Vital Measurement Parameters

Figure 3. ADXL362 power consumption as a function of the output data rate.

A wearable device is worthless for measuring vital parameters without having a notion of what the human body is doing. For that reason, motion detection and profiling are important. Some use cases like optical heart rate monitoring are very sensitive to motion, and motion can destroy the accuracy of the measurement completely. For that reason, motion also needs to be tracked to compensate for artifacts. Motion sensors will help to track movement and, where needed, motion can be compensated in the final outcome of the readings. The ADXL362 low-power motion sensor has a 3-axis MEMS sensor with an integrated 12-bit ADC to detect motion in the X-, Y-, and Z-axes. The output data rate (ODR) of the ADC represents the power dissipation of the sensor, which is 3 μA at the full ODR of 400 Hz per axis. In Figure 3, a plot of the power dissipation as a function of the output data rate is shown.

This sensor can also be used as a motion activation switch. There is a possibility to reduce the sampling rate to just 6 Hz. Every 150 ms, the sensor wakes up and measures the motion activity. Without motion, it goes straight back to sleep for another 150 ms. At the moment, motion is being detected at a g-force equal to or higher than the preprogrammed threshold level. For at least the minimum time programmed, the sensor generates an interrupt or enables a power switch to turn on the application. With this mode, the sensor is consuming only 300 nA, and can run for years on a single coin cell battery. All the use cases summarized make the motion sensor a must-have in a wearable device.

Temperature sensing is another vital parameter; the GEN II wearable has two temperature sensors embedded. The wrist-worn device uses NTCs to measure both skin temperature and the temperature inside the device — there are multiple methods to measure temperature via sensors contacting the body. The NTCs are powered and conditioned by discrete circuitry and the 16-bit ADC finally converts the signals into the digital domain.

Bringing it all Together

The GEN II device makes use of two processors. This is not absolutely needed but provides more flexibility. The interface board with BLE radio has one processor and the same device is used on the sensor board to be able to run autonomously. The ultra-low-power ADuCM-3029 has been integrated to collect sensor data and run the algorithms.

The core is a 26-MHz Cortex-M3 with a rich peripheral set, onboard memory, and an analog front end. There are four operating modes; in full operation, the chip consumes 38 μA per MHz. If processing power is not needed, the device can run in flexi-mode in which the analog front end is running, peripherals are active, and the measured signals can be stored in memory through DMA. This mode consumes 300 μA, making the chip very attractive for low-power, battery-operated systems. There are several security features embedded for code protection and a hardware accelerator for cryptographic functions.

Selection of Use Cases

The GEN II wearable device can be used for many purposes. The sensors can be integrated in smart watches but the range of functions, including accurate heart rate monitoring and activity measurement/ calorie burn, are also helpful for sport watches. The tradeoff between a smart watch and sport watch is mainly made between accuracy vs. battery lifetime.

The device can be used to measure stress or emotional state. Usually a combination of measurements is used to get a reliable reading such as skin impedance together with heart rate variability and temperature. Blood pressure monitoring is another interesting use case. This is a very important parameter but most of the systems are cuff-based, which are hard to integrate in a wearable and continuous system. There are certain techniques that can be used to measure blood pressure without the need for a cuff. One technology is by making use of the pulse-wave transmit time (PTT). This requires ECG measurement in combination with PPG measurement. The sensors inside the GEN II wearable device can support this.

The last key market is related to elderly care and independent living. There is huge need for systems that can help caregivers monitor certain parameters remotely. This wearable device supports 95% of the features needed. The system monitors several vital parameters. It can track if people are moving or walking but is also able to detect falls. The missing piece in the wearable design is an emergency button but this is a matter of connecting one I/O pin on the processor to a switch on top of the device.

Conclusion

The GEN II device has many high-performance sensors and features embedded in a small, wearable system. Besides the electronic design, many mechanical aspects have also been taken into consideration. This makes the platform very attractive to design companies and device manufacturers focusing on the semi-professional sports market, the medical market, and companies involved in systems for smart buildings, independent living, or elderly care. All parameters can be measured simultaneously but algorithms need to complement the application to support the use cases. Instead of building hardware before testing and validating the algorithms, this device will give developers and device manufacturers a quick start.

This article was written by Jan-Hein Broeders, Healthcare Business Development Manager for Analog Devices’ Healthcare Business in Europe, the Middle East, and Africa. For more information, visit here.