Application Notes—Analog Optical Isolators
0.5 db of full signal is one time constant, which is usually only a few
Switching
milliseconds. The step change of a switch has been transformed into a
rapid but smooth increase in signal level. In addition, the possibility of
turn-on in the middle of a peak has been eliminated.
Mechanical switching of low level audio signals requires the use of
switches with precious metal contacts. Sudden changes in signal can
cause the speakers to thump and damage may occur if the speaker is
underdamped. A simple way to avoid these problems is to use an AOI
in place of a mechanical switch. In the circuit of Figure 1d, the initial
resistance of the LDR cell is so high that amplifier gain is essentially
zero. A step change in forward current through the LED is translated
into a slower time change in the cell resistance. The resistance drops
to 10 times the final value in one millisecond or less. As the resistance
continues to drop, the final value is approached exponentially. Express
in terms of conductivity:
Turn-off is slower and depends on the ratio of R1 to the final value of
photocell resistance. A high ratio will slow down the turn-off and speed
up the turn-on.
This circuit can be extended into a matrix as shown in Figure 2. While
a 3 x 3 matrix is shown, the number of nodes is not limited. Individual
inputs can be summed into a single output or connected to more than
one output. A matrix can be made very compact with the output
amplifiers mounted very close to reduce pickup. The op-amps
eliminate any crosstalk between the inputs since the summing point is
at virtual ground.
G = G0[1 – exp(–t ⁄ tc)] mhos
and:
R = 1 ⁄ G ohms
The controls for the matrix are usually remotely located. The DC
current through the LEDs may be controlled by switches, manual
potentiometers, or a computer. The matrix may be used for simple ON-
OFF gating, summing of several signals, or proportional control. When
proportional control is used, the output should be continuously
supervised to correct for changes in signal level due to photocell
resistance variation from temperature, light adapt history, and self
heating.
where:
G = conductance, mhos
t = time, ms
tc = time constance of the photocell, ms
If R1 is made equal to nine times the final value of resistance, the
response to 50% signal will occur in 1.0 ms. The time to get to within
Figure 2. Switching Matrix
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