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| 型號： | VT900 |
| 廠商： | PerkinElmer Inc. |
| 英文描述： | Photoconductive Cells and Analog Optoisolators (Vactrols) |
| 中文描述： | 光導(dǎo)細(xì)胞和模擬光隔離器（Vactrols） |
| 文件頁數(shù)： | 40/76頁 |
| 文件大?。?/td> | 1240K |
| 代理商： | VT900 |
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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:
I
NJ
= Johnson noise current, amps RMS
k = Boltzmanns 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 randomvariations superimposed on
this current due to randomfluctuations in the emssion 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:
I
NS
= shot noise current, amps RMS
e = electron charge, 1.6 x 10
-19
I
dc
= 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:
I
NF
= flicker noise, amps
K = a constant that depends on the type of material
and its geometry
I
dc
= 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
termed “pink noise”. Unfortunately, the constant (K) can only be
determned empirically and may vary greatly even for simlar devices.
Flicker noise may domnate when the bandwidth of interest contains
frequencies less than about 1kHz.
, it is
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 semconductor 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 mnimumdistortion 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.
I
NJ
4kTBW
(
)
R
=
I
NS
2eI
dc
BW
=
I
NF
KI
dc
BW f
=
1 f
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