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Connectivity Among Major Analog Design Considerations in Health Wearables

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Development of very low powered analog body sensors, digital microcontrollers, and innovative power and battery management circuits are driving the growth of wearable healthcare products. Applications have expanded from activity tracking to continuous monitoring of blood-oxygen levels, blood-glucose levels, body temperature, and more. Nearly all of the human body signals traditionally monitored in a clinical environment can now be collected by a wearable product, very often with close to the same level of precision and at much lower price points.

No surprise, then: According to projections by the market research firm IHS, global wearable product shipments are expected to exceed 200 million units in 2019—doubling their volume over a six-year period.

Nonetheless, a number of issues related to reliability and accuracy still must be addressed before wearables become ingrained into the daily lives of even more people. These devices need to be highly reliable, as readings may be used for lifestyle adjustments or as an early warning sign of illness. To do so, biosensors must be designed to overcome measurement challenges stemming from factors such as a rugged environment, sweat, motion, and ambient light.

The Right Connectivity

A key requirement for any wearable device is connectivity. Seamless wireless connectivity has pretty much become a given for today’s wearables. Wireless transfer allows data transmission to larger display screens or to remote data collection facilities. Low-power Bluetooth (BLE) is an emerging standard for this purpose. In addition, near field communication (NFC) provides limited-range wireless connectivity that is well-suited for short content transfers such as configuration information and logged data retrieval.

When faced with developing a product such as a new fitness band, for example, the engineer needs to consider how much data will need to be transferred, how frequently, and over what range it will be sent. If the quantity of data that needs to be transmitted reaches megabytes, then the designer might well consider using Bluetooth Classic or Wi-Fi.

Range is another determining factor. BLE can typically communicate over 30 meters in line of sight. What is more, use case factors come into play, such as whether the device communicates to a smartphone that forwards data to the cloud for analysis.

Able to Take Some Knocks

Many wearable systems are designed to be worn during sports and other rugged activity. Ruggedness is relative; the requirements for a life-saving device are different from those of an activity monitor worn by a bicyclist.

Reliability under real-world conditions means dealing with environments electronics do not usually have to live in. These components include low-power, analog front end (AFE) solutions for multi-parameter monitoring as well as embedded analog parts such as op amps, current sense amps, filters, data converters, etc., all of which are necessary to interface real-world signals to digital systems.

In particular, the electrical outputs from body sensors have very low magnitude, in the millivolt and microvolt ranges. As such, many of the sensors that are practical for wearable health applications are being combined with amplification and conversion circuits within a single die or package so that they output either a higher-level analog signal or a serialized digital signal.

Example: Dealing with Flicker

Photoplethysmography (PPG) is an uncomplicated and inexpensive optical measurement method often used for heart rate monitoring purposes and pulse oximetry (a test used to measure the oxygen level of the blood) readings. PPG is a non-invasive technology that uses a light source and a photodetector at the surface of skin to measure the volumetric variations of blood circulation.

Unfortunately, in use the optical sensor can pick up ambient light. This can be particularly troublesome as indoor lighting commonly contains a flicker, which can interfere with the PPG signal. Based on where they are in the world, indoor lights may flicker with fundamental frequencies at 50Hz or 60Hz. This rate is close to the frequency at which PPG signals are sampled. Left uncorrected, ambient flicker can produce a different bias offset for each sample.

Diagram: mission of wearable PPG Figure 1: The primary mission of a wearable PPG circuit is to maximize the signal-to-noise ratio (SNR) while conserving expended power. (Source: Maxim ) Diagram: Maxim Max 30112 Figure 2: Simplified block diagram of the Maxim MAX 30112. (Source: Maxim)

To counteract these effects, advanced PPG ICs now have intelligent signal paths. Algorithms, too, have grown more sophisticated. As a result, designers are now able to include PPG in a variety of form factors, including earbuds, rings, necklaces, head and arm bands, bracelets, watches, and smartphones.

Whatever the form factor, wearable sensors must be able to perform reliably while overcoming the effects of common noise and error sources. Environmental noises for PPG sensors typically fall into two major categories: optical and physiological.

Optical noise refers to changing characteristics of the optical path as seen by the sensor that are unrelated to light absorption by the volume of blood observed. Likewise, a physiological change could alter blood flow and volume in the tissue, which in turn changes the PPG signal.

Featuring advanced correlated sampling techniques designed specifically to attenuate any 50Hz/60Hz flickering components, the Maxim MAX30112 heart rate detection solution for wrist applications can mitigate the corruptive effects of flickering on the PPG signal. It operates on a 1.8V main supply voltage, with a separate 3.1V to 5.25V LED driver power supply. The device supports a standard I2C compatible interface, as well as shutdown modes through its software with near-zero standby current, allowing the power rails to remain powered at all times.

Time-Saving Tools

A wearable healthcare device is an autonomous, noninvasive system that performs a specific bio-medical function. These devices track heartbeat, body heat, blood oxygen, and electrocardiogram (ECG) signals. The sensor reacts to some sort of physical input and responds by generating a signal, typically in voltage or current form. This signal is cleaned and smoothed out to make it easier to read, sampled at a reasonable rate, then converted into a signal readable by processors.

With all of these requirements, building a wearable healthcare product can be challenging and time-consuming. Fortunately, tools like Maxim's Health Sensor Platform 2.0 brings the ability to monitor ECG, heart rate, and body temperature to a wrist-worn wearable, saving months in development time. Using tools such as this, almost all the signals that are traditionally monitored in a clinical environment now can be obtained by a wearable product.

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