Tiny Sensor Brings Oximetry to Wearables
Keith Cress posted on January 15, 2016 |
The MAX30102 is an ultra low-power oximeter and heart rate sensor.

Design engineers working on wearables should take note of the new MAX30102. 

This fully-integrated sensor module can perform pulse oximetry (i.e., monitor a person’s blood oxygen level) as well as heart rate when placed against the skin.

(Image courtesy of MAXIM-IC.)
(Image courtesy of MAXIM-IC.)
The device uses the now standard method of comparing hemoglobin oxygen loading by detecting the ratio of infrared and visible red light transmission through the body.

How Blood Oxygen Levels Are Measured

Blood oxygen levels in healthy humans should normally span between 95 and 99 percent loading of the hemoglobin found in arterial blood; that's the blood moving out from the heart and lungs. Making this measurement requires a lot of detailed information.

A typical oxymeter.

A typical oxymeter.

As one's hemoglobin loads up with oxygen, the blood will absorb more infrared light and pass more visible red light.  As the hemoglobin loses its oxygen, it absorbs more visible red light and passes more infrared. 
These two color readings are typically each done many times per second. If one notes the ratios, they can then use tables to determine blood oxygen levels.

One other key issue is that these measurements need to made only for the arterial blood, which carries the maximally oxygenated hemoglobin, and not the venous oxygen-depleted blood. 

Fortunately, the arterial blood always pulsates, as it's coming directly from the beating heart. This is why the technique is called pulse oximetry, as the system needs to see the pulse to know it’s measuring arterial blood. 

Features of the MAX30102 Pulse Ox and Heart Rate Sensor

Some vital statistics of this new sensor are its:

  •  Ultra-low power consumption: running on 1.8V logic power and 5V only for driving the integrated LEDs
  • Small solution size: being a 14-pin optical module (5.6mm x 3.3mm x 1.55mm) which integrates LEDs, photodetectors, integrated glass optics, and low-noise electronics
  • Temperature Range: -40 to +85°C (-40 to +185°F)

A unique feature of this sensor is that it doesn't need to shine clear through a finger or earlobe, it can operate against a wrist, like a watch.  This is a reflective solution.

This MAX30102 communicates through a standard I2C interface and can be dropped to a shutdown power of 0.7uA to help with battery life. It also allows programmable sample rates for dynamic energy savings.

MAXIM has also created a useful evaluation board to help with development needs.  It includes an accelerometer for potential fitness applications.

They've even worked out a robust Motion Artifact Resilience ability.  Motion artifacts are errors in readings caused by blood flow changes not related to pulse but rather from muscle use, like swinging an arm or adjusting a grip.

With 26 million smart garments estimated to ship this year, 2016 looks to be a banner year for wearables. Being able to add oximetry into step counters and other fitness-oriented devices gives users access to a host of new and vital information about their health.

If you're interested in the MAX30102 and looking for the details, register with MAXIM-IC on their website so you can fulfill their NDA requirements and secure a full data sheet.

Check it out here.

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