VT33CT datasheet(40/76 Pages) PERKINELMER

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Characteristics of Analog Optical Isolators

Some major characteristics of Johnson noise are that it is:

Independent of frequency and contains a constant power density

per unit of bandwidth.

Temperature dependent, increasing with increased temperature.

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:

Independent of frequency.

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 I-V curve of the

photocell.

4kTBW

() R

⁄

2eI

dcBW

⁄

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