Solutions Evolve for Medical Wearable Designs

MCUs, Analog ICs and Sensors

With extreme low-power and wireless connectivity as leading design needs, mobile and wearable medical devices continue to evolve. Solutions for designers have taken shape in the form of microcontrollers, analog ICs and sensors.

To keep pace with those demands, makers of microcontrollers (MCUs), analog ICs and sensors have rolled out a new crop of solutions over the past 12 months, each designed for the needs of mobile and wearable medical systems.

BLOOD-PRESSURE MONITORING
Exemplifying these trends, in November Maxim Integrated introduced a solid-state blood-pressure monitoring solution to more conveniently track this critical health indicator. Until now, accurate blood-pressure monitoring could only be achieved with bulky and mechanical cuff-based medical devices. Design engineers can develop blood-pressure trending solutions with Maxim’s MAXREFDES220# reference design.

Comprised of a complete integrated optical sensor module, an MCU sensor hub and a sensing algorithm, the MAXREFDES220# optical blood-pressure trending solution can be embedded into smartphones or wearables (Figure 1). It enables users to place their finger on a device for 30-45 seconds while resting to measure blood pressure anywhere and anytime. The reference design includes the MAX30101 or MAX30102 high-sensitivity optical sensor, as well as the MAX32664D sensor hub IC with built-in algorithms.

The MAX30101 (using three LEDs) and MAX30102 (using two LEDs) pulse-oximeter and heart-rate optical sensor modules combine photodetectors, LEDs and analog front-end (AFE) electronics with an integrated cover glass. This integrated optical design and lenses-on-top approach enables optimal signal-to-noise ratio (SNR) for a small, lower cost and power-efficient module. It comes in a tiny 5.6mm × 3.3mm 14-pin optical package with an I²C interface to the sensor hub IC.

The MAX32664D sensor hub IC includes firmware, which controls the sensor and executes the algorithms that measure cuffless blood pressure, heart rate and blood-oxygen levels. Its ultra-small size (1.6mm × 1.6mm) can easily fit into small devices to seamlessly and effortlessly connect to a device’s host microcontroller.

The MAXREFDES220# provides industry-best accuracy that meets Class-II regulatory limits. For example, under resting-only measurement conditions, the solution provides the following accuracies: Systolic Error: Mean = 1.7mmHg, Std Dev = 7.4mmHg; Diastolic Error: Mean = 0.1mmHg, Std Dev = 7.6mmHg. For reference, Class-II regulatory limits are |Mean Error| ≤ 5mmHg, and Std Dev ≤ 8mmHg. This solution requires calibration every four weeks to maintain the above accuracies. This is based on independent testing and a limited number of subjects at resting condition.

SWITCH IC FOR ULTRASOUNDS
Ultrasound systems require high levels of resolution and high-integration to facilitated portable design requirements. Meeting these needs, in October, STMicroelectronics (ST) released the STHV64SW, a 64-channel high-voltage analog-switch IC offering high integration for advanced ultrasound systems, probes and other applications.

The IC comprises a shift register for logic control signals, self-biased high-voltage MOSFET gate drivers and N-channel MOSFET switches capable of providing up to ±3A peak output current (Figure 2). The switches respond quickly, with 1.5µs turn-on time, and low quiescent current saves power when turned off. Low on-resistance with low distortion and crosstalk ensure high signal integrity. Thermal shutdown and under-voltage lockout (UVLO) are built-in to ensure safe operation.

ST says it created this advanced device leveraging proprietary BCD6s-SOI (silicon on insulator) and BCD8s SOI process technologies to combine precise analog circuitry (Bipolar), low-voltage CMOS logic, and robust DMOS power stages on the same chip. The STHV64SW can operate with various combinations of high-voltage supplies up to -100V/+100V, 0V/200V, or -200V/0V.

According to the company, The STHV64SW has already been designed into innovative high-tech equipment such as ultrasonic flaw detectors for industrial non-destructive testing (NDT), as well as into affordable and portable handheld medical echography devices that are raising standards of pre-natal care available to remote and rural communities.

The highly integrated 64-channel analog-switch IC comes in a small FCBGA (flip chip ball grid array) package as does the recently announced companion 16-pulser STHV1600 IC, enabling system manufacturers to increase channel density for superior ultrasound image resolution, using minimal board space. The STHV64SW is packaged as a BGA-196 device.

HEALTHCARE SENSORS
Last summer, Maxim rolled out a pair of sensors aimed specifically at medical wearables. The MAX30208 is a clinical-grade digital temperature sensor that enables new wearable health and fitness use cases at half the power. And the MAXM86161 is an in-ear heart-rate monitor provides best-in-class SNR at lowest power and 40% less space for continuous heart-rate and SpO2 measurements, according to Maxim (Figure 3).

To provide value, wearable health and fitness monitors require greater accuracy in measuring human biometrics such as body temperature and heart rate, but device designers have been limited by sensor accuracy for small, battery-powered, body-worn devices. Maxim’s two new continuous-monitoring body sensors provide higher degrees of accuracy in measuring vital signs such as temperature, heart rate and blood-oxygen saturation (SpO₂).

The MAXM86161 in-ear heart-rate monitor and pulse oximeter is the market’s smallest fully integrated solution that delivers highly accurate heart-rate and SpO₂ measurements from hearables and other wearable applications, says Maxim. It is optimized for in-ear applications with its small package size and best-in-class SNR—3dB improvement with a band limiting signal for PPG (photoplethysmography) use cases compared to closest competitor, according to the company. This enables development of devices that cover a wider range of use cases. MAXM86161 delivers approximately 35% lower power to extend battery life of wearables. In addition, an integrated analog front-end (AFE) eliminates the additional AFE typically needed to procure a separate chip and connect to the optical module.

CLINICAL GRADE MEASUREMENT
The MAX30208 digital temperature sensor delivers clinical-grade temperature measurement accuracy (±0.1°C) with fast response time to changes in temperature. It also meets the power and size demands of small, battery-powered applications such as smartwatches and medical patches. It simplifies the design of battery-powered, temperature-sensing wearable healthcare applications. Easier to use than competitive offerings, it measures temperature at the top of the device and does not suffer from thermal self-heating like competitive solutions. MAX30208 is compatible with up to four I²C addresses to enable multiple sensors on the same IC bus. The MAX30208 can be attached to either a PCB or a flex printed circuit (FPC).

MAX30208 delivers ±0.1°C accuracy in the range of 30°C to 50°C and eliminates thermal self-heating, a factor that affects measurement accuracy in competitive devices. MAXM86161 cancels ambient light for greater accuracy and provides highest SNR (Nyquist SNR is 89 dB; 100 dB SNR with averaging). In addition, Maxim provides algorithms for motion compensation to increase measurement accuracy.

To extend battery life of wearables, the MAXM86161 consumes approximately 35% lower power versus the closest competitor, with less than 10μA operating power and 1.6μA in shutdown mode. Compared to the closest competitive solution, the MAX30208 consumes only half the power (67μA operating current during active conversion vs. 135 μA) under a representative use case. MAXM86161 is available in an OLGA package (2.9mm × 4.3mm × 1.4mm), which is 40% smaller than the closest competitor. MAXM86161 includes three LEDs—red and infrared for SpO₂ measurement and green for heart rate. MAX30208 is available in a 10-pin thin LGA package (2mm × 2mm × 0.75mm).

DUAL-FUNCTION SENSORS
High integration is an ongoing theme for mobile and wearable medical system designs. Part of the game is getting multiple functions on one chip. Along such lines, Infineon Technologies offers its XENSIV DPS368, a miniaturized digital barometric pressure sensor that is capable of measuring both pressure and temperature (Figure 4). It offers an ultra-high precision of ±2 meters and a low current consumption for precise measurement of altitude, air flow and body movements. This makes the DPS368 ideal for mobile applications and wearable devices offering, for example, activity tracking and navigation. Additionally, the sensor can be used in home appliances for airflow control, in drones for flight stability and in medical devices such as smart inhalers.

Due to its robust package, it can withstand operation 50 meters under water for one hour (IPx8) and protects the sensing cells against dust and humidity. As a result, the board handling in an assembly line is also facilitated. The 8-pin LGA package with its small dimensions of 2.0mm x 2.5mm x 1.1mm saves up to 80% space compared to other waterproof sensors, says Infineon.

The pressure sensor element is based on a capacitive sensing principle that guarantees high precision even during temperature changes. The internal signal processor converts the output from the pressure and temperature sensor elements to 24-bit results. Calibration coefficients stored in the sensor are used in the application to convert the measurement results to high accuracy pressure and temperature values. DPS368 provides quick feedback due to a measurement rate of up to 200Hz and fast read-out speed. The integrated FIFO memory can save up to 32 measurement results, allowing for power-savings on system level.

The XENSIV DPS368 sensor features an average low power consumption of 1.7μA for pressure measurements at 1Hz sampling rate. In standby mode, this is reduced to 0.5μA. The sensor operates at pressure ranges from 300 to 1200hPa (hectopascals) and temperature ranges from -40°C to +85°C with a temperature accuracy of ±0.5°C. Sensor measurements and calibration coefficients are available through the serial I²C or SPI interface.

BLE SOC WITH NFC
Low-power SoCs with wireless connectivity have become a key ingredient in many new wearable medical device designs. In January, NXP Semiconductors fed such demands by announcing the availability of its QN9090 and QN9030 Bluetooth 5 SoC with hardware compatible options for 802.15.4, Multiprotocol RF and optional NFC technology.

The QN9090 and QN9030 devices are powered by an Arm Cortex-M4 running at 48MHz and include up to 640KB onboard flash and 152KB SRAM, providing storage space and flexibility for complex applications and safe over-the-air (OTA) updates (Figure 5). The QN Series devices help accelerate development and time-to-market for developers creating products with rich features for diverse IoT applications such as personal healthcare devices, sports and fitness trackers and connected appliances.

As NXP’s BLE (Bluetooth Low Energy) SoC with NFC integrated on chip, the QN9090T and QN9030T variants support out-of-band wireless communications to enable numerous use cases. By tapping an IoT device based on the QN9090T to a smartphone, tablet or other NFC reader device, a BLE connection can be quickly established, greatly simplifying the pairing process. The built-in NFC NTAG eliminates the need for the tag to be powered and creates additional opportunities for diagnostics or device commissioning in stages of the device life cycle.

The devices are well suited to wearable Bluetooth, battery-operated applications, thanks to their 4.3mA Rx current and 7.3mA Tx current at +0dBm. The SoC has a 48MHz Arm Cortext-M4 core, 640KB of embedded flash and 152KB of SRAM. The embedded NFC NTAG reduces system board footprint and cost of manufacturing with digital and analog integration. The devices’ rich set of MCU capabilities includes various low power modes, digital MIC interface with wake up on audio event and Quad SPI NOR flash memory controller for high-density data or code storage.

The 2.4GHz BLE 5.0 transceiver supports 2Mbps and up to 8 concurrent Bluetooth connections with antenna diversity support. Integrated power amplifiers with exceptionally high transmit power (up to +11dBm) to make long-distance transmission possible. A wide temperature range if -40°C to +125°C makes the devices applicable in various environments.

MCU EMBEDDED SECURITY
Embedded security is a critical feature for many healthcare wearable devices. The more such functionality can be integrated on the device’s MCU the better. Along such lines, Renesas Electronics has introduced the RX23W, a 32-bit MCU featuring Bluetooth 5.0 for IoT endpoint devices such as home appliances and healthcare equipment (Figure 6).

By combining BLE  5.0 with Renesas’ Trusted Secure IP on its popular high-performance RX MCU family, Renesas offers embedded system designers an optimized single-chip solution for system control and wireless communication, while also providing a more secure way to answer the Bluetooth security risks such as eavesdropping, tampering and viruses.

The RX23W is based on Renesas’ RXv2 core, which provides high computational performance with improved FPU and DSP functions and operates at a maximum clock frequency of 54MHz. The RX23W provides full BLE support including long-range and mesh networking functions and achieves the industry’s lowest level reception mode peak power consumption at 3.0mA, says Renesas.

Furthermore, it integrates a rich set of peripheral functions that are indispensable for IoT equipment, including security, touch key, USB, and CAN functions. These functions allow the RX23W to implement both system control and Bluetooth wireless functions for IoT endpoint equipment such as home appliances, health care equipment, and sports and fitness equipment on a single chip. In addition, the RX23W’s Bluetooth mesh functions make it optimal for industrial IoT equipment collecting sensor data within a factory or a building.

The RX23W supports long distance communication (long range: 400m), 2Mbps data throughput, and Bluetooth mesh networking, supporting the full functionality of BLE 5.0 Low Energy. Furthermore, the RX23W achieves a 3.0mA reception mode peak current—the industry’s lowest level, according to Renesas—and a reception sensitivity level of -105dBm at 125Kbps.

In addition to a Bluetooth 5.0 basic protocol stack package, Renesas provides API functions that conform to all standard profiles, including a Health Thermometer Profile (HTP), an Environment Sensing Profile (ESP), and an Automation I/O profile (AIOP). These allow users to start prototype development and evaluation quickly and can speed up the user’s development process.

The RX23W integrates Renesas’ Trusted Secure IP (TSIP) as part of its built-in hardware security engine. The TSIP driver uses strong encryption key management with hardware accelerators to securely boot customers’ IoT devices and protect them from security threats. Cryptographic Algorithm Validation Program (CVAP) certification is expected. The RX23W is available now in both 7mm × 7mm 56-pin QFN and 5.5mm × 5.5mm 85-pin BGA packages with 512KB of on-chip flash memory.

TINY BLUETOOTH RADIO
For wearable healthcare devices that need to partition the Bluetooth radio separately, ON Semiconductor offers its RSL10 family of Bluetooth 5 certified radio SoCs with a ready-to-use 6mm × 8mm × 1.46 mm System-in-Package (SiP) module. Supporting BLE wireless profiles, RSL10 devices can be easily designed into any “connected” application including sports/fitness or healthcare wearables, smart locks and appliances (Figure 7).

The RSL10 SIP features a built-in antenna, RSL10 radio, and all required passive components in one complete, miniature solution. Certified with the Bluetooth Special Interest Group (SIG), the RSL10 SIP significantly reduces time-to-market and development costs by removing the need for any additional RF design considerations.

With the 2Mbps speeds possible with Bluetooth 5 alongside low power consumption, the RSL10 family provides advanced wireless functionality without compromising battery application life. RSL10 consumes just 62.5nW while in Deep Sleep mode, and 7mW peak receive power. RSL10’s energy efficiency was validated in 2018 by the EEMBC’s ULPMark where it became the first device in the benchmark’s history to break 1,000 ULP Marks and produced Core Profile scores more than twice as high as the previous industry leader. The RSL10 SIP is offered in a 51-pin 6mm ×8mm ×1.46mm package.

Mobile and wearable medical device designers are hungry for highly integrated, low-power, wirelessly connected solutions to meet their high demands. Makers of MCUs, analog ICs and sensors will continue to address those demands with new innovative products.

RESOURCES
Infineon Technologies | www.infineon.com
Maxim Integrated | www.maximintegrated.com
NXP Semiconductors | www.nxp.com
ON Semiconductor | www.onsemi.com
Renesas Electronics | www.renesas.com
STMicroelectronics | www.st.com

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • MARCH 2020 #356 – Get a PDF of the issue

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