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參數(shù)資料
| 型號(hào)： | VT900 |
| 廠商： | PerkinElmer Inc. |
| 英文描述： | Photoconductive Cells and Analog Optoisolators (Vactrols) |
| 中文描述： | 光導(dǎo)細(xì)胞和模擬光隔離器（Vactrols） |
| 文件頁數(shù)： | 65/76頁 |
| 文件大?。?/td> | 1240K |
| 代理商： | VT900 |
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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 mllisecond 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
mlliseconds. 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 mddle of a peak has been elimnated.
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 limted. 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
elimnate any crosstalk between the inputs since the summng 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, summng 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 fromtemperature, light adapt history, and self
heating.
Figure 2. Switching Matrix
G
G
0
1
exp
t
–
(
tc
)
–
[
]
mhos
=
and: R
1 G ohms
=
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