Photoplethysmography (PPG) is a popular optical technique for heart rate monitor (HRM) and peripheral capillary oxygen saturation (SpO2). It’s simple and convenient because you just need toLEDAnd a photodetector (PD) can be connected to the body.

The basic theory of PPG is based on the change of light absorption of human tissue, which is related to the change of relative concentration of oxygenated hemoglobin and deoxyhemoglobin. Changes in blood volume caused by cardiac contraction and relaxation can be used to estimate arterial oxygen saturation.

Realize the functions that cannot be realized by traditional parameter measurement

Refer to our latest reference design for more information, “Bluetooth support” ® 5 multi wavelength optical HRM and SpO2 monitoring reference design. “

The PPG signal is susceptible to motion artifacts, depending on the wavelength of the light source. The absorption of light and therefore the depth of penetration into the tissue depends on the wavelength. The absorption of longer wavelengths such as infrared and near infrared (NIR) is relatively low, which can achieve deeper tissue penetration. Light with shorter wavelengths such as green and blue is strongly absorbed by melanin; Therefore, the penetration depth into the tissue is relatively shallow. Therefore, red and near-infrared light PPG are subject to artifacts, while green and blue light PPG have relatively no artifacts.

Fig. 1 shows the skin penetration depth of an optical signal with a wavelength of 400 to 1000 nm.

Multi wavelength optical measurement realizes the functions that cannot be realized by traditional SpO2 measurement

Figure 1: shows the optical penetration depth of a series of light sources with different wavelengths( δ)

Multi wavelength measurement and monitoring can be achieved using multiple LEDs and PDs in a multiplexing configuration. Each diode and detector can support different wavelengths and can also be multiplexed (in different sampling stages) to monitor different parameters.

Using Bluetooth ® 5 multi wavelength optical HRM and SpO2 monitoring reference design adopts afe4420 single chip biosensor front end, and supports four LED and four time division multiplexing photodetector inputs at the same time. It integrates a complete receiver chain of flexible LED drivers and photodetectors.

Signal acquisition can be completed in up to 16 phases, and each phase can flexibly allocate led and Pd (see Figure 2). It works with cc2640r2fsimplelink ™ Bluetooth low-power wireless microcontroller (MCU) communication, which integrates arm through serial peripheral interface or I2C ® Cortex ®- M3 and 2.4-GHz radio frequency (RF) transceivers. The design supports the use of joint test behavior to organize connections to capture wired data and send it to capture wireless data through Bluetooth 5.

Multi wavelength optical measurement realizes the functions that cannot be realized by traditional SpO2 measurement

Figure 2: reference design block diagram of multi wavelength optical HRM and SpO2 monitoring

The main features of the design include:

  1. ·Raw PPG data used to calculate heart rate, SpO2 and other related parameters. Afe4420 has unique integration, low power consumption and size in the market, and provides flexibility in ultra-low power consumption mode and integrated first in first out function, which can keep MCU in sleep mode to prolong battery life.
  2. ·Use cc2640r2f to realize wireless connection. The cc264r2f integrates arm Cortex-M3 and 2.4-GHz RF transceivers, 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. The built-in low battery detection algorithm helps to reduce external components in wearable applications.
  3. ·Low power operation with button battery. The design adopts a single 3-V, 500 mA cr3032 button battery, which can run continuously for 100 hours after testing; In case of uninterrupted transmission, the battery life is 30 days.

Designed for medical, personal health care and fitness applications, this reference design is accompanied by design guidelines, schematics, layouts and bill of materials documents to help you quickly evaluate and accelerate product development. It facilitates real-time monitoring and data logging functions and supports optimization for different configurations.

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