Characteristics of Analog Optical Isolators
Capacitance
The equivalent circuit for the output photocell is a resistor in parallel
with the capacitance. The capacitance arises from the topside
metallization of the electrodes which form a coplanar capacitor. The
value of this capacitance is largely determined by the size of the
ceramic base. For lower capacitance, a smaller cell is needed. The
capacitance is so small (3.0 pF, typical on catalog AOIs) that it is
negligible in most applications. However, there are applications such
as wideband or high frequency amplifiers in which the capacitance
needs to be considered. At 4.5 MHz, the video baseband frequency,
the photocell capacitive reactance is only 12 kilohms. If the phase shift
of the signal is to be kept below 10°, the highest useful cell resistance
is only 2.0 kilohms. At high AOI input drive, where the cell is drive
below 1.0 kilohm, the capacitance can increase additionally from 2 to
10 times, possibly due to distributed effects.
Summary
Analog Optical Isolators have many unique features, such as:
1. High input-to-output isolation.
2. True resistance element output.
3. Wide dynamic range (low “on” resistance/high “off” resistance).
4. Low drive current.
5. Low distortion.
These features are primarily dependent on which input element and
output element photoconductive material is used in the Vactrol AOI.
Thus, there is a wide variety of Vactrols to choose from for your
application.
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PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
