Pulse Oximeters

Description

Pulse oximetry detects oxygen saturation in the bloodstream. The reading is useful as a general health monitor because the degree of oxygen saturation can be an early detector of medical problems involving the heart and lungs.

Operation is based on the light-absorption characteristics of hemoglobin in the blood. Oxygenated hemoglobin absorbs more infrared light than red light, and deoxygenated hemoglobin absorbs more red light than infrared light. Thus, red and infrared LEDs in the oximeter alternately transmit light, and a photodiode receives the light that is not absorbed. The ratio of the red and infrared light received by the photodiode is used to calculate the percentage of oxygen in the blood. Based on the pulsatile nature of arterial blood flow, the pulse rate and strength are also determined and displayed during the measurement cycle.

The reflected light signal is algorithmically processed to determine the degree of oxygen saturation. Most pulse oximeters display the oxygen level and pulse as they are sensed. Some products provide logging, and some provide wireless communication to and from a host device for a larger display.

Older products used a sensor cable that attached to a base pulse oximeter. Modern designs digitize the signals within a single enclosure, negating the need for a base unit in many cases. A modern design will also optionally feature a standardized wireless interface like low-power Bluetooth to send data to a capture or display device like a tablet or smartphone.

Today, accurate consumer-grade pulse oximeters are moderately priced and come with a variety of features such as data logging. Logging allows monitoring of oxygen levels, for example, to detect sleep apnea, and allows tracking of heart rate over time, which can help determining the existence of some heart conditions.

Please click on the "design consideration," "circuits," and "block diagram" tabs for more information on how to design a modern pulse-oximetry monitor.


From an analog design perspective, the most challenging part of the design is building the light transmitter and detection circuit. Usually two wavelengths of light are used—ultraviolet and red—so two transmitters and two detector circuits are needed. In addition, usually only one of the two LEDs is illuminated at one time; this functionality is built into the control circuit. Once the microcontroller accurately receives the signal, the internal algorithm processes the signals to determine the oxygenation factor and the heart rate.

LED Transmit and Receive Path


As can be imagined, the received signal levels are very small, so the design requires very accurate and precise detection circuitry. Today, most high-volume pulse ox sensors utilize highly integrated circuitry that contains both transmitting LEDs and photodetectors for both wavelengths. Sensors are available from multiple sources that provide complete detection and conversion to digital within a single IC. Unless a design has requirements beyond those of the standard pulse ox application, it is best to select an integrated light sensor with a digital output.

For those who need to design their own sensor signal chain, the following diagrams can provide some guidance.

Figure 1. Simplified red and infrared LED bias and control circuit.

This driver design allows easy setting of the drive currents by setting a DAC. The two phase signals are digital on and off to control turn the LEDs on and off. For more information, refer to Application Note 4671: Improve Sensor Performance and SNR in Pulse Oximeter Designs.

Figure 2. Simplified photodiode receive path circuit.

In Figure 2, the key specifications for the TIA amplifier are extremely low input current, input current noise, and input voltage noise, as well as high-voltage operation. These characteristics are necessary to maximize the SNR so that the small currents of interest can be measured amid the large ambient currents. High-voltage operation means that a larger feedback resistor can be used to easily amplify the ambient and received LED current before removing the ambient portion with a highpass filter. The remaining small signal of interest is then amplified to maximize the ADC's dynamic range. This gain stage should be programmable to compensate for changing environmental factors and the aging of optical components.

The key specifications for the ADC are high SNR and sample rate. The sample rate should be fast enough to capture the modulated signal and perform the required digital processing, which is different for each manufacturer.

Because the optical properties of an LED can drift over time, it is important that a pulse oximeter have a built-in method of calibration.

Product Selection

The MAX30100 is an integrated pulse oximetry and heart-rate monitor sensor solution. It combines two LEDs, a photodetector, optimized optics, and low-noise analog signal processing to detect pulse oximetry and heart-rate signals.

View the block diagram for product recommendations.

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Microcontroller


The microcontroller selection will depend on the degree of analog functionality and precision needed, which will depend on the sensor signal-chain construction—whether it is a single-chip sensor with a digital output, or a proprietary sensor with a custom analog signal chain. If the input from the sensor is serial digital, then a lower cost processor similar to a low–power, 32-bit ARM controller would suffice. Other functions to consider include display driving capability if a display is included in the device.

Product Selection

The MAX32600 microcontroller is based on the industry-standard ARM® Cortex®-M3 32-bit RISC CPU operating at up to 24MHz. It includes 256KB of flash memory, 32KB of SRAM, a 2KB instruction cache, and integrated high-performance analog peripherals. This product is a good fit if very precise analog processing needs to be integrated onto a single microcontroller to meet product size requirements.

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Power


Depending on the design, the power will be supplied by a single-cell Li+ battery for mobile or wearable devices, or a more traditional power from line via a "wall wart" type transformer.

For a mobile or wearable device, the power supply will consist of either battery only (replaceable ) or rechargeable batteries combined with a Micro-USB port for recharge power and for data.

In both cases the power rails are supplied through either individual step-down, step-up, or LDO voltage converters.

Product Selection

The MAX14676/MAX14676A ICs are battery-charge-management solutions ideal for low-power wearable applications. These devices include a linear battery charger with a Smart Power Selector™, ModelGauge™ fuel gauge, and several power-optimized peripherals. They feature an ultra-low-power buck regulator with a quiescent current of 900nA (typical) and 74% efficiency with 10μA output.

For more design information, please review the application notes specified to the right, view the circuits listed under the "circuits" tab, and view the block diagrams listed under the "block diagrams" tab.

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RTC Integrated Pulse OX Sensor µC Buttons Backlight ESD Protection Multi-Rail PMIC with Battery Management Battery Authentication
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