Accurate heart rate monitoring for wearable devices

Developing a good optical heart rate monitoring solution for wrist-based wearables can be a challenge for many designers. After all, it’s a complex endeavor that requires optical and mechanical expertise, low-power electrical design, and algorithmic capabilities. Many companies have ideas for great new products, but not all have all the resources needed to make their ideas a reality. At the same time, there are market opportunities waiting to be seized, given the growing consumer demand for comfortable and convenient ways to keep tabs on key vital signs.

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

Optical sensors use light to support a process called photoplethysmography (PPG), in which light interrogates a portion of tissue. As light travels through matter, it is reflected, scattered, absorbed, diffused, or altered in some way. In the cardiovascular system, as the cardiovascular pulse wave moves from the heart and propagates through the body, this activity causes cyclical dilation of arteries and arterioles in the subcutaneous tissue. Optical heart rate monitoring systems use PPG to optically measure the volume changes of blood in tissue during the cardiac cycle. The light received through the tissue corresponds to the changes in blood volume. However, the quality of the detected PPG signal is affected by the optical properties of the skin.

Integrating optical technology into products such as in-ear sports headphones and smartwatches can bring the comfort and convenience that many consumers are looking for. Given the creative thinking out there, it’s exciting to imagine the types of form factors that may emerge for healthcare wearables in the future.

Of course, a big impact on design is that getting accurate heart rate readings from some parts of the body is more challenging than others. In-ear optical measurements tend to be more precise because the ear is perfused with blood. The wrist, on the other hand, has relatively low perfusion and is prone to high motion, especially during activity, which creates more noise when processing and calculating readings. This is why motion compensation is an important consideration when developing a heart rate monitoring application. Signal-to-noise ratio (SNR), ambient light cancellation, and overall power consumption of the end device are also important.

Quickly transform your ideas into market-ready products

To simplify the design process for wearable devices that monitor vital signs, Maxim has introduced two new wearable platforms for health and fitness applications. The MAX-HEALTH-BAND is an evaluation and development platform that uses optical technology to monitor heart rate and an accelerometer to measure activity. An important benefit of the platform is that it provides raw data from sensor measurements that is not available in currently available wearable products. The MAX-HEALTH-BAND streams raw data from sensors or processes the raw data to output heart rate, heart rate variability, step count, activity classification, and calorie burn. The platform is available for a white box license and can save up to six months of time in developing highly accurate, small, and power-efficient wearable health and fitness applications. The MAX-HEALTH-BAND is based on small, energy-efficient ICs and includes:

The MAX86140 is an ultra-low-power optical pulse oximeter/heart-rate sensor with three programmable high-current LED drivers that can be configured to drive up to 19 LEDs. On the receiver side, the device features a single optical readout channel. It also provides a low-noise signal conditioning analog front end (AFE) including a <>-bit analog-to-digital converter (ADC), ambient light cancellation circuitry, and a picket fence detection and replacement algorithm. The picket fence algorithm provides device users with a consistent level of accuracy under a variety of lighting conditions. For example, as a jogger wearing a fitness tracker walks through a tree-lined park with distributed light, the algorithm detects and counteracts changes in ambient light to eliminate any effects that switching between shadows and bright light may have on heart rate detection.

MAX20303 wearable power-management IC (PMIC) with eccentric rotating mass (ERM)/linear resonant actuator (LRA) haptic driver with automatic resonant tracking, micro-quiescent current boost and buck regulators, linear Li-ion battery charger, micro-quiescent current low-dropout (LDO) regulator, and optional fuel gauge

Maxim's motion compensation algorithm extracts useful data from PPG signals

MAX-ECG-MONITOR is an evaluation and development platform for monitoring clinical-grade ECG and heart rate, providing wet electrode patches for clinical applications and chest straps for fitness applications. The platform uses the MAX30003 ultra-low-power, clinical-grade, integrated biopotential analog front end (AFE) to provide ECG waveforms and heart rate detection. Unlike currently available single-point measurement tools, MAX-ECG-MONITOR provides continuous measurements that can be used for trending or predictive 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 applications (with long battery life) for different ECG-based use cases that display heart rate signals at rest or during high-speed exercise. Its built-in heart rate detection function includes interrupt functionality, eliminating the need to run heart rate algorithms on the microcontroller, enabling robust RR detection with very low power consumption in high-speed exercise environments. To extend battery life, the platform operates at 1μW at an 85.1V supply voltage and features a configurable interrupt function that allows the microcontroller to wake up only on every heartbeat, reducing overall system power consumption.

Achieving a healthier world

Wearable health and fitness applications have the potential to enhance well-being in unprecedented ways. By giving device users and healthcare professionals access to insightful data, these technologies can help us be more proactive, take preventative measures, and react more quickly to red flags. Platforms such as MAX-HEALTH-BAND and MAX-ECG-MONITOR are just part of Maxim's overall efforts to create a healthier world.