Signal-To-Noise Ratio
In a vast majority of applications, one is required to measure physical quantities that change with time. When a transducer is used, the primary variable in the experiment is converted to an electrical voltage. The use of a voltage is particularly convenient because of the ease with which it can be manipulated with respect to filtering, amplification, differentiation and integration, correlation, digitization and graphical display. A certain class of transducers produce charge and current as their output. In practice, these are also converted to a voltage before the data is recorded.
The set of voltage values as a function of time that corresponds to the primary process variable is called the signal . The voltage values acquired by the hardware is seldom identical to the signal and the difference between them is called noise. Noise in measurements arises from ambient disturbances such room vibration and air currents, the finite size of the transducer and material inhomogeneities in it, and noise transmitted from the electrical mains supply. For a meaningful measurement we require the signal-to-noise ratio  to be large. Here  and  are measures of the signal and noise respectively. A typical working value of  is 10; a value of 0.1 is considered poor.
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