Signal conditioning often consists of converting an analog signal from a sensor in the physical world into a digital signal that can be used for data collection, controlling a process, performing calculations, producing display readouts, and other purposes. Analog sensors measure many types of physical phenomena such as temperature, pressure, force, flow, motion, position, pH (acid/base), and light intensity. Sensor signals usually can’t be converted directly into digital data because their outputs are relatively small voltages, currents, or resistance changes that must be conditioned before being converted into digital data. Conditioning takes the form of amplifying, buffering, or scaling the analog signal to make it suitable for input to an analog-to-digital converter (ADC). That ADC then digitizes the signal and sends it to a microcontroller or other digital device for further data processing within the system. The key to making this chain work is selecting operational amplifiers that interface properly with the various types of sensors used in measurements. A designer must then pick an ADC that has the characteristics to process the signal from the input network and produce a digital output that satisfies the resolution, accuracy, and sampling rate of the data acquisition system.
Sensors—The Starting Point
Sensors are classified according to the type of physical variable they measure: thermocouples, resistance temperature detectors (RTDs), and thermistors measure temperature; strain gages measure pressure or force; pH electrodes measure the acidity or alkalinity of a solution; and PIN photodiodes are used as photodetectors to measure light intensity. Sensors can be further classified as active or passive. An active sensor requires an external source of excitation (voltage or current), whereas a passive sensor generates its own electrical voltage without excitation. Common active sensors are RTDs, thermistors, and strain gages, while thermocouples and PIN diodes are passive sensors. To determine what specifications are necessary for the amplifier to interface with a sensor, a designer must consider the major sensor characteristics shown to the right