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Pulse Oximeter design solution from ON Semiconductor

Breath in…Breath out…21% of air around us consists of oxygen. But have you ever wondered why we need oxygen? The simplest answer is every organ in your body will not function without it, heart, brain and liver being the most important consumers. Hemoglobin is a protein in your red blood cells that carries oxygen molecules to organs, tissues and cells of your body.

Oxygen saturation, or sometimes called as O2, is a term used to describe how much of hemoglobin is saturated with oxygen. Pulse oximetry is often applied as a non-invasive testing method for monitoring oxygen saturation level in the arterial blood. Pulse oximeters tend to be implemented as a part of comprehensive vital sign monitoring equipment at hospitals, but recently they became more available as stand-alone medical and fitness devices. 

In this article, we present pulse oximeter design solution from ON Semiconductor. The system diagram illustrated below will help us to highlight some of the important aspects of the oximeter design.

ON Semiconductor Pulse Oximeter Design – Block Diagram

Transmitter and Detector

Oxygen content in the bloodstream is detected by transmitting and detecting light through a thin part of the body, such as finger, ear or, in case of new born babies, toes. Transmitter circuit usually consist of a wavelength switch, which is used to alternate between red and ultraviolet waves, and a LED driver. A linear constant current regulator (CCR) is a simple, yet robust solution for regulating current in LEDs. NSI45025 (737-7981) is a CCR that is designed to have a slightly negative temperature coefficient, which means that the current through the device decreases as the temperature increases. This ensures that LEDs that are in series with the CCR are protected from overcurrent.

The amplitude of the received signal from LEDs is very small since most of the transmitted light is absorbed by tissues and bones. On top of that, ambient light noise present in the received signal challenges further decoding of the oxygenation rate. Therefore, the signal, containing both signal of interest and the ambient noise, needs to be amplified, and then ambient portion can be removed using a high pass filter. A good product recommendation for amplifier would be NCS2001 (787-8800) low voltage operational amplifier with a rail-to-rail common mode input voltage range and with rail-to-rail output drive capability. The amplifier has a 1.0 MHz gain bandwidth product, 2.2 V/us slew rate and is operational over a power supply range less than 0.9 V to as high as 7V.

The signal chain for detection is completed by implementing an ADC with dedicated voltage reference component. LM285 and LM385 series of micropower voltage regulator diodes are excellent low-post options that are available in 1.235V and 2.5V versions. The LM285 is specified over a −40°C to +85°C temperature range while the LM385 is rated from 0°C to +70°C. For space saving alternative, consider NCP51460 (719-2642) high performance, low power precision voltage reference that does not need an output capacitor to demonstrate a stable performance.

Processor/Memory

8-bit core microcontroller will be sufficient for implementation of a basic pulse oximeter, though 16/32-bit would be preferable for added functionality such as data manipulation or driving the display. The  Bluetooth® 5 certified RSL10 radio (172-3391) features an Arm® Cortex®-M3 processor for clocking speeds up to 48 MHz. The radio also provides low-power Bluetooth Low Energy connectivity to smartphones, enabling cloud services. EEPROM memory can be used to store sensor calibration coefficients and user data in pulse oximeter devices. ON Semiconductor offers a wide range of low-power serial EEPROM devices with densities from 1kb to 2Mb.

I/O Peripherals and Protection

The oxygen content data can be send to computer via USB, RS-232 or Ethernet interfaces. ON Semiconductor offers protection devices from common hazards such as electromagnetic interference (EMI), electrostatic discharge (ESD) and overvoltage /overcurrent. NUF2101 (805-2021) and NUF2042 (805-1879) EMI filters with integrated ESD protection are intended to be used in downstream and upstream USB 1.1 ports, respectively. NUP4304 (802-4310) low capacitance diode array is an ideal option for ESD protection of data lines in USB 1.1 and 2.0 applications. ESD5B (791-6038) series of micro-packaged diodes offers a compact solution for protecting sensitive components from ESD in portable applications. NUP5120 (178-7634) diode array can protect up to 5 lines from surge and ESD. NCP360 (805-1992) positive overvoltage protection circuit is designed to withstand up to +20V, whereas NCP361(802-1632) has also the capability of protecting the equipment when overcurrent occurs.

User Interface

Depending on the size of end device, small to medium sized LED or LCD display can be placed to show the oxygen content data in real-time. Backlight LEDs will have to be added if LCD display is selected. ON Semicoductor offers a wide range of backlight LEDs in both series and parallel LED configurations. CAT4137(792-2067) CMOS boost converter can drive up to 5 white LEDs in series providing uniform brightness and matching. Alternatively, CAT4238 (789-3859) is perfect for driving larger LCD display panels with 10 white LEDs in series. The series connected LEDs requires a bulky inductor, hence, you might want to try charge pump-based configuration with LEDs connected in parallel. CAT3604V (789-3812) and CAT3648 (789-3824) are high efficiency fractional charge pumps that can drive up to four LEDs and provide dimming capabilities. The latter supports 32 distinct dimming levels via 1-wire EZDim™ pulse-dimming interface.

Battery and Power

The device can be powered through an external power source, but also use rechargeable backup batteries. Standard rectifiers can be used to build battery charger circuit. ON Semiconductor offers 1N400x  and 1N540x rectifiers with average rectified forward current of 1A and 3A, respectively.

Power from both sources needs to be managed to ensure that energy is saved when device is not in use. Load switches can be used to connect and disconnect the load from the source. NCP4545 (821-5281) load switch with integrated delay and slew rate control provides component reducing solution for power management applications.

Voltage regulation is another crucial aspect of power management. Regulated and controlled voltage provided by voltage regulator ensures that various circuits inside the electronic devices are being supplied with constant voltage. Low dropout voltage regulators (LDO) are low cost and small size option for voltage regulation in battery powered devices. ON Semiconductor LDO regulators with ultra-high-power supply rejection ratio (PSRR) are available in different output current ratings:  NCP110 (185-8036) – 200mA, NCP163 (186-1292) – 250mA, NCP161 (161-2593) – 450mA, NCP133 (842-7854) – 500mA, NCP167 (186-1293) – 700mA. Alternatively, step-down (buck) switching regulators can be used for more efficient, yet high output current implementation. LM2595 (805-1324) , NCP1597B (786-2442) and NCP1599 (7919683) are good options for synchronous buck converters that operate at from 1A to 3A output current.

Same as the damage that occurs in the electronic device due to overvoltage, undervoltage or out of specification voltage at the rail might lead to performance issues or system failure. The supply rail of LDOs or DC-DC converters needs to be above a certain threshold voltage before it can power up correctly. NCP300 (806-0732) voltage detectors can be utilized to monitor the voltage level at the supply rail and reset the voltage regulator circuit in case it does not reach pre-specified threshold level.

Are you involved in medical device design and manufacture?

Let us know in the comments if you are involved in the response to the COVID-19 response in pulse oximeter design.

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6 May 2020, 15:50