Optical heart rate measurement at the earbud

November 18, 2019 //By Christoph Kämmerer
heart rate
Advancements in sensor technology have transformed how and where people diagnose their vitals and health. Portable, noninvasive measurement techniques permit fast and simple measurements that can be performed while we go about our daily lives. But although this diagnostic technology has become very popular in the fitness industry, there were limits to its accuracy that we have only recently overcome.

Fitness trackers enable the measurement of heart rate and other vitals that can help users set exercise routines. They often have built-in motion sensors that can detect movement patterns to help distinguish between walking, running, and swimming, which allows them to work as pedometers.

For comfort and convenience in everyday life, the measurements are typically made on the wrist because the sensors can be housed in accessories such as watches, jewelry, and wrist straps. However, this position is not optimal for measurement quality. Heart rate detection is limited by motion artifacts and difficult because relatively high muscle mass limits access to arteries.

In contrast, the ear is much more suitable for optical heart rate measurements. The earlobe is already used by medical experts for the measurement of blood oxygen levels. However, up to now, this has not been fully exploited on a consumer level because ear-based measurement devices have limited space and require a large battery due to very high power consumption. But with the introduction of highly integrated, lower power consumption chips, Analog Devices has developed a solution that overcomes these problems. It is now possible to integrate a functioning vital sign measuring device into typical in-ear headphones. The improved responsivity opens up complete new fields of applications and possibilities.

The underlying measurement method is of an optical nature. Short pulse signals from up to three LEDs are used for the measurement. The LED current can be up to 370 mA at a minimum pulse width of 1 µs. The optimum wavelength of the LED is selected according to the measuring position and the measurement method. Whereas only superficial arteries can be measured on the wrist and hence green light is selected here, infrared light and a greater penetration depth as well as a higher SNR can be used on the ear. A photodiode, whose detector area is directly related to its responsivity, measures the reflected light. It thus measures both the signal and the background noise. The downstream analog front end provides for a higher SNR. It functions as a signal filter and converts the detected current into a voltage and thus into a digital format. The algorithm includes, besides the reflection measurement, a correction for filtering out motion artifacts by means of an accelerometer.

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