“Photoplethysmography (PPG) is a popular optical technique used for heart rate monitors (HRM) and peripheral capillary oxygen saturation (SpO2) measurements. It’s easy and convenient as just attach the LED and photodetector (PD) to the body.
Photoplethysmography (PPG) is a popular optical technique used for heart rate monitors (HRM) and peripheral capillary oxygen saturation (SpO2) measurements. It’s easy and convenient as just attach the LED and photodetector (PD) to the body.
The basic theory of PPG is based on changes in light absorption by human tissues, which correlate with changes in the relative concentrations of oxyhemoglobin and deoxyhemoglobin. Changes in blood volume caused by systole and diastole can be used to estimate arterial oxygen saturation.
PPG signals are susceptible to motion artifacts, which depend on the wavelength of the light source. The absorption of light and thus the depth of penetration into the tissue depends on the wavelength. Longer wavelengths such as infrared and near-infrared (NIR) have relatively low absorption, allowing deeper tissue penetration. Shorter wavelengths of light such as green and blue are strongly absorbed by melanin; therefore, the penetration depth into tissue is relatively shallow. Thus, red and near-infrared light PPGs are subject to artifacts, while green and blue light PPGs are relatively free of artifacts.
Figure 1 shows the depth of skin penetration for optical signals with wavelengths from 400 to 1,000 nm.
Figure 1: Shows the optical penetration depth (δ) for a range of different wavelength light sources
Using multiple LEDs and PDs in a multiplexed configuration enables multi-wavelength measurement and monitoring. Each diode and detector can support different wavelengths and can also be used in a multiplexed fashion (at different sampling stages) to monitor different parameters.
The Bluetooth® 5 Multi-Wavelength Optical HRM and SpO2 Monitoring Reference Design features the AFE4420 single-chip biosensing front-end, supporting four LEDs and four time-multiplexed photodetector inputs simultaneously. It integrates a complete receiver chain of flexible LED drivers and photodetectors.
Signal acquisition can be done on up to 16 phases, with flexible assignment of LEDs and PDs per phase (see Figure 2). It communicates with the CC2640R2FSimpleLink™ Bluetooth Low Energy wireless microcontroller (MCU), which communicates via the serial peripheral interface or I2C integrates Arm® Cortex®-M3 and 2.4-GHz radio frequency (RF) transceivers. The design supports capturing wired data using a JTA connection and sending it over Bluetooth 5 to capture wireless data.
Figure 2: Block diagram of multi-wavelength optical HRM and SpO2 monitoring reference design
Key features of the design include:
・ Raw PPG data for calculation of heart rate, SpO2 and other related parameters. The AFE4420 is unique in the market with its level of integration, low power consumption and size, and offers the flexibility of ultra-low power modes and an integrated first-in, first-out feature that keeps the MCU in sleep mode for extended battery life.
・ Use CC2640R2F for wireless connection. The CC264R2F integrates an Arm Cortex-M3 and 2.4-GHz RF transceiver that supports Bluetooth 4.2 and 5.0 profiles and can also be used as a host processor. Internal DC/DC converters help improve overall system efficiency and extend battery life. A built-in low-battery detection algorithm helps reduce external components in wearable applications.
・Use a coin cell battery for low-power operation. The design uses a single 3-V, 500 mA Cr3032 coin cell battery and has been tested for 100 hours of continuous operation; 30 days of battery life with uninterrupted transmission.
Designed for medical, personal health care, and fitness applications, this reference design comes with design guidelines, schematics, layouts, and bill of materials files to help you quickly evaluate and accelerate product development. It facilitates real-time monitoring and data logging capabilities and supports optimization for different configurations.