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VTL5C1 Datasheet(PDF) 65 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 65 Page - PerkinElmer Optoelectronics |
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65 / 76 page  60 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 GG 0 1 exp t – tc ⁄ () – [] mhos = and: R 1 G ohms ⁄ = |
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