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ADA4424-6 Datasheet(PDF) 11 Page - Analog Devices |
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ADA4424-6 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADA4424-6 Rev. C | Page 11 of 16 APPLICATIONS INFORMATION CVBS OUTPUT The composite video signal (CVBS_OUT) is produced by passively summing the Y and C channels (see Figure 1), after amplification by their respective gain stages. Each signal experiences a 6.2 dB loss from the passive summation and is subsequently amplified by 6.2 dB in the fixed stage following the summer. The net signal gain through the composite video path is, therefore, 0 dB, and the resulting composite signal present at the ADA4424-6 output is the sum of Y and C with unity gain. CORNER FREQUENCY SELECTION The component channels of the ADA4424-6 allow for a 1 dB filter corner frequency of either 25 MHz or 39 MHz/34 MHz. The FC_SEL pin operates as described in Table 5. Table 5. ED/HD Bandwidth Selection FC_SEL (Pin 12) HD/ED −1 dB Corner Frequency (Typ) Low (0) ED 25 MHz High (1) HD 39 MHz (HY); 34 MHz (Pb, Pr) INPUT DC OFFSET CANCELLATION The luma channels (Y_IN, HY_IN) of the ADA4424-6 are capable of detecting and cancelling dc input offsets of up to 1.1 V. Four distinct modes of detection/cancellation are available. These are selected via the MODE1 and MODE0 pins. The chroma (C_IN) and color difference (HPb_IN, HPr_IN) inputs do not support offset cancellation. It is recommended that these inputs be ac- coupled. Automatic Detection/Cancellation Mode There are two modes of automatic operation. The primary mode samples the input signal between the rising edge of the horizon- tal sync pulse and the start of active video (back porch), averages the value over the sampling interval, and subtracts it from the output signal. This is the more accurate method and is able to reduce the input-referred offsets to less than 25 mV. An alternate method is available for copy-protected signals, where sampling the back porch may not provide a reliable dc average. This method detects the input negative sync tip, and clamps it to a fixed value (−214 mV for SD, and −300 mV for ED/HD). Sample intervals for SD and ED are shown in Figure 16, and the HD sample intervals are shown in Figure 17. BLACK LEVEL SYNC TIP SAMPLE INTERVAL BACK PORCH SAMPLE INTERVAL Figure 16. Back Porch and Sync Tip Sample Intervals (SD/ED) BLACK LEVEL SYNC TIP SAMPLE INTERVAL BACK PORCH SAMPLE INTERVAL Figure 17. Back Porch and Sync Tip Sample Intervals (HD) |
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