• The wireless sensor nodes are the central element in a wireless sensor network (WSN).
• It is through a node that sensing, processing, and communication take place.
• •It stores and executes the communication protocols and the data-processing algorithms
• The quality, size, and frequency of the sensed data that can be extracted from the network are influenced bythe physical resources available to the node
• Therefore, the design and implementation of awireless sensor node is a critical step.
The node consists of sensing, processing, communication, and power subsystems.
The designer has a plethora of options in deciding how to build and put together these subsystems into a unified, programmable node.
• The processor subsystem is the central element of the node and the choice of a processor determines the tradeoff between flexibility and efficiency –in terms of both energy and performance.
• There are several processors as options: microcontrollers, digital signal processors, application-specific integrated circuits, and field programmable gate arrays
Sensor node is shown in the figure 1.
Figure 1: Architecture of a wireless sensor node
• Some sensors have their own built-in ADC which can be directly connected with the processor through a standard chip-to-chip protocol.
• Most microcontrollers have one or more internal ADCs to interface analog devices.
I. The Sensing Subsystem
• The sensing subsystem integrates one or more physical sensors and provides one or more analog-to-digital converters as well as the multiplexing mechanism to share them.
Nowadays, there are a plethora of sensors that measure and quantify physical attributes at a cheap price.
• A physical sensor contains a transducer, a device that converts one form of energy into another form of energy, typically into an electrical energy (voltage).
• The output of this transducer is an analog signal having a continuous magnitude as a function of time.
• Therefore, an analog-to-digital converter is required to interface a sensing subsystem with a digital processor.
Note: Serial Peripheral Interface SPI and Inter-IC bus
Analog-to-Digital Converter
The analog-to-digital converter (ADC) converts the output of a sensor which is a continuous, analog signal –into a digital signal. This process requires two steps:
1. The analog signal has to be quantized (i.e., converted from a continuous valued signal into a discrete valued signal; discrete both in time and magnitude).
• The most important decision at this stage is to determine the number of allowable discrete values.
• This decision in turn is influenced by two factors: (a) the frequency and magnitude of the signal; and (b) the available processing and storage resources.
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2. The sampling frequency. In communication engineering and digital signal processing, this frequency is decided by the Nyquist rate.
• In wireless sensor networks, however, the Nyquist rate does not suffice. Oversampling is required because of noise.
• The prevailing consequence of the first step is the quantization error while the second isaliasing
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