4 - Sensor Characteristics

Abstract

A sensor can be regarded as a black box where the important thing is the relationship between the input signal and the output signal. The theoretical relationship between input and output signals in a sensor is established through a transfer function. Other characteristics of sensors include accuracy, calibration error, hysteresis, nonlinearity, saturation, repeatability, dead band, resolution, output impedance, excitation, dynamic range, and reliability. Sensors and their associated circuits are used to measure various physical properties, such as temperature, force pressure, flow position, light intensity, and electric and magnetic fields. The sensor output must be conditioned and processed to provide a valuable measurement. Therefore, sensors must have their output signal conditioned with signal conditioners and several analog or digital signal processing circuits. They are of two types: active and passive. Digital techniques are becoming more and more popular in processing sensor outputs in data acquisition, process control, and measurement. Resistive elements are some of the most common sensors. Resistive elements can be made sensitive to temperature, strain, and light. The challenge of selecting the right amplifier for a particular signal-conditioning application is complicated by the wide selection of processes, architectures, and such amplifiers. Three noise sources in an op-amp circuit include: voltage noise, current noise, and the Johnson noise. Many sensors have output impedances greater than several mega ohms and the associated signal conditioning circuits must be designed to meet the requirements of low bias current, low noise, and high gain. Examples of high-impedance sensors include photodiode preamplifier, piezoelectric sensors, charge output sensors, and charge-coupled devices.