The analogue subsystem - 2:

Sensors are used to measure physical parameters by producing an output current or voltage signal that represents that parameter. For example, a thermocouple produces a voltage that is proportional to the temperature at the junction of two dissimilar metals. Similatily, a piezo-resistive strain gauge produces an output voltage that is proportional to strain on a resistance element, which in turn is proportional to an applied displacement, force or pressure. While the number of different sensors is large and varied, there are only a few different forms of sensor output circuit configuration. These forms must be properly matched to the input of the eireuit that follows the sensor, or trouble will result.