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handbook of modern sensors

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5.9 Noise in Sensors and Circuits 209<br />

Fig. 5.46. Types <strong>of</strong> noise: (A) noise-free signal; (B)<br />

additive noise; (C) multiplicative noise.<br />

(A)<br />

(B)<br />

(C)<br />

Multiplicative noise affects the sensor’s transfer function or the circuit’s nonlinear<br />

components in such a manner as the V s signal’s value becomes altered or modulated<br />

by the noise:<br />

V out =[1 + N(t)]V s , (5.78)<br />

where N(t) is a function <strong>of</strong> noise. An example <strong>of</strong> such noise is shown in Fig. 5.46C.<br />

Multiplicative noise at the output disappears or becomes small (it also becomes additive)<br />

when the signal’s magnitude nears zero. Multiplicative noise grows together<br />

with the signal’s V s magnitude. As its name implies, multiplicative noise is a result <strong>of</strong><br />

multiplication (which is essentially a nonlinear operation) <strong>of</strong> two values where one<br />

is a useful signal and the other is a noise-dependent value.<br />

To improve noise stability against transmitted additive noise, quite <strong>of</strong>ten <strong>sensors</strong><br />

are combined in pairs; that is, they are fabricated in a dual form whose output signals<br />

are subtracted from one another (Fig. 5.47). This method is called a differential technique.<br />

One sensor <strong>of</strong> the pair (it is called the main sensor) is subjected to a stimulus<br />

<strong>of</strong> interest s 1 , while the other (reference) is shielded from stimulus perception.<br />

Since additive noise is specific for the linear or quasilinear <strong>sensors</strong> and circuits,<br />

the reference sensor does not have to be subjected to any particular stimulus. Often,<br />

it may be equal to zero. It is anticipated that both <strong>sensors</strong> are subjected to identical<br />

transmitted noise (noise generated inside the sensor cannot be canceled by a differential<br />

technique), which it is said is a common-mode noise. This means that noisy

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