Effects of circuit noise

All signal detectors are required to detect the signal against a background of noise . Therefore, the signal-to-noise ratio must be optimized or, put another way, for maximum sensitivity the noise has to be minimized. The sensitivity of any detector is determined by the noise level in the amplified output signal. In the case of a pyroelectric detector and its associated circuitry, the principal sources of noise are Johnson noise, amplifier noise and thermal fluctuations. 

The noise level can be expressed in terms of the power incident on the detector necessary to give a signal equivalent to the noise. If the noise voltage is A EN then the noise equivalent power (NEP) is defined by 

Because it is preferable to have a quantity that increases in value as the performance of the system is improved, the detectivity D is defined as 

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