Detectors noise equivalent input power

The achievable signal-to-noise ratio Vs/Vn, which is, in principle, limited by the noise of the incident radiation. It may, in practice, be further reduced by inherent noise of the detector. The detector noise is often expressed by the noise equivalent input power (NEP), which means an incident radiation power that generates the same output signal as the detector noise itself, thus yielding the signal-to-noise ratio S/N = 1. In infrared... 

This is 10" to 10 times higher than the noise equivalent input power of standard IR detectors. 

The detectivity D [cm/s / -W ] gives the obtainable signal-to-noise ratio Vg/Vn of a detector with the sensitive area A and the detector bandwidth Af, at an incident radiation power of P = 1 W. Because the noise equivalent input power is NEP = P V /Vg the detectivity of a detector with the area 1 cm and a bandwidth of 1 Hz is D = 1/NEP. 

The fundamental performance parameter of any detector is its noise equivalent power (NEP). This is simply the input irradiance power necessary to achieve a detector output just equal to the noise. This NEP is dependent on a number of detector and signal variables such as modulation frequency and wavelength (the input signal is defined as sine wave modulated monochromatic light), and detector area, bandwidth and temperature. 

It is evidently insufficient to consider only the response of a detector when analysing its usefulness for a particular application. It is generally necessary to analyse both intrinsic and extrinsic noise signals and compare them with the response. The result of this comparison can be expressed in many different ways. One of the most useful is the noise-equivalent power nep which is the power of an rms signal input (in watts) required to give a response equal to the total rms noise voltage AVN. Then ... 

Typical detectivity values as a function of wavelength for PbS photoconductive and various photovoltaic detectors. is a figure of merit defined as A /NEP, where A is the detector area and NEP is the noise-equivalent power, the rms radiant power in watts of a sinusoidally modulated input incident on the detector that gives rise to an rms signal equal to the rms dark noise in a 1-Hz bandwidth. Data from Hughes Aircraft Company. 

This signal should be larger than the noise equivalent power NEP (this is the input power of the detector which gives the same detector output as the noise). 

Noise equivalent power, NEP That optical power which produces a detector output current equal to the total noise current. A detector with input power, NEP, produces an output signal-to-noise ratio of unity. 

The major noise sources for a typical pyroelectric detector are the dielectric or Johnson noise, the amplifier current and voltage noise, and the thermal noise, caused by fluctuations in the power flow from the element to its heat sink. Each of these has an equivalent voltage generated at the amplifier input V y (given by equation (5.9)), and and Vj. respectively. These combine to give the total equivalent input noise according to the equation... 

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