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When Every μA Counts! by Jan-Hein Broeders, Analog Devices Measuring multiple parameters, accurate read- ings, and having a long battery lifetime—these are the most critical parameters for a wearable health device. Since the first pedometer came on the market 10 years ago, a lot has been changed. Initially, the measurement was focused on just step counting. Studies have been done for decennia with the outcome that 10,000 steps a day would give a good balance on calorie intake vs. the amount of calories burned. In the meantime, additional functions and features have been added to the wearable device such as measuring heart rate, heart rate variability, body temperature, and skin conductance. The wearable device, which initially was meant for sports and wellness purposes, is now moving slowly toward a more medical market. With this transition, we have to rely more on the accuracy of the measurements and battery lifetime. The longer the device runs from a single battery charge, the easier it will be to get it adopted by the user. In this article, a new generation of products for wearable health devices is described, including tips and tricks on how they can make your system more reliable and power efficient. PPG FOR HEART RATE MEASUREMENT When it comes to our health, one of the most important organs in our body is the heart. It can be seen as the engine of our human sys- tem. Without a well-performing heart, we can face serious health issues. Monitoring the heart function for that reason is a key priority. There are many good reasons for checking our heart rate that go beyond the number of beats per minute. Besides that, we can retrieve a signifi- cant amount of additional information 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 is not good, and neither is a fast changing heart rate, which could be an indicator of a cardiac disease such as atrial fibrillation. Besides monitoring heart frequency, there is another parameter called heart rate variability (HRV). When people are relaxed, their heart won't beat with a fixed number of beats per minute, but you 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 people get stressed or get a startled response, the adrenaline level in the body goes up and the heart starts pumping with a very monotonic frequency. For that reason the parameter HRV is important to monitor. The classical way for retrieving cardiac signals is by biopotential measurement with an electro- cardiogram (ECG); however, this is not easy to integrate in a wearable device. A trend for measuring heart rate, other than bio- potential, is by making use of an optical principle. This technology has existed for quite some time and is called photoplethysmogram (PPG). PPG 4 NOVEMBER 2022 | www.eetimes.eu COVER STORY