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VSP2270 Datasheet(PDF) 11 Page - Texas Instruments |
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VSP2270 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 19 page VSP2270 SLES019 – DECEMBER 2001 11 www.ti.com PRINCIPLES OF OPERATION optical black (OB) level clamp loop To extract the video information correctly, the CCD signal must be referenced to a well-established optical black (OB) level. The VSP2270 device has an auto-calibration loop to establish the OB level, using the optical black pixel output from the CCD imager. The input signal level of the OB pixels is identified as the real OB level, and the loop must be closed while CLPOB is active. During the effective pixel interval, the reference level of the CCD output signal is clampled to the OB level by the OB level clamp loop. To determine the loop time constant, a required off-chip capacitor must be connected to pin 28 (COB). The time constant T is given the following equation: T + C 16384 I min where, C is the capacitor value connected to pin 28 (COB). Imin is the minimum current (0.15 µA) of the control DAC in the OB level clamp loop, and 0.15 µA is equivalent to 1 LSB of the DAC output current. When C is 0.1 µF, the time constant T is 40.7 µs. The slew rate SR is given the following equation: SR + Imax C where, C is the capacitor value connected to pin 28 (COB). Imax is the maximum current (153 µA) of the control DAC in the OB level clamp loop, and 153 µA is equivalent to 1023 LSB of the DAC output current. Generally, the OB level clampling at high-speed causes clamp noise or white streak noise. However, the noise is reduced by making C large. On the other hand, a large C requires a much longer time to restore from the stand-by mode or right after the power goes ON. Therefore, 0.1 µF to 0.22 µF is considered the reasonable value range for C. However, the value depends on the application environment. Make careful adjustments by the trial and error method. The OB clamp level (the pedestal level) is programmable through the serial interface, refer to the serial interface section for details. Table 1 shows the relationship between input code and the OB clamp level. The user can choose the active polarity of CLPOB (active high or active low) through the serial interface, refer to the serial interface section for details. The default value of CLPOB is active low. Upon power on, this value is unknown. For this reason, it must be set to the appropriate value by using the serial interface, and reset to the default value by strobing pin 45 (RESET). The description and the timing diagrams in this data sheet are all based on a polarity of active low (default value). |
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