In this session, we invited Engineer Feng from a professional medical smart wearable device company to share practical case studies through Exclpoint, a technology-driven distributor. This company has been a long-term client of Exclpoint, primarily engaged in the research, development, production, sales, and service of medical diagnostic products. Engineer Feng possesses extensive academic and practical experience.
Project Requirements
Wearable products are small in size, requiring components with small packaging and high integration.
■ Ultra-low power consumption, powered by rechargeable battery;
■ It has a continuous heart rate monitoring function;
■ It has a function to continuously monitor blood oxygen saturation;
■ It has a step counting function that can store more than a month's worth of data, which will not be lost when the power is off.
Heart rate monitoring program
Heart rate monitoring can be based on electrocardiogram (ECG) signal detection. The periodic beating of the human heart is precisely controlled by bioelectrical signals. Once these ECG signals are captured, heart rate can be calculated. However, ECG detection requires acquiring potentials at multiple points on the body, leading to complex hardware implementation and high costs. One approach is a resonant detection scheme, where a sensor detects changes in arterial pressure to calculate heart rate. However, this scheme requires placing the device on a part of the body where the pulse is most pronounced, posing a challenge for wearable devices. Another approach is photoelectric detection, based on the principle that blood reflects red light and absorbs green light. During a heartbeat, blood flow through the skin increases, leading to increased absorption of green light; during the interval between heartbeats, blood flow in the blood vessels decreases, resulting in less green light absorption. When the device uses a green LED in conjunction with a light-sensitive photoelectric sensor, it can detect blood flow through various parts of the skin at any given time. By statistically analyzing the data and using the highest point of blood flow each time as a cycle, heart rate can be calculated. Furthermore, the device can be designed in the form of a watch for convenient wrist wear.
A photoelectric heart rate monitoring scheme (also known as photoplethysmography, PPG) requires hardware modules for light source driving, photoelectric conversion, analog-to-digital conversion, and data processing. Generally, a constant current source (with adjustable current and a specific frequency switch) is chosen for the light source driving; photoelectric conversion uses photodiodes, followed by current-to-voltage conversion and signal conditioning using operational amplifiers; finally, an A/D chip performs analog-to-digital conversion and sends the data to an MCU for processing. If discrete components are used in the hardware scheme, the number of chips will be large, and space will be limited; therefore, integrated components are preferable. Analog Devices (ADI) offers the ADPD105, a multi-functional optoelectronic measurement front-end chip. It features a fully integrated AFE, ADC, LED driver, and timing core, providing superior ambient light suppression without the need for photodiode filters. Flexible digital interfaces, including SPI and I2C, are available. It boasts low power consumption with an ultra-low 1.8V analog/digital operating power supply and a standby current of only 0.3μA, making it suitable for battery-powered devices. The flexible sampling frequency range is 0.122 Hz to 3820 Hz. It supports up to three LED constant current sources with a peak current of 370mA per channel. Available in a WLCSP chip-scale package, its extremely small size makes it ideal for space-constrained applications. Its internal functional block diagram is shown in Figure 1.
Blood oxygen saturation monitoring protocol
Photoplethysmography (PPG) can not only be used for heart rate detection, but also for measuring blood oxygenation through certain algorithm processing, providing a good solution for non-invasive blood oxygenation monitoring.
Principle: Oxyhemoglobin (HbO2) and hemoglobin (Hb) both have certain absorption characteristics for light with wavelengths of 600-1000 nm. Hb has a higher absorption coefficient for light between 600-800 nm, while HbO2 has a higher absorption coefficient for light between 800-1000 nm. Therefore, the PPG signals of HbO2 and Hb can be detected using red light (600-800 nm) and near-infrared (IR) light (800-1000 nm) respectively. The corresponding ratios are then calculated through program processing, thus obtaining the blood oxygen saturation value.
The ADPD105 utilizes two LED constant current sources to drive a red LED and a near-infrared LED respectively, while another constant current source drives a green LED for heart rate detection. A single photodetector can be used, time-division multiplexing, to separately acquire heart rate and blood oxygen signals. This achieves efficient data acquisition with minimal components and the lowest overall cost. Additionally, the recently released ADI ADPD410X, a newer product in the ADPD series, offers higher integration and sampling accuracy, supporting PPG, ECG, and EDA measurements. It can serve as a functional expansion solution for this project.
Step counting function solution
The most commonly used step counting solution employs a three-axis accelerometer. By recognizing the three-axis dynamic data during walking through algorithms, step counting can be achieved. This solution is based on wearable devices, therefore chip power consumption and size are crucial considerations. The ADXL363 is an ultra-low-power three-sensor combination product, consisting of a 3-axis MEMS accelerometer, a temperature sensor, and an ADC input for synchronously converting external signals. The entire system consumes less than 2µA at an output data rate of 100 Hz, and 270 nA in motion-triggered wake-up mode; it operates within a 1.6~3.5V power supply range, easily compatible with the power supplies of other devices; the ADXL363 also provides access to the internal ADC for synchronous conversion of external analog inputs. Its compact LGA package is ideal for wearable device applications. Its internal functional block diagram is shown in Figure 2.
MCU Selection
This solution requires the selection of an MCU that balances high performance and low power consumption. Analog Devices' ADuCM3029 boasts ultra-low power characteristics: Active mode (fully enabled): < 30μA/MHz (typical), Sleep mode (with SRAM reservation): < 750nA (typical), Shutdown mode (optional RTC active): < 60nA (typical); an ARM® Cortex®-M3 processor with integrated MPU, capable of handling high-performance algorithms with ease; 256 KB embedded flash memory with integrated ECC for convenient offline data storage; a low-voltage supply (1.74V to 3.6V) allows the use of a coin cell battery; a full range of on-chip and off-chip peripherals including SPI, UART, IIC, Timer, DAC, DMA, and Watchdog for ease of use; a built-in 32kHz oscillator and a 26MHz high-frequency oscillator, and also supports external clock sources; and two compact WLCSP/LFCSP models are available, suitable for space-constrained applications.
Alternatively, the high-performance ADuCM4050 can be used as a high-end upgrade option, offering better performance and lower power consumption in sleep and shutdown modes: Active mode (fully enabled): < 41μA/MHz (typical), Sleep mode (with SRAM reservation): < 650nA (typical), Shutdown mode (optional RTC active): < 50nA (typical); ARM Cortex-M4F processor, 52 MHz, with FPU, MPU, ITM, and SWD interfaces for enhanced compatibility; EEC upgraded to 512KB for easier storage of more offline data;
Power Solution
Wearable devices require a rechargeable power source, typically a lithium battery, thus necessitating a lithium battery management chip. The LTC4065L is a complete constant current/constant voltage linear charger for single-cell lithium batteries. Due to its small size (DFN package 2mm*2mm), it can accurately regulate low charging current, making it ideal for low-capacity lithium batteries. It can also operate via USB standard power supply and features robust charging anomaly protection: automatic recharging, low battery charge regulation (tricky charging), and soft-start function (for limiting inrush current).
In this design, the MCU, accelerometer, and analog front-end chip can all operate within the 1.8V range. However, an LDO is needed to step down the 3.7V lithium battery voltage. Due to the presence of analog circuitry, a low-noise LDO is required. The LT 3042 offers ultra-low noise (0.8µVRMS, 10 Hz to 100 kHz), ultra-high power supply rejection ratio (minimum 79 dB @ 1MHz), ultra-low operating voltage drop (350mV), adjustable output (0 to 15V), and a compact DFN package (3mm x 3mm).
