Low Power, Long Range, ISM Wireless Measuring Node(AD)

Ideally, a wireless measurement node is low power, has good range, and is easily interfaced to different sensors. Through the combination of three Analog Devices, Inc., parts, an intelligent measurement node with an average current consumption of <70 ?A, a range of almost 1 km (in free space), and a data rate of one transmission/minute can be achieved, while also main-taining a 16-bit ADC performance (see Figure 1). This makes the circuit suitable for battery power and such applications as automation and remote sensing. The system consists of a low power temperature measurement node that wakes once a minute, measures temperature, transmits this measurement at 10 kbps to the base node, and then returns to sleep. The base node continuously listens for a package from the measurement node and sends this information to the PC via the UART for display in HyperTerminal. The ADuC7060 precision analog microcontroller has a low power ARM7 core as well as a myriad of precision analog functions. The onboard multiplexer, digitally programmable gain amplifier (PGA), voltage reference, programmable current sources, and 24-bit sigma-delta ADC allow almost any temperature and bridge sensors to be directly connected. In this case, a 4-wire Pt100 (100 ? platinum RTD) temperature sensor was chosen. Further details on the measuring circuit can be found in the AN-0970 Application Note. The wireless band chosen for this application is the sub-GHz, license-free ISM (industrial, scientific, medical) band. The ADF7020 transceiver, which supports bands in the 431 MHz to 478 MHz as well as the 862 MHz to 956 MHz frequency ranges, is, therefore, a natural choice. This low power transceiver requires very few external components, is easily connected to the ADuC7060 precision analog microcontroller, and offers excellent performance. The ADP121 voltage regulator provides the 2.5 V supply from two 1.5 V batteries. The very low quiescent current of this voltage regulator (11?A at no load) is paramount in maximizing battery lifetime.    Figure 1. Low Power, Long Range, ISM Wireless Measurement Node (Simplified Schematic: All Connections and Decoupling Not Shown)