Healthy Gains Thanks to Innovative Wireless Sensors

Ultra-low-power radio technology is enabling wireless wearable technology featuring long battery life and small form factor for medical and other fields.

In today’s wearable technology, long battery life and small form factor are critical design requirements for wireless sensing and monitoring devices. Radio transceivers embedded within these wearable devices need to support continuous data streaming with extremely low power consumption. This is especially critical for wearable platforms systems that are used in environments where frequent battery replacement would be difficult and impractical. Although systems previously required AA or AAA batteries, many can now operate using +3V coin cell batteries. Making this possible are ultra-low-power short-range radio transceivers whose circuit design has been optimized for power efficiency across several key parameters. With improved ultra-low-power consumption, which allows the use of tiny coin cell batteries, wearable applications can be further reduced in size with the use of radios available in a smaller chip-scale package (CSP).

Radio Requirements for Wearable Technology
Several factors must be considered when selecting a short-range radio transceiver capable of optimizing power efficiency in wearable technology. Power supply voltage is particularly important. Many of today’s sensors can operate from a +3V supply voltage that allows wearables to operate using a single low-cost, readily available coin cell battery. Other key power supply considerations include the ability to maintain transceiver and receiver performance and the use of a low supply current profile without excessive peaks to fit supply impedance.

Another key issue is peak current. All wireless-based sensor networks require operation at a predetermined duty cycle to save power and restrict radio space usage. Low peak current consumption in the radio transceiver reduces constraints on the wireless sensor’s power supply. Low sleep current is also critical for low duty extended battery life.

The choice of frequency also influences power consumption. Available frequency bands within the industrial, scientific, and medical (ISM) radio band are 2.4 GHz or sub-GHz frequencies. The most prevalent 2.4 GHz protocols are Wi-Fi, Bluetooth, and ZigBee. In low-power and lower-data-rate wireless monitoring applications, however, sub-GHz wireless systems offer several advantages, including reduced power consumption and longer range for a given power.

The Friis equation quantifies the superior propagation characteristics of a sub-GHz radio, showing that path loss at 2.4 GHz is 8.5 dB higher than at 900 MHz. This translates into 2.67 times longer range for a 900 MHz radio because range approximately doubles with every 6 dB increase in power. To match the range of a 900 MHz radio, a 2.4 GHz solution would need greater than 8.5 dB of additional power. Sub-GHz ISM bands are mostly used for proprietary low-duty-cycle links and are not as likely to interfere with each other. The quieter spectrum means easier transmissions and fewer retries, which is more efficient and saves battery power.

Choice of the transceiver device package is a key parameter for any miniaturized platform. Not only are package dimensions critical, but so are the pin array configuration and the RF circuit impedance match necessary for the final design. CSP is an ideal platform that allows for miniaturization of the PCB circuit and high-density layouts on both rigid and flexible substrates. Pinouts on the CSP’s ball grid arrays allow for much simpler layouts and simplified RF matching circuits, using fewer components on RF ports. Figure 1 shows an example of a CSP package with a simplified RF circuit with embedded loop antenna.

Figure 1: Simplified RF Circuit Design with embedded loop antenna using CSP package.

Figure 1: Simplified RF Circuit Design with embedded loop antenna using CSP package.

Balancing Power and Performance Through Proper Circuit Design
It is challenging to achieve desired radio performance capabilities while meeting requirements for extremely low power consumption and small package size. The careful choice of radio architecture and building blocks is critical to meeting communication requirements and power consumption mandates.

One example of a solution derived from a careful balance of these trade-offs is the ZL70550 transceiver from Microsemi. Housed in an approximately 2 mm × 3 mm CSP, it has standard two-wire and serial peripheral interfaces for control and data transfer using any standard microcontroller. Combined with the ZL70550 transceiver, the resulting solution can be used to develop a wireless sensor solution that can run continuously from a CR series coin.

The ZL70550 choice is ideal for low-power applications with an ultra-low current of 2.4 mA in RX mode, 2.75 mA in TX mode, and an industry-leading ultra-low sleep current of 10 nA. The ZL70550’s low-power performance enables low-cost button cell or small lithium ion batteries to support continuous data streaming in wearable devices. Figure 2 shows the Microsemi ZL70550 in a low-power application.

Figure 2: Block diagram of a typical wireless sensor based on the ZL70550 transceiver.

Figure 2: Block diagram of a typical wireless sensor based on the ZL70550 transceiver.

With the advent of micro-power batteries combined with advances in ultra-low-power transceiver technology, it is now possible to build smart, flexible wireless sensors. Proper transceiver selection is critical for addressing a variety of key design issues so that wearable wireless devices can perform continuous monitoring of bio-signals for long periods while using a single small battery. Today’s ultra-low-power transceivers deliver a combination of performance and power efficiency by balancing the trade-offs associated with the use of inversion techniques to achieve the highest possible gain from a low current.

Herman-Morales---Microsemi_webHerman Morales is a Technical Business Development Manager for Microsemi Corporation’s Ultra-Low-Power Medical Products. Morales has held key applications and design positions at Skyworks, SiGe Semiconductor, and Magnavox Defense Systems. He holds a Bachelor of Science in Electrical Engineering, with an option in Biomedical and Clinical Engineering from California State University, Long Beach.

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