Characteristics of Analog Optical Isolators
Some major characteristics of Johnson noise are that it is:
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:
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.
INF
=
KIdcBW ⁄ f
Johnson noise is defined by the following equation:
where:
I
= flicker noise, amps
INJ
=
(4kTBW) ⁄ R
NF
K = a constant that depends on the type of material
and its geometry
where:
I = dc current, amps
dc
BW = bandwidth of interest, Hertz
f = frequency, Hertz
I = Johnson noise current, amps RMS
k = Boltzmann’s constant, 1.38 x 10
NJ
-23
T = temperature, degrees Kelvin
R = photocell resistance
BW = bandwidth of interest, Hertz
Unlike thermal or shortnoise, flicker noise has 1/f spectral density and
in the ideal case for which it is exactly proportional to 1 ⁄ f, 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.
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.
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.
“Shot” noise is:
1. Independent of frequency.
2. Dependent upon the direct current flowing through the photocell.
Distortion
Shot noise is defined by the following equation:
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.
INS
=
2eIdcBW
where:
I
= shot noise current, amps RMS
NS
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.
-19
e = electron charge, 1.6 x 10
I = dc current, amps
dc
BW = bandwidth of interest, Hertz
35
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
