VT83CT datasheet(65/76 Pages) PERKINELMER

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Application Notes—Analog Optical Isolators

Switching

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:

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

0.5 db of full signal is one time constant, which is usually only a few

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.

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.

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.

Figure 2. Switching Matrix

0 1

exp

–

⁄

()

–

[] mhos

and:

1 G ohms

⁄

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