The wearable device has been one of the hottest products after the invention of the mobile phone. Except for the function of step calculation, common electronic bracelet and smart watch have integrated the function of heart rate monitoring in order to improve the added value and competitiveness of their products. This text will introduce the principle and solutions of optical heart rate monitoring design.
Modules with different integration levels based on requirements
Manufacturers of wearable electronic devices are ceaselessly adding heart rate monitoring functions for their health and fitness products. These heart rate monitoring sensors have included some detached components in the highly integrated modules, such as analog front end (AFE), photodetectors and light-emitting diodes (LEDs) etc. These modules support more convenient implementation model that they can add heart rate monitoring function into wearable products. Meanwhile, they can reduce the cost and complexity as well as promote the ceaseless reduction of the cost for heart rate monitoring sensor.
The applications of each heart rate monitoring are not identical due to the reason that system developers must consider many design requirements like product comfort, sensing precision, system cost, power consumption and sunlight rejection, multiple skin type management, motion rejection, development time and physical size etc. All these design factors will influence the mode of system integration, such as adopting highly integrated module solution or adopting the architectures integrated with more discrete components.
Designing and achieving an optical Heart Rate Monitoring (HRM) system (also known as Photo Plethysmo Graphy technology, PPG for short) is a complex project with many fields involved. Design factors include human engineering, signal processing and filtering, optical and mechanical design, low noise signal receiving circuit and low noise current pulse creation.
The basic method for measuring heart rate signal adopts the optical mode that extracts heart rate pressure waves from human tissue. Through observing the transmission way of the ray after entering skin, as well as blood expansion and contraction motions caused by heart rate pressure waves, the optical signals in human tissues injected from green LED can be modulated. Received signals are greatly reduced after crossing the skin, which will be received and sent to an electronic subsystem for management by a photodiode. Amplitude modulating signals derived from the pulse will be detected (filtering out motion noise), analyzed and displayed.
One of the basis methods of heart rate monitoring system design is adopting a customized microcontroller (MCU) which is used for controlling pulse signal generation of the external LED drivers and reading current output data in a discrete photodiode. The current output of the given photodiode must be converted to voltage and outputted to analog-to-digital (A/D) blocks.
Current pulsed applied in a heart rate system is usually between 2mA and 300mA. The detailed value depends on the subject’s skin color and the sunlight, which must be counteracted for desired signals. Only a little infrared radiation in the sunlight can be reduced while penetrating skin tissue, which is different from the ray generated by expected green LED. Thus it can submerge the signal of the green light, unless the adopted green light is very strong or an expensive infrared ray blocking filter has been added. The intensity of green LED ray penetrating into skin is usually 0.1 to 3 times sunlight intensity.
Si118x optical sensor of Silicon Labs can achieve heart rate monitoring design in a relatively convenient way so that the engineers only need to focus on the optical part of the design including how to match various optical units of the circuit board and the system with the skin. Although this method can achieve high-powered heart rate monitoring solutions, it is not an expected miniaturized or highly efficient scheme of some designers.
In order to achieve smaller solutions, LED die and control silicon must be integrated into a single package, in which all necessary functions are integrated including optical units and lens for improving LED output. The Si117x optical sensor of Silicon Labs is a solution with a higher integration level.
This kind of heart rate monitoring design does not need extra LEDs because the LED and photodiode have been integrated into the module which can be directly installed under the optical ports, such as the rear cover of wearable products like a smart watch etc. This dominant method makes the distance between LED and photodiode shorter than discrete device design, which supports the operation of extremely low power consumption due to less light loss in skin penetration. The integration of multiple LEDs can also solve the light leakage problem between the LED and photodiode, with the result that designers do not need to add light blocking for the printed circuit board.
Silicon Labs also provides heart rate algorithm applied in Si117x/8x optical sensor which can run in most processors after being compiled. If developers of wearable products do not have the resource for developing the algorithm, they can get this kind of software from third party supplier in the manner of software licensed basis.
Of course, the designer is responsible for deciding the suitable integration level for various heart rate monitoring applications finally. Designers can use highly integrated module solution and purchase a licensed algorithm to simplify design procedure and accelerate product launching. Developers with deep recognition for optical sensing as well as abundant time and resources can choose the application of separated components (sensors, photodiodes and lens etc.) and their own system integration, and even develop their exclusive heart rate monitoring algorithm to add market segmentation for the products.