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SN28835FS Datasheet(PDF) 5 Page - Texas Instruments |
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SN28835FS Datasheet(HTML) 5 Page - Texas Instruments |
5 / 17 page SN28835 1/2-INCH NTSC TIMER SOCS023B – FEBRUARY 1991 5 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 variable-integration-time mode In addition to the normal TV mode of operation, the SN28835 timing generator offers an optional variable-integration mode for use with the TC244 and TC245 CCD area-array image sensors. The variable-integration mode is selected by applying a low-logic level to GPS. This low-logic level disables the vertical-drive (VD) signal from controlling, internal to the timer, the image-area and storage-area parallel transfer signal (GP). Prior to the start of a new integration period, the charge that has accumulated in the image area must be transferred out. To flush this previous signal or dark-current charge from the image area, GP is pulsed low two times. Each low pulse generates 256 image-area and storage-area gate and transfer signals that shift the unwanted charge into the clearing drain. This clearing function should be performed during the high time of the VD signal (see Figure 3 through Figure 12). The new integration period continues as long as GP remains high. GPS must be held at a low-logic level to prevent VD from controlling GP internally. The integration ceases and the readout occurs when VD and GP are pulsed low simultaneously; this is accomplished by taking GPS to a high-logic level. The readout timing is dependent on the vertical-drive pulse; this means that the total-integration time is a multiple of 1/60 of a second plus the time interval between the last GP low pulse and the next VD low pulse. The image readout occurs within the normal 1/60-second readout interval. If the integration time is less than 1/60 of a second, normal output operation occurs; if the integration time is greater than 1/60 of a second, a frame buffer may be required to capture the image. Integration times greater than 1/60 of a second result in image degradation at temperatures greater than 25 °C due to dark-current generation. The degradation is seen as a decrease in dynamic range (contrast) and an increase in noise. It is recommended that the image sensor be cooled for long-exposure operation. The dark-current generation is reduced by a factor of two for each 7 °C temperature decrease. The sensor operates at – 30 °C. Cooling can be accomplished by using a thermoelectric or Peltier cooler attached to the image sensor. Condensation on the header must be prevented by isolating the cooled sensor from moist air. Vacuum isolation is preferred; however, the continual flushing of dry nitrogen across the header can also prevent condensation. X1 X2 34 35 C1 ≈ 40 pF C2 ≈ 40 pF SN28835 NOTE: The SN28835 is designed for use with a crystal oscillator. The X1 and X2 terminals should not connect directly to external driver outputs. Figure 2. Connection of an External Crystal Oscillator to the SN28835 |
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