which includes the subtraction of the offset as a result of the electrode half-cell potential. The second stage is combined with a third-order, low-pass filter. It is second-order Sallen Key working in unity followed by an additional low-pass filter. The objective of this filter is to reject all EMG-related signals coming from muscle activity.
The operating frequency of the biopotential front end depends on the use case. For a normal heart rate monitor, where just QRS detection is needed, the operating frequency range is much less compared to an ECG monitor where more information is required, such as timing and amplitude data from the P-wave vs. QRS-Complex vs. T-wave. The band of interest can be configured by external resistors and capacitors. To support flexibility, the GEN II wearable device has the ECG front end connected to the embedded electrodes, configured in a sports bandwidth, supporting a band of interest from 7 Hz to 25 Hz. The second AD8233 that can be operated in combination with external electrodes is configured to monitor signals from 0.5 Hz up to 40 Hz. In principle, nearly any bandwidth can be selected; however, this requires modifications of the hardware by changing R and C settings.
Depending on the required accuracy, output can be sent to the 12-bit successive approximation register (SAR) ADC embedded in the Cortex®-M3 processor on the sensor board, or digitization can be done by the standalone 16-bit SAR ADC. Tradeoffs can be made and depend on either accuracy or battery lifetime.
At the back side of the device are two electrodes. These have a double function: in addition to ECG measurement, these also can be used for electrodermal activity (EDA) or galvanic skin response (GSR). This is related to the conductivity of the skin, which is momentarily changed by emotion, coming from either an internal or external stimulus — skin impedance changes, for instance, as a result of stress or epilepsy. The GEN II device is able to detect this minute change in conductivity. The system is making use of an ac excitation signal that is applied over the two dry electrodes. Wet electrodes can be used as well and will be better; however, this device is just making use of two embedded dry stainless steel electrodes. The main advantage of using an ac excitation signal is that this will not polarize the electrodes.
The receive signal chain represents a transimpedance amplifier, followed by the 16-bit, SAR ADC. The ADC sampling rate is much higher than the excitation rate for performance reasons. The ADC output is followed by a discrete Fourier transform (DFT) engine, running on the ADuCM3029 processor, to represent the complex impedance. The measurement principle described above is capable of measuring skin impedance or skin conductance at a high signal-to-noise ratio and a very good suppression of 50 Hz/60 Hz environmental noise. The circuit around this measurement principle is completely built with discrete components. The main reason for this design decision is flexibility and accuracy at a rather low power dissipation.