Characteristics of Analog Optical Isolators
applied, the photocells resistance drops very fast, typically reaching
63% (1-1/e conductance) of its final values in under 10 msec.
Transfer Characteristics
The light output of an LED is proportional to the input drive current, I .
F
When the light is removed, the resistance increases initially at an
exponential rate, approximately tripling in a few milliseconds. The
resistance then increases linearly with time.
Some LEDs will begin to radiate useful amounts of light output at
forward currents as low as 10 µA. These same LEDs can be driven at
50 mA with no degradation in performance.
The fast turn-on and slow turn-off characteristics can be used to
advantage in many applications. This is especially true in audio
applications where a fast turn-on (attack) and a slow turn-off (release)
is preferred. For example, the typical AOI can be made to turn-on in
100 to 1000 µsec. In a limited circuit this is fast enough to catch high
peak amplitudes but not so fast as to cause obvious clipping. The turn-
off will take as much as 100 times longer so the audio circuit will return
to a normal gain condition without a disturbing “thump” in the speaker.
A transfer curve of output resistance versus input light current for a
typical AOI is shown in Figure 1. AOIs not only possess a large
dynamic range, but the output resistance tracks the input current in a
somewhat linear manner over a range of two or more decades.
This characteristic makes the AOI suitable for use in a very broad
range of applications, especially in audio circuits where they are used
for switching, limiting, and gating. For a more extensive discussion on
AOIs in audio circuits, refer to Application Notes #1.
Response Time
AOIs are not high speed devices. Speed is limited by the response
time of the photocell. With rise and fall times on the order of 2.5 to
1500 msec, most AOIs have bandwidths between 1 Hz and 200 Hz.
Figure 2. Resistance vs. Time
Noise
The sources of electrical noise in the output element of AOIs are the
same as for any other type of resistor.
Figure 1. Transfer Curves (25°C)
One source of noise is thermal noise, also known as Johnson or
“white” noise, which is caused by the random motion of free electrons
in the photoconductive material.
One of the characteristics of photocells is that their speed of response
1
increases with increasing levels of illumination. Thus the bandwidth of
Vactrols is somewhat dependent upon the input drive level to the LED.
In general, the higher the input drive the wider the bandwidth.
The turn-off time and turn-on time of photocells are not symmetrical.
The turn-on time can be an order of magnitude faster than the turn-off
time. In the dark (no input), the resistance of the cell is very high,
typically on the order of several megohms. When light is suddenly
1. For a more comprehensive discussion on the turn-on and turn-
off characteristics of photocells and how response time is effect-
ed by light level, see the Photoconductive Cell section of this cat-
alog.
34
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
