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VTL5C1 Datasheet(PDF) 39 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 39 Page - PerkinElmer Optoelectronics |
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39 / 76 page  34 Characteristics of Analog Optical Isolators Transfer Characteristics The light output of an LED is proportional to the input drive current, IF. 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. 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 1. Transfer Curves (25°C) One of the characteristics of photocells is that their speed of response increases with increasing levels of illumination .1 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 applied, the photocells resistance drops very fast, typically reaching 63% (1-1/e conductance) of its final values in under 10 msec. 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. 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. 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. 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. 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. |
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Similar Description - VTL5C1 |
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