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Take over the forehead temperature gun! Which chip solutions will the popular oximeter bring on fire?

Release on : May 7, 2021

Take over the forehead temperature gun! Which chip solutions will the popular oximeter bring on fire?

Affected by the continued fermentation of the global epidemic, following medical products such as forehead thermometers and ventilators, the demand for blood oximeters has also seen explosive growth. Since blood oxygen saturation is one of the important diagnostic indicators of this new coronary pneumonia, the demand for oximeters that can measure blood oxygen saturation will usher in a short-term increase of 3-5 times. At the same time, some industry insiders believe that after the epidemic, it is expected that the demand for medical blood oxygen monitoring equipment will continue to grow substantially in the next few years, and the growth rate is expected to remain between 25% and 30%.
Principle of oximeter

Generally speaking, oximeters can be divided into two categories. One is the in-hospital equipment used in the hospital, which is generally integrated on the vital sign monitor to obtain blood oxygen data through the blood oxygen probe; the other is used outside the hospital The small finger clip oximeter can detect blood-oxygen data directly on the finger for a few seconds.
The oximeter generally consists of a microprocessor, memory (EPROM and RAM), two digital-to-analog converters that control LEDs, a device that filters and amplifies the signal received by the photodiode, and digitizes the received signal to provide it to the microprocessor The LED and photodiode are placed in a small probe that is in contact with the patient’s fingertip or earlobe. Among them, the pulse oximeter generally also includes a small liquid crystal display.


The main measurement indicators of the oximeter are pulse rate, blood oxygen saturation, and perfusion index (PI). Take a small finger clip oximeter as an example. The principle is: by driving a red LED (660nm) and an infrared LED (910nm) in turn, the blue line indicates that when the hemoglobin does not contain oxygen molecules, the receiving tube will reduce hemoglobin. In the induction curve, it can be seen from the graph that reduced hemoglobin has relatively strong absorption of 660nm red light, while the absorption length of 910nm infrared light is relatively weak. The red line represents the induction curve of hemoglobin and red blood cells with oxygen molecules when the receiving tube responds to oxygenated hemoglobin. It can be seen from the figure that the absorption of 660nm red light is relatively weak, and the absorption of 910nm infrared light is relatively strong.
In blood oxygen measurement, reduced hemoglobin, and oxygenated hemoglobin, by detecting the difference between the two kinds of light absorption of different wavelengths, the measured data difference is the most basic data for measuring blood oxygen saturation. In the blood oxygen test, the two most common wavelengths of 660nm and 910nm, in fact, to achieve higher accuracy, in addition to the two wavelengths, there are even up to 8 wavelengths. The main reason is that the human hemoglobin is in addition to reducing hemoglobin. In addition to oxyhemoglobin, there are other hemoglobins. We often see carboxyhemoglobin. The more wavelengths, the higher the accuracy of the oximeter.
Inventory of oximeter chip design schemes
In order to better understand the oximeter market, the editor compiled the mainstream oximeter chip design schemes on the market for readers (if there are deficiencies, welcome to add).

01, Renesas Electronics

The Renesas Electronics oximeter solution is equipped with a highly integrated sensor module OB1203 and a high-precision analog RA2A1 MCU. It is optimized for the application characteristics of handheld oximeters and achieves a perfect balance between performance and power consumption. Renesas Electronics RA2A1 MCU provides a complete set of signal conditioning and measurement simulation functions for the solution. Its analog functions include 16-bit SAR ADC, 24-bit Sigma-Delta ADC, comparator, operational amplifier, and DAC. The system uses a lithium battery charging system that combines 16-bit ADC, operational amplifier, and PWM.

Renesas electronic oximeter solution

At the same time, the Renesas Electronics oximeter solution uses a single-cell lithium-electronic USB charging design. With the powerful analog function of the RA2A1 ARM microcontroller, it can simply realize the USB charging of the lithium-ion battery without the need for additional Components. The voltage and current inside the system can be monitored in real-time, and the charging method can be adjusted as needed.
The highly integrated sensor OB1203 equipped with Renesas’ oximeter solution is the smallest optical biosensor module in the industry. It has a fully integrated biosensor for reflective photoplethysmography. Heart rate and blood oxygen saturation can be determined through appropriate algorithms. (SpO2), respiratory rate, and heart rate variability. OB1203 integrates the light source and driver into a single optical optimization package. The main microcontroller RA2A1 can directly obtain PPG data through IIC communication.
At the same time, the solution also provides a low-power design that automatically powers off after 10 seconds without finger contact. With the low-power features of the RA2A1 MCU and the biosensor module OB1203, the power consumption of the entire system has been further optimized.

Renesas electronic oximeter solution

Features of Renesas’ electronic oximeter solution:
1. Single battery Li-ion USB charging • Simple realization of Li-ion battery USB charging • Real-time monitoring of voltage and current
2. Low-power system • Low-power MCU RA2A1, low-power biosensor module OB1203• Automatically cut off the power after 10 seconds without a finger
3. Easy to control • I2C communication with OB1203 and OLED • When approaching, generating new PPG data and FIFO is full, OB1203 triggers an interrupt • AD and PWM control battery charging
4. Measuring range• Blood oxygen saturation: 70%~100%• Pulse: 25bpm~200bpm

02, Texas Instruments

The AC/DC adapter subsystem of the Texas Instruments pulse oximeter solution converts the AC power line into an isolated regulated DC output; the sensor front-end subsystem drives the red/IR led, and converts the photodiode input into a buffer, obtained So that the system processor can process it.

Texas Instruments Pulse Oximeter Solution

Features of Texas Instruments Pulse Oximeter Solution:
1. The integrated LED driver and photodiode signal conditioning circuit can detect light absorption with a high degree of accuracy, thereby simplifying the design and helping the final product to maintain a small form factor.
2. High-efficiency power management solutions help to extend battery life, thereby reducing the frequency of users replacing or recharging the battery.
3. A low-power microcontroller, which can process sensor measurements and calculate oxygen saturation and heart rate.

03, NXP

NXP provides ultra-low-power MCUs that support LCD displays, suitable for portable pulse oximeters, and can track the oxygen content in the patient's blood.
These devices provide a simple non-invasive method for monitoring the percentage of oxygen-saturated hemoglobin in the blood. The LCD display will show blood oxygen saturation and heart rate.
The pulse oximeter can be used as a standalone portable device or as part of a large patient monitoring system.

NXP Pulse Oximeter

NXP pulse oximeter supporting device

04, ADI

ADI provides a complete range of high-performance linear, mixed-signal, MEMS, and digital signal processing technologies for pulse oximeter design.

ADI pulse oximeter design scheme

Pulse oximeter includes transmitting path, receiving path, display, and backlight, data interface, and audio alarm. The emission path includes red light LED, infrared light LED and DAC used to drive the LED. The receiving path includes photodiode sensors, signal conditioning, analog-to-digital converters, and processors.

Design considerations and main challenges of pulse oximeter system:
When designing a pulse oximeter system, many problems need to be solved, such as low blood perfusion, exercise and skin humidity, stray light interference, carboxyhemoglobin, and methemoglobin interference.
1. Low blood perfusion (small signal level). Photodiode measurements require signal conditioning with a wide dynamic range and low noise gain in order to capture pulse events. The transmit and receive paths require high-quality, low-noise LED driver circuits with high-resolution DACs and high-precision analog front-end circuits with high-resolution ADCs.
2. Exercise and skin humidity. The motion will cause artifacts, which can be solved by software algorithms or use accelerometers such as ADXL345 to detect and solve them.
3. Stray light interference. Use photodiodes to respond to red light and infrared light, it is easily interfered by ambient light. Therefore, the algorithm used to filter out the red light and infrared light target signals is very important, which means that the signal processing is more complicated. In this case, you need to use a DSP with higher signal processing capabilities.
4. Carboxyhemoglobin and methemoglobin. Carbon monoxide (CO) easily binds to hemoglobin, making the blood more like red HbO2, resulting in an artificially high SpO2 value. The iron in the home base is in an abnormal state and cannot carry oxygen (Fe+3 instead of Fe+2), resulting in a decrease in hemoglobin and a false low SpO2 reading. Using more wavelengths can improve accuracy, but this requires a higher-performance digital processing DSP, and the processing time is critical.

05, Renajie Electronics

Renaje Electronics uses time-division multiplexing to drive photodiodes and periodically light up two LEDs to detect two optical signals on the same transmission path. In order to obtain two independent signals, at the same time, a control signal synchronized with the LED driving pulse is used to control the use of a holding circuit to realize signal separation, and two signals of infrared and red light are obtained respectively. LED drive timing pulses and sample-and-hold circuit switching timing pulses are provided by the GPIO of RJM8L151.

Finger Clip Pulse Oximeter Scheme

The analog signal processing unit completes the processing of the signal collected by the photodiode, including the functions of signal conversion, sample, and hold, amplification, filtering, and compensation. Since the signal output by the photodiode is a current signal, it must be converted into a voltage signal before it can be processed by the subsequent circuit. Moreover, the photodiode works in a reverse bias state, and its junction resistance is relatively large, and the output current is relatively small. Therefore, an operational amplifier with a higher input impedance should be selected as the TIA for I/V conversion.
The output of the current-voltage conversion circuit is two kinds of optical time-division multiplexed signals. To separate the two optical signals, two independent sample-and-hold circuits are used, and the GPIO of RJM8L151 is used to control and switch the two-channel sample-and-hold circuits. , And to be synchronized with the control pulse of the LED drive pulse.
Since the separated AC signal is very weak, in order to eliminate the DC component and high-frequency interference, a band-pass filter is used in the circuit to process the red light and infrared signals. The band-pass filter is composed of high-pass and low-pass parts. The high-pass part uses an RC filter network to filter out the DC component, and its cut-off frequency is set to 0.23 Hz. The low-pass part uses a second-order low-pass filter circuit with a cut-off frequency of 0.48 Hz.
Ruinajie Electronics has launched the RJM8L151 series of ultra-low-power MCUs, which are very suitable for battery-powered IoT terminal devices. The RJM8L151 series not only has an excellent performance in running and standby power consumption but also has a built-in 12-bit high-precision successive approximation ADC and a 2-channel multi-function comparator, which is extremely beneficial to the high-precision and real-time detection of weak signal sensors. The rich peripheral interfaces of RJM8L151 make it more convenient to expand various communication modules and functional modules. RJM8L151 is based on the enhanced Harvard architecture CPU core and multi-stage pipeline instruction system. The processing performance of the same clock frequency is 3 times that of the traditional 8051. It adopts Keil uVision or IAR integrated development environment to develop and debug application code.

RJM8L151 series MCU hardware block diagram

Features of Renajie electronic finger clip oximeter solution:

1. The normal operating voltage range of RJM8L151 is 1.62V to 5.5V, which is very suitable for the direct power supply of 2 and 3 dry batteries, eliminating the need for additional LDO circuits. In addition, the standby current of RJM8L151 is as low as 0.5uA, which can maintain the normal internal RTC. Timing, while keeping the SRAM data unchanged, this performance can greatly reduce the system's requirements for battery capacity and size.
2. RJM8L151 wakes up from a low-power state in less than 5us, which can realize fast sleep and wake-up low-duty cycle work, which in turn greatly reduces the system power consumption.
3. RJM8L151 has 4 clock sources: internal high-speed clock, internal low-speed clock, external high-speed clock and external low-speed clock. The clock control module of RJM8L151 divides these clock sources through flexible configuration to achieve different power consumption and performance requirements.
4. In terms of analog circuits, RJM8L151 has a 7-channel 12-bit successive approximation ADC, a sampling conversion rate up to 1MSPS, and supports external reference voltage input.
5. RJM8L151 has designed a wealth of timing modules, including two 16-bit basic timers, and a 16-bit general-purpose timer that supports input capture/output comparison/PWM output functions.

06, Ying Ruien

The role of the analog switch RS2105 is mainly to switch between red LED and infrared LED lights, which are time-sharing multiplexed and modulated with a certain frequency. This requires an op-amp chip (such as RS321) to cooperate with the internal DAC of the microcontroller to generate a constant current output signal because if the LED lamp is to work at a constant current to obtain a relatively stable wavelength, its emission current usually needs to reach tens of milliamperes. Therefore, it is required that the on-resistance of the analog switch should be ≤1Ω, and the on-resistance of the RS2105 analog switch used in this solution is 0.6Ω.

Ying Ruien household finger clip oximeter principle diagram

At the photosensitive receiving end, the op-amp RS622 is used to realize photoelectric conversion, which requires the input bias current Ib of the op-amp to be at the pA level. In addition, the bandwidth of the op-amp needs to be above 5MHz to meet the fast response. The low noise series of op-amps can Better improve the resolution of the system. In terms of power supply topology, one is to use two dry batteries (3V) and then boost to 5V and then step down for each circuit of the system. The other has a lithium battery that can be recharged and used. The low power consumption LDORS3236 is suitable for both of these A topological systems.

07, ZLG Ligong Technology

The main control adopts the HC32L series of Huada Semiconductor, the chip adopts the Cortex M0+ core, the main frequency range is 32~256K Flash, 4~32K RAM, the power supply range is 1.8~5.5V, and the current is only 0.5uA in the case of deep sleep, which can extend the battery Service life, so it is very suitable for portable product applications.

ZLG Ligong Technology portable oximeter monitoring solution

The Bluetooth module acts as a communication bridge between the MCU and the mobile phone and transmits the data collected by the MCU to the mobile phone

A new generation of BLE5.0 transparent transmission module with power consumption, low cost, and small size. Dot-matrix LCD is mainly used for the real-time display of collected data.

BLE5.0 transparent transmission module

Introduction of BLE5.0 transparent transmission module:
1. The maximum payload data transmission rate can reach 94KB/s;
2. Two low-power consumption modes are supported. In low power consumption 2 mode, the average current is the only 250nA;
3. LE Secure Connections supporting BLE4.2 to prevent eavesdropping by unauthorized third-party devices;
4. Two-way communication between the serial port and MCU;
5. A rich AT command set is preset internally, which is easy to operate, shortens the user development cycle, and speeds up the product launch;
6. Applicable to smart furniture, instrumentation, health care, sports measurement, automotive electronics, and other fields.
08, Zhengjiu Electronics
The design plan of Zhengjiu electronic oximeter adopts the 32-bit microprocessor HK32F103C8T6 of the Hangshun chip ARM Cortex™-M3. The processor has a built-in 160MHz high-speed oscillator, 20KB SRAM, 64KB FLASH, and up to 37 interruptible and wake-up IO ports. Abundant peripheral interfaces, including two IIC, two high-speed SPI, three USART, USB2.0 Full Speed ​​interface, CAN 2.0A/B bus interface controller.

Zhengjiu electronic oximeter design scheme
In this solution, the OLED is connected to the main control HK32F103C8T6 through the I2C interface to display the interface and various data. SPDI34 is connected to the main control HK32F103C8T6 through a high-speed SPI interface, and the main control unit reads the data of SPDI34 for relevant processing and displays it on the OLED. The Bluetooth module is connected to the main control HK32F103C8T6 through the USART interface, and the relevant blood oxygen, pulse rate, and other data processed by the main control can be read by mobile phones, tablets, and other devices through the Bluetooth module.
Features of Zhengjiu electronic oximeter solution:
1. The product adopts a two-color OLED display with multiple display methods;
2. One-key measurement;
3. Bluetooth data transmission;
4. Ultra-low power consumption, with two AAA batteries that can be used continuously for more than 40 hours;
5. Low battery alarm;
6. Automatic shutdown: automatic shut down after 8 seconds without operation;
7. Small size, lightweight, and easy to carry;

09, Apex

Apex’s self-developed APM32F103 series MCUs feature low power consumption, high performance, and rapid transplantation, which can help customers speed up research and development, improve product performance, and ease the pressure on the supply and demand of medical electronic equipment such as blood oximeters.
It adopts the latest version of Arm® Cortex®-M3 core, the highest operating frequency is 96MHz, Flash is up to 512Kbytes, SRAM is up to 128Kbytes, built-in FPU floating-point arithmetic unit has powerful calculation processing capabilities in terms of signal speed and accuracy, and can be read in a very short time Take and calculate the corresponding measurement data, and display the blood oxygen value by driving the liquid crystal.

Integrate RTC, PWM, SDIO, EMMC (enhanced external memory controller), and digital peripherals and communication resources such as I2S, high-speed SPI, master-slave I2C, USART, USB, CAN, etc. The optimized and upgraded USB2.0 can support the simultaneous use of USB and CAN connections when the system clock is 96MHz, which is conducive to the realization of the diversified control and communication requirements of the oximeter.
Supports three ultra-low-power modes of operation, standby, and sleep. The standby power consumption current is within 3uA, which can effectively extend the life of the battery and achieve long-term standby; the IO drive capacity is up to 25mA, which meets the power consumption and reliability of the oximeter Sexual performance requirements. Three 12-bit A/D converters with 1us conversion time, 12-channel DMA controller, and up to 112 fast I/O ports can quickly acquire data and improve the sensitivity and accuracy of oximeter measurement.