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Measuring multiple parameters, accurate readings, and having a long battery lifetime: these are the most critical parameters for a wearable device that monitors cardiovascular health. Wearable devices that were initially meant for sports and wellness purposes are now being designed for medical purposes, measuring critical health parameters such as heart rate and heart rate variability (HRV). With this transition, the accuracy of the measurements and device's battery lifetime become more important design considerations. This article examines this new generation of wearable health devices, including how to make a system more reliable and more power efficient.

PPG for Heart Rate Measurement

Wearable devices that were initially meant for sports and wellness purposes are now being designed to measure critical health parameters such as heart rate and heart rate variability.

When it comes to health, one of the most important organs in the body is the heart. Without a well-performing heart and heartbeat, people can face serious health issues. For that reason, monitoring the heart function is a key priority. There are many good reasons for checking heart rate that go beyond the number of beats per minute. In addition, a significant amount of additional information can be gained from the behavior of the heart in terms of the frequency as function of activity. When more activity is asked from the body, the heart rate should go up to bring more nutritious- and oxygenated-blood to the cells. A continuous high heart rate or a fast-changing heart rate can be indicators of a cardiac disease such as atrial fibrillation.

The ADPD174 is an optical system in a single package.

Besides monitoring heart frequency, another parameter, monitoring HRV, is also important. When a person is relaxed, the heart won't beat with a fixed number of beats per minute, but it should experience a slight variation around the heart frequency, something in the range of 3 beats per minute. This variation is an indicator for being relaxed. At the moment that people get stressed or get a startled response, the adrenalin level in the body goes up and the heart starts pumping with a very monotonic frequency. For this reason, the HRV parameter is important to monitor.

The classical way for retrieving cardiac signals is by biopotential measurement with an electrocardiogram (ECG); however, this is not easy to integrate into wearable devices. A trend for measuring heart rate other than biopotential is by making use of an optical principle. This technology has existed for quite some time and is called photoplethysmogram (PPG). PPG technology mainly has been used in systems for measuring oxygen saturation in the blood (SPO2). To measure SPO2, two wavelengths of light are sent through a particular part of the body (usually the finger or earlobe), measuring the percentage of oxygenated hemoglobin versus the total amount of hemoglobin. This technology is also commonly used in wearable systems such as small wrist-worn devices, where unlike a biopotential measurement, it is possible to pick up the heart rate using a single measurement point. The ADPD174 from Analog Devices is an optical subsystem, which has been designed to support these applications (see Figure 1).

Reflective Versus Transmission

This graph indicates the required LED forward voltage versus LED current.

The SPO2 measurement is usually performed with a clip on the finger or earlobe. Light is sent through a part of the body and at the opposite site, the received signals are being measured by a photodiode. This transmission technique measures the amount of received light or light that is not absorbed. This principle is best in class in terms of signal performance versus the amount of power spent.

Integration of transmissive measurement, however, isn't an easy task in a wearable system where comfort is key. Therefore, reflective measurement is more commonly used in wearable systems. In a reflective optical system, light is sent into the surface of the tissue, whereupon a part is absorbed by the red blood cells, and the remaining light is reflected back to the tissue surface and measured by a photosensor. In a reflective system, the receive signals are up to 60 dB weaker, so it is important to pay attention to electrical and optical aspects of the transmit and receive signal chain.

Electronic and Mechanical Challenges

During a heartbeat, the flow and volume of blood is changing, resulting in scattering of the amount of reflected light received. The wavelength of the light that is used for measuring the PPG signals can vary depending on a number of factors, the first being the type of measurement. The discussion in this article is limited to the measurement of just heart rate and variation. For this measurement, the required wavelength depends not only on the location on the body where the measurement is being taken, but also on the relative perfusion level, temperature of the tissue, and color tone of the tissue. Arteries are not located on top of the wrist, so for wrist-worn devices, pulsatile components are picked up from veins and capillaries just under the skin surface. The wavelength of a green light in these applications is most effective. Where there is sufficient blood flow, like the upper arm, temple, or ear canal, red or infrared wavelengths will be more effective as they penetrate deeper into the tissue. Because red or infrared LEDs require a lower forward voltage, they are also suitable for wearable applications where battery power and size is always an issue. For applications where coin cell batteries are used, these LEDs can be driven directly from the battery voltage.

Unfortunately, green LEDs need a higher forward voltage that requires an additional boost converter, and so these have a negative impact on a system's overall current consumption. Figure 2 shows the required forward voltage for different LED colors as function of the current. If Green LEDs are still required, the ADP2503 buck/boost converter could be of help to support a higher LED forward voltage up to max 5.5 V, operated from an input voltage, which can go as low as 2.3 V.

When trade-offs such as sensor-position and LED color are being made, the next step is to select the most appropriate optical solution. Many types of analog front ends (AFEs) — both discrete built or fully integrated — are available, but there are also many photosensors and LEDs. To minimize design efforts and to shorten time to market, ADI built ADPD174, a fully integrated optical subsystem for reflective optical measurement (see Figure 3). The module is 6.5 × 2.8 mm, which makes it suitable for wearable systems.

The module is built around a big photodiode, two green LEDs, and an IR LED. The onboard mixed signal application-specific integrated circuit (ASIC) includes an analog signal processing block, a SAR-type ADC, a digital signal processing block, an I2C communication interface, and three free programmable LED current sources. The system drives the LEDs and measures the corresponding optical return signal with its 1.2 mm2 photodiode. The biggest challenge for measuring PPG with a wearable device is overcoming interferers like ambient light and artifacts generated by motion.

Ambient light can greatly influence the measurement results. Sunlight is not too difficult to reject, but light from fluorescent and energy-saving lamps, which include AC components, are difficult to cancel. The ADPD174 optical module has a two-stage ambient light rejection function. After the photosensor and input amplifier stage, a band-pass filter is integrated, followed by a synchronous demodulator, to offer best-in-class rejection for ambient light and interference from DC up to 100 kHz. The ADC has a resolution of 14 bits and up to 255 pulse values can be summed to get a 20-bit measurement. Additional resolution up to 27 bits can be achieved by accumulating multiple samples.

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