Accurate Heart-Rate Monitoring for Wearables

For many designers, developing a good optical heart-rate monitoring solution for wrist-based wearables can be a challenge. It’s a complex endeavor, after all, requiring optical and mechanical expertise, low-power electrical design, and algorithm capabilities. Many companies have ideas for great new products, but not all of them have all of the resources needed to bring their ideas to fruition. At the same time, given consumers’ growing appetite for comfortable and convenient ways to keep tabs on key vital signs, there are market opportunities waiting to be seized.

We now see on the market heart-rate monitoring capabilities in form factors such as chest straps, in-ear instruments, wrist-based wearables, rings, and even clothing. Chest straps typically use an electrical pulse to read the heart rate, while some of the other options rely on optical technology. Electrical technology tends to be the more accurate of the two methods, but optical technology is becoming increasingly precise.

Optical sensors utilize light to support a process called photoplethysmography (PPG), where light interrogates a section of tissue. As it passes through matter, light gets reflected, dispersed, absorbed, scattered, or altered in some way. In the cardiovascular system, when the cardiovascular pulse wave moves from the heart and propagates through the body, this activity causes the arteries and arterioles in the subcutaneous tissue to distend periodically. An optical heart-rate monitoring system uses PPG to generate an optical measurement of the volumetric change of blood in the tissue during the cardiac cycle. Light received through the tissue corresponds with blood volume variation. The quality of PPG signals detected is, however, affected by the optical properties of the skin.

Integrating optical technology into products like in-ear sports headphones and smartwatches results in the comfort and convenience that many consumers seek. Given the creative minds out there, it’s exciting to imagine the types of form factors for healthcare wearables that might emerge in the future.

A big influence on designs, of course, is the fact that obtaining an accurate heart-rate reading is more challenging from some areas of the body than others. In-ear optical measurements tend to be more precise because the ear has blood perfusion. The wrist, on the other hand, has relatively lower perfusion and is prone to high motion, especially during activity, creating more noise to factor in when processing and computing the readings. That’s why motion compensation is an important consideration when developing heart-rate monitoring applications. Also important are signal-to-noise ratio (SNR), ambient light cancellation, and the end device’s overall power consumption.

Turn Your Ideas Into Market-Ready Products Quickly

To ease the design process for wearables that monitor vital signs, Maxim has launched two new wearable platforms for health and fitness applications. MAX-HEALTH-BAND is an evaluation and development platform that uses optical technology to monitor heart rate and an accelerometer to measure activity (see Figure 1). An important benefit of this platform is that it provides raw data from the sensor measurements that currently available wearable products are unable to offer. MAX-HEALTH-BAND streams raw data from sensors or processes raw data to output heart rate, heart-rate variability, step count, activity classification, and calorie consumption. Available for white-box license, the platform can eliminate up to six months for developing highly accurate, small, and power-efficient wearable health and fitness applications. MAX-HEALTH-BAND is based on small, power-efficient ICs including:

• The MAX86140 ultra-low-power optical pulse-oximeter/heart-rate sensor, which features three programmable high-current LED drivers that can be configured to drive up to six LEDs. On the receiver side, the device has a single optical readout channel. It also offers a low-noise signal conditioning analog front-end (AFE), including a 19-bit analog-to-digital converter (ADC), an ambient light cancellation circuit, and a picket-fence detect-and-replace algorithm. The picket-fence algorithm provides the device user with a consistent accuracy level in a variety of lighting conditions. For example, as a jogger wearing a fitness tracker runs through a tree-lined park with distributed light, an algorithm detects and cancels out the change in ambient light to eliminate any effect that the switching between shadows and bright light might have on heart-rate detection
• The MAX20303 wearable power management IC (PMIC), which features an eccentric rotating mass (ERM)/linear resonant actuator (LRA) haptic driver with automatic resonance tracking, micro quiescent current boost and buck regulators, a linear lithium-ion battery charger, micro quiescent current low-dropout (LDO) regulators, and an optional fuel gauge
• Maxim's motion-compensated algorithms, which extract useful data based on PPG signals

MAX-HEALTH-BAND makes it easier to extract highly accurate vital signs and raw data from health sensors for wearable designs.

MAX-ECG-MONITOR is an evaluation and development platform for monitoring clinical-grade ECG and heart rate, available in a wet electrode patch for clinical applications and a chest strap for fitness applications (see Figure 2). The platform features the MAX30003 ultra-low-power, clinical-grade, integrated biopotential analog front end (AFE), which provides ECG waveforms and heart-rate detection. Unlike currently available single-spot measurement tools, MAX-ECG-MONITOR provides the continuous measurements that can be useful for trend- or predictive-type applications. Using the platform, you can run your own fitness or medical ECG-based applications and algorithms. As part of the Movesense ecosystem, the platform runs an open API, so you can create unique in-device apps—with long battery life—for different ECG-based use cases that show heart-rate signals at rest or during high-motion activity. Its built-in heart-rate detection includes an interrupt feature that eliminates the need to run a heart-rate algorithm on a microcontroller, resulting in robust R-R detection in a high-motion environment at extremely low power. To extend battery life, the platform operates at 85µW at 1.1V supply voltage and features configurable interrupts that allow the microcontroller to wake only on every heart beat to reduce overall system power.

MAX-ECG-MONITOR analyzes data and accurately tracks heart signals as well as user status signals such as temperature and motion.

Enabling a Healthier World

Wearable health and fitness applications have the potential to enhance well-being in ways that haven’t been possible before. By giving device users and healthcare professionals access to insightful data, these technologies can help us be more proactive, take preventive measures, and react more promptly to red flags. Platforms such as MAX-HEALTH-BAND and MAX-ECG-MONITOR are just a part of Maxim’s overall efforts to enable a healthier world.