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MAX4029EWP Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX4029EWP Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 13 page Therefore, a 6k Ω resistor will produce a 1.13V key- clamp voltage as shown in Figure 2. The clamp voltage (VCLAMP) is measured at the output; the voltage at the input is VCLAMP (sync tip or key clamp) divided by the gain (+2V/V) + VBE. In order for these clamps (sync tip or key) to work proper- ly, the input must be coupled with a 0.1µF capacitor (typ) with low leakage (<1µA to 2µA, max). Without proper cou- pling, the clamp voltage will change during the horizontal line time causing the “black level” to vary, changing the image brightness from left to right on the display. In addi- tion to the capacitor, a low resistance ( ≤75Ω) is required on the source side to return the capacitor to ground. The clamps used here are active devices with the coupling capacitor serving two functions; first, as a charge reser- voir to maintain the clamp voltage, and second, as the compensation capacitor for the clamp itself. If an input is not used, it must be terminated to avoid causing oscilla- tions that could couple with another input. In general, a sync-tip clamp is used for composite video (Cvbs), gamma corrected primaries (R’G’B’), and the luma signal (Y) in S-video. A key clamp is preferred for component color difference signals (Pb and Pr), linear primaries (RGB in PCs), and chroma (C) in S-video. The rule is to sync tip clamp a signal if sync is present and key clamp all others. Several examples are given in the Clamp/Key-Clamp Settings for Video Formats section. Clamp/Key-Clamp Settings for Video Formats Tables 1 and 2 provide the clamp settings on the MAX4028/MAX4029 to interface with various video formats. Low-Power, High-Impedance Disable Mode All parts feature a low-power, high-impedance disable mode that is activated by driving the DISABLE input low. Placing the amplifier in disable mode reduces the quiescent supply current and places the output imped- ance at 2k Ω typically. Multiple devices can be paral- leled to construct larger switch matrices by connecting the outputs of several devices together and disabling all but one of the paralleled amplifiers’ outputs. Triple/Quad, 2:1 Video Multiplexer-Amplifiers with Input Clamps 8 _______________________________________________________________________________________ KEY-CLAMP REFERENCE VOLTAGE vs. RKEYREF RKEYREF (k Ω) 11 9 7 5 3 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 113 Figure 2. Key-Clamp Reference Voltage vs. RKEYREF Table 1. MAX4028 Clamp Settings for Video Formats INPUT FORMAT CLAMP/KEY 1 Cvbs1 Clamp 2 Cvbs2 Clamp 3 Cvbs3 Clamp INPUT FORMAT CLAMP/KEY 1G’ Clamp 2B’ Clamp 3R’ Clamp R, G, B have sync on all. INPUT FORMAT CLAMP/KEY 1 Gs Clamp 2 B Key 3 R Key Gs, B, R have sync only on Green. INPUT FORMAT CLAMP/KEY 1 Y Clamp 2 C Key 3 Cvbs Clamp INPUT FORMAT CLAMP/KEY 1 Y Clamp 2 Pb Key 3 Pr Key |
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