Pulse rate and blood-oxygen (SpO2) levels are easily measured by the hardware, software, and algorithms of the MAXREFDES117# reference design
Wearable devices for personal health and wellness are a fast-growing market. There are the wristband devices (typified by Fitbit and Garmin, among others), but they are only a starting point. There's also tremendous interest in more advanced wearable devices for cardiac monitoring of heart rate and blood-oxygen (SpO2) levels, as these provide significant insight into heart function and health. For maximum insight, this monitoring needs to be done 24/7 (or nearly so), during both activity and rest states.
What's interesting is that measurement of heart rate and blood-oxygen levels presents two very different situations, but they are converging. Obviously, heart rate is a non-invasive, easy-to-measure parameter, at least in principle: all you need is access to the subject's wrist, a watch to measure the time, and perhaps someone else to take the reading. However, despite the apparent ease of doing it, it can’t be done continuously, though it can be done frequently – but the subject has to be in a sitting (quiescent) mode.
Blood-oxygen readings present a very different story. [Oxygen saturation refers to the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood.] Until the 1970s, it could only be measured using the invasive procedure of taking a blood sample and then sending it to a lab. This is clearly an unpleasant and even risky scenario in many ways, and obviously limits the frequency of updates (and the cost is substantial, too). However, using optical attenuation readings through the skin and blood vessels at two different wavelengths, it is possible to determine SpO2 levels accurately, continuously, and in real time.
Why Consider a Reference Design?
If all that was needed for an effective pulse oximetry system was a few LEDs, photosensors, and ICs, it would seem to be a fairly simple design and the end of the story. However, the reality is far more complicated, as is almost always the case in most real-world test and measurement situations, especially if it is a medical application.
That's why Maxim developed the MAXREFDES117# reference design, a low-power, optical heart-rate module which includes integrated red and IR LEDs and a power supply. This tiny board (just 0.5in × 0.5in/1.25cm × 1.25cm, Figure 1) can be placed on a finger or earlobe to accurately detect heart rate. It includes a heart-rate/SpO2 sensor (MAX30102), a step-down converter (MAX1921), and an accurate level translator (MAX14595), Figure 2.
What are the challenges of heart-rate sensing that the MAXREFDES117# reference design addresses? First, the basic sensor data is corrupted by noise (such as minute user movement) as well as other aberrations. Further, the relationship between light transmission and the two parameters being measured–especially SpO2–is not a simple table look-up or curve fitting between data and final answers. Instead, it is quite complicated, with calibration, compensation, and determination of some equation constants, plus allowance for component variations, temperature coefficients, and other factors. Even test and verification of results is a challenge.
But compact, complete hardware is only part of the needed design. This module works with both Arduino and the ARM mbed platforms for quick testing, development, and system integration. It also includes a basic, open-source heart-rate and SpO2 algorithm in the example firmware.
The inclusion of the heart-rate algorithm, which users may modify and improve as desired, is a major part of the reference design's story. In the AACN Procedure Manual for Critical Care, Sandra L. Schutz details why the path from raw sensor reading to meaningful, accurate readout is a complex and subtle one. It depends on the application scenario (such as sedate v. active), which is why the algorithm itself may need to be fine-tuned by the OEM; a product for an athlete in motion is different in form factor, user interface, and even algorithm specifics than one designed solely for a subject at rest. These scenarios fit under the category of motion rejection. Admittedly, MAXREFDES117# does not have an accelerometer nor subsequent motion rejection.
To get the reference design operational, the user has to do one simple thing: just add a 2V to 5.5V supply; dissipation is around 5mW, with current demands in the low single-digit mA range. The BOM, schematics, layout files, and Gerber files are all available from the Design Resources tab at Maxim's site. If that's not enough, the reference design's boards are even available for purchase from Maxim.
The Maxim reference design is so complete that users can package it as-is in their final product, thus avoiding the need to "reinvent the wheel" (so to speak) of a basic, ready-to-go design for a photoplethysmogram (PPG-optical)-based heart-rate/SpO2 sensor. Alternatively, they can use it as a part of a larger medical device, which simultaneously senses, measures, and monitors multiple health and activity parameters.