Our approach to instrumentation is to treat sensors as circuit elements. We will call a sensor passive when it requires an external power source, i.e. behaves like a passive circuit element such as resistor, capacitor or inductor. We will call a sensor active when it derives its power from the stimulus which it is measuring. In the circuit an active sensor appears like a signal source, with a Thevenin or Norton equivalent circuit. Please note that there is some confusion regarding the use of the terms active and passive in the instrumentation literature, where the opposite definition is often used, i.e. a sensor is called active when we need to supply power to a sensor in order to use it. We prefer to use a term consistent with the notions of activity and passivity in circuit engineering.