Threshold and detectivity
No sensor will respond to arbitrarily small signals. Signals in the range between zero and the sensor threshold min x will not cause the output of the sensor to change. The existence of a threshold is related to nonlinearity and noise. A stimulus which is too small for the output to exceed the noise floor is considered to be smaller than the threshold. Nonlinearity can play a role as well. Consider an enhancement mode MOSFET as a voltage sensor (MOSFETs are used as very high impedance voltage or charge probes in high end “active” oscilloscope probes). Clearly such an instrument cannot respond to voltages smaller than the MOSFET threshold voltage. A sensor will also fail to respond to stimuli which are arbitrarily large. A sensor will necessarily have a range or a full scale max x . The full range of a sensor can be limited by compression or by clipping. (Note that clipping is an extreme example of compression!) Since both compression and clipping are manifestations of nonlinearity we conclude that all sensors are non-linear.
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