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Portable Pulse Oximeter Solution from Renesas Electronics

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Pulse oximetry is a non-invasive measurement technique that uses absorption characteristics of different wavelengths of light when it travels through the human tissue. By attaching a pulse oximeter device to the thin part of the body (mostly a finger or ear), vital health parameters of the subject such as oxygen saturation (SpO2) and heart rate can be monitored for early detection of life-threatening conditions.

This design features a portable, battery-operated pulse oximeter from Renesas Electronics. We will discuss the individual components of the proposed design with product recommendations and resources to get started. 

Block diagram

The block diagram below illustrates the functional components of the portable pulse oximeter solution from Renesas.

Figure 1. Block diagram of pulse oximeter design from Renesas

Sensing

The sensing element of the pulse oximeter design uses an  OB1203 sensor module that integrates an ambient light sensor, RGB colour sensor, a proximity sensor as well as an optical biosensor for reflective photoplethysmography. The latter allows monitoring of vital health parameters, such as human heart rate, oxygen saturation (SpO2), respiration rate, and heart rate variability, by illuminating the skin and then measuring the light absorption of saturated versus unsaturated blood through the skin. The module comes in the industry’s smallest (4.2 × 2 × 1.2mm), a 14-pin package. The reliable and hypoallergenic glass lid allows the use of this module for applications that require direct skin contact. 

The OB1203 sensor module consists of infrared (IR) and red LEDs for excitation and a sensor IC. The LEDs can be controlled externally through LED1 and LED2 pins, however, the IC contains trimmed LED source with 250mA maximum current drive for a more integrated solution. There are three distinct blocks within the sensor IC, including a light sensor, proximity/biosensor and digital block, as well as some peripheral circuits, such as two internal oscillators, a current source, and voltage reference.

Figure 2. The internal structure of OB1203 module (renesas.com)

The Light Sensor block includes five photodetectors arranged in a matrix array and with dedicated analogue-to-digital converters (ADC) for faster and more accurate performance. It can be configured to run in Light Sensor Mode (Green, Clear and Comp) or in Color Sensor Mode (Red, Green, Blue, Clear and Comp). The Comp channel receives data from a metal-covered photodiode used to measure dark current and compensate the reading of the light sensors for temperature changes.

The Proximity and Biosensor block utilizes a single photodiode, which is located below the light sensor array. It measures the amount of reflected energy in the red and infrared range from a target object using the LED cathode/driver outputs on the LED2 and LED1 pins, respectively. The transmitter is realized with an infrared LED (peak wavelength of approximately 940nm) and a red LED (peak wavelength of approximately 700nm) that are integrated into the OB1203 module. This block features dedicated ambient light and crosstalk/offset cancellation circuits to avoid any optical interferences.

Lastly, the Digital Block incorporates a state machine, communication interface, digital filters, registers and fuses for calibration purposes. The OB1203 sensor module relies on the I2C communication protocol for reporting the data to the MCU.

MCU

The pulse oximeter design features Renesas’ RA2A1 (192-7200) (192-7194) microcontroller based on Arm® Cortex®-M23 core with 256kB Flash memory and 32kB SRAM. The RA2 series belongs to RA family of 32-bit MCUs and it is ideal for low power and cost-sensitive applications such as portable pulse oximeters.

The RA2A1 MCU also incorporates a dedicated analogue front-end with 16-bit SAR ADC, a 24-bit Sigma-Delta ADC, DACs and operational amplifiers. This allows seamless integration of Li-ion battery management solution into the pulse oximeter design for prolonged battery life. The MCU can monitor status parameters such as temperature, voltage and current in the battery, hence protecting it from potential hazards (e.g. overcurrent, overvoltage and overheating protection).

Figure 3. RA2A1 MCU overview (renesas.com)

To get started with this MCU, try the EK-RA2A1 (192-7184) evaluation kit along with Renesas’ Flexible Software Package (FSP). 

Power

The proposed pulse oximeter is designed to operate with Li-ion battery and supports recharging through the USB power supply. The charging circuit consists of a combination of switches, Zener diode, an LC filter, whereas the control system is managed by the MCU. The DC voltage is supplied to charge the battery by rectifying the switching voltage of the buck converter. The 5V from either USB charger or the battery is then converted into the voltage level required to power the MCU and the sensor module. The recommended devices for this purpose are ISL80510 (193-1524) or ISL80505 (121-7270) high-performance low-dropout (LDO) voltage regulators that are capable of sourcing up to 1A or 500mA output current, respectively. Additionally, ISL80510EVAL1Z (193-1522) evaluation kit is available through RS Components. 

Summary of products

Product type

Product

Description

Document

RS stock no

MCU

RA2A1

32-bit, 48- MHz RA MCUs, 256kB flash memory, 32kB SRAM, scalable from 32pin to 64 pin packages

Datasheet

(192-7200)

(192-7194)

Optical Sensor

OB1203

High-Performance Solid-State MEMS Flow Sensor

Datasheet

Stock number TBA

LDO

ISL80505

Low dropout regulator, 500mA

Datasheet

(121-7270)

ISL80510

Low dropout regulator, 1A

Datasheet

(193-1524)

 

ISL80510EVAL1Z

High-Performance 1A LDO Voltage Regulator Evaluation Board

Datasheet

(193-1522)

I am an electronics engineer who is passionate about technology. In my spare time, I like travelling, blogging and learning new languages. よろしくお願いします.

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