ALLPCB
Overview
Photoplethysmography (PPG) is a widely used optical technique for heart rate monitoring (HRM) and peripheral capillary oxygen saturation (SpO2) measurement. It is simple to implement: LEDs and a photodetector (PD) are attached to the body to collect signals.
The PPG signal is based on changes in light absorption by tissue related to the relative concentrations of oxyhemoglobin and deoxyhemoglobin. Blood-volume changes caused by cardiac systole and diastole can be used to estimate arterial oxygen saturation.
Wavelength Dependence and Motion Artifacts
PPG signals are susceptible to motion artifacts, and the severity depends on the light wavelength. Light absorption and thus tissue penetration depth depend on wavelength. Longer wavelengths such as red and near-infrared (NIR) are absorbed relatively less and penetrate deeper into tissue. Shorter wavelengths such as green and blue are strongly absorbed by melanin, resulting in shallower penetration. As a result, red and NIR PPG are more affected by motion artifacts in some cases, while green and blue PPG may exhibit fewer motion-related artifacts.
Figure 1 shows skin penetration depth for wavelengths from 400 to 1000 nm.
Figure 1: Optical penetration depth (δ) across a range of wavelengths
Multispectral, Time-Multiplexed Measurement
Using multiple LEDs and PDs in a time-multiplexed configuration enables multispectral measurement and monitoring. Each LED and detector can support different wavelengths, and LEDs can be multiplexed during different sampling phases to monitor distinct parameters.
A reference design for multispectral optical HRM and SpO2 monitoring with Bluetooth 5 uses the AFE4420 single-chip biosensor front end, supporting four LEDs and four time-division multiplexed photodiode inputs. The AFE integrates flexible LED drivers and a complete receiver chain for photodetectors.
The signal acquisition can be configured across up to 16 phases, with flexible assignment of LEDs and PDs per phase. The design pairs the AFE4420 with the CC2640R2F SimpleLink Bluetooth low-energy wireless microcontroller, which integrates an Arm Cortex-M3 and a 2.4 GHz RF transceiver. Communication between the AFE and the MCU is via a serial peripheral interface or I2C. The MCU includes an internal DC/DC converter to improve system efficiency and built-in low-battery detection functionality.
Figure 2: Block diagram of the multispectral optical HRM and SpO2 monitoring reference design
Key Features
- Raw PPG data for heart rate, SpO2, and related parameters. The AFE4420 provides high integration, low power, and compact size, plus ultra-low-power modes and integrated FIFO to allow the MCU to remain in sleep mode and extend battery life.
- Wireless connectivity via the CC2640R2F. The CC2640R2F integrates an Arm Cortex-M3 and a 2.4 GHz RF transceiver with support for Bluetooth 4.2 and 5.0 profiles, and can act as the host processor. An internal DC/DC converter improves overall system efficiency and battery life. Built-in low-battery detection algorithms reduce the need for external components in wearable applications.
- Low-power operation from a coin cell. The design uses a single 3-V, 500 mA Cr3032 coin cell; tested continuous operation is 100 hours, and with intermittent transmission the battery life is 30 days.
Applications and Design Resources
The reference design targets medical, personal health, and fitness applications. It includes design notes, schematics, layout files, and a bill of materials to support evaluation and accelerate product development. The design supports real-time monitoring and data logging, and can be optimized for different sensor and system configurations.
