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VTL5C1 Datasheet(PDF) 40 Page  PerkinElmer Optoelectronics 

VTL5C1 Datasheet(HTML) 40 Page  PerkinElmer Optoelectronics 
40 / 76 page 35 Characteristics of Analog Optical Isolators Some major characteristics of Johnson noise are that it is: 1. Independent of frequency and contains a constant power density per unit of bandwidth. 2. Temperature dependent, increasing with increased temperature. 3. Dependent on photocell resistance value. Johnson noise is defined by the following equation: where: INJ = Johnson noise current, amps RMS k = Boltzmann’s constant, 1.38 x 10 23 T = temperature, degrees Kelvin R = photocell resistance BW = bandwidth of interest, Hertz A second type of noise is “shot” noise. When a direct current flows through a device, these are some random variations superimposed on this current due to random fluctuations in the emission of electrons due to photon absorption. The velocity of the electrons and their transit time will also have an effect. “Shot” noise is: 1. Independent of frequency. 2. Dependent upon the direct current flowing through the photocell. Shot noise is defined by the following equation: where: INS = shot noise current, amps RMS e = electron charge, 1.6 x 10 19 Idc = dc current, amps BW = bandwidth of interest, Hertz The third type of noise is flicker of 1/f noise. The source of 1/f noise is not well understood but seems to be attributable to manufacturing noise mechanisms. Its equation is as follows: where: INF = flicker noise, amps K = a constant that depends on the type of material and its geometry Idc = dc current, amps BW = bandwidth of interest, Hertz f = frequency, Hertz Unlike thermal or shortnoise, flicker noise has 1/f spectral density and in the ideal case for which it is exactly proportional to , it is termed “pink noise”. Unfortunately, the constant (K) can only be determined empirically and may vary greatly even for similar devices. Flicker noise may dominate when the bandwidth of interest contains frequencies less than about 1 kHz. In most AOI circuits noise is usually so low that it is hardly ever considered. One notable exception is in applications where large voltages are placed across the cell. For a typical isolator, it takes 80 to 100V across the photocell before the noise level starts to increase significantly. Distortion Analog Optical Isolators have found wide use as control elements in audio circuits because they possess two characteristics which no other active semiconductor device has: resistance output and low harmonic distortion. AOIs often exhibit distortion levels below 80 db when the voltage applied to the photocell output is kept below 0.5V. Figure 3 shows the typical distortion generated in typical AOIs. The distortion depends on the operating resistance level as well as the applied voltage. The minimum distortion or threshold distortion shown in Figure 3 is a second harmonic of the fundamental frequency. The actual source of this distortion is unknown, but may be due to some type of crossover nonlinearity at the original of the IV curve of the photocell. I NJ 4kTBW () R ⁄ = I NS 2eI dcBW = I NF KI dcBW f ⁄ = 1f ⁄ 
Similar Part No.  VTL5C1 

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