Characteristics of Analog Optical Isolators
Storage Characteristics
The instantaneous output resistance of any AOI is somewhat
dependent on the short term light history of the photocell output
element. With no applied input current or voltage, the output element is
in the dark. Dark storage causes the cell to “dark adapt”, a condition
which results in an increase in the photocell’s sensitivity to light. When
first turned on, an AOI which has experienced a period of dark
adaption will exhibit a lower value for “on” resistance, at any given drive
condition, than the same device which has been continuously on.
The output resistance of an AOI which has been biased “on” is
considered to be constant with time (neglecting long term aging
effects). After the removal of the input drive, the photocell begins to
experience dark adaption. The cell’s rate of increase in sensitivity is
initially high but eventually levels off with time in an exponential
manner. Most of the dark adapt occurs in the first eight hours, but with
some AOIs for sensitivity can continue to increase for several weeks.
When an AOI which has been sitting in the dark is turned on, the cell
immediately begins returning to its light adapted state. For any given
device, the rate of recovery is dependent on the input light level.
The type of photoconductive material is the major factor determining
the magnitude of these changes. Lower resistivity materials show
greater initial and final changes but their rate of change is faster.
These light/dark history effects are pronounced at both high and low
input levels. However, at high input levels, the photocell light adapts
quite rapidly, usually in minutes.
Figure 1 shows the transfer curves for an AOI after 24 hour storage
with no input and then after it has been operated with rated input for 24
hours. Because of these “memory” phenomena, it is best to use these
parts in a closed loop circuit to minimize the effects of these changes.
Open loop proportional operation is possible if the application can
tolerate variations. The use of the VTL5C2 and VTL5C3 with their
more stable characteristics will help.
Storage at low temperature has no operating effect on AOIs. Units may
be stored at temperatures as low as -40°C. Lower temperatures may
cause mechanical stress damage in the package which can cause
permanent changes in the AOI transfer characteristics.
The chemistry of the photoconductive materials dictates a maximum
operating and storage temperature of 75°C. It should be noted that
operation of the photocell above 75°C does not usually lead to
catastrophic failure but the photoconductive surface may be damaged,
leading to irreversible changes in sensitivity.
The amount of resistance change is a function of time as well as
temperature. While changes of several hundred percent will occur in a
matter of a few minutes at 150°C, it will take years at 50°C to produce
that much change.
In most applications, operation is intermittent. At elevated
temperatures, the resistance of the cell rises during the turn-on period
and recovers during the turn-off period, usually resulting in little net
change. However, if the AOI is stored at elevated temperatures for
many hours with no input signal, there is a net reduction in output
resistance. There will be some recovery during operation over time but
it is not possible to predict the rate or to what degree. Elevated
temperatures do not produce sudden catastrophic failure, but changes
in the device transfer curve with time must be anticipated.
Temperature Range
Operating and storage temperature range is limited at the lower end by
the reduction of dark resistance of the cell and at the upper end by
rapid aging. At low temperatures, the response time of the output cell
increases. The temperature at which this becomes pronounced
depends on the photoconductive material type. Isolators using low
resistivity materials, as in the VTL5C4, will show this lengthening of
response time at -25°C. Higher resistivity materials such as used in the
VTL5C3 and VTL5C6 do not slow down excessively until temperatures
get below -40°C. This characteristic is completely reversible with the
response time recovering when the temperature rises.
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PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
