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ADV7128 Datasheet(PDF) 5 Page - Analog Devices |
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ADV7128 Datasheet(HTML) 5 Page - Analog Devices |
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5 / 8 page  ADV7128 REV. 0 –5– TERMINOLOGY Color Video (RGB) This usually refers to the technique of combining the three pri- mary colors of red, green and blue to produce color pictures within the usual spectrum. In RGB monitors, three DACs are required, one for each color. Gray Scale The discrete levels of video signal between reference black and reference white levels. A 10-bit DAC contains 1024 different levels, while an 8-bit DAC contains 256. Raster Scan The most basic method of sweeping a CRT one line at a time to generate and display images. Reference Black Level The maximum negative polarity amplitude of the video signal. Reference White Level The maximum positive polarity amplitude of the video signal. Video Signal That portion of the composite video signal which varies in gray scale levels between reference white and reference black. Also referred to as the picture signal, this is the portion which may be visually observed. CIRCUIT DESCRIPTION AND OPERATION The ADV7128 contains one 10-bit D/A converter, with one in- put channel containing a 10-bit register. Also integrated on board the part is a reference amplifier. Digital Inputs Ten bits of data (color information) D0–D9 are latched into the device on the rising edge of each clock cycle. This data is pre- sented to the 10-bit DAC and is then converted to an analog output waveform. See Figure 2. CLOCK DATA ANALOG OUTPUTS I OUT DIGITAL INPUTS D0–D9 Figure 2. Video Data Input/Output All these digital inputs are specified to accept TTL logic levels. Clock Input The CLOCK input of the ADV7128 is typically the pixel clock rate of the system. It is also known as the dot rate. The dot rate, and hence the required CLOCK frequency, will be determined by the on-screen resolution, according to the following equation: Dot Rate = (Horiz Res) × (Vert Res) × (Refresh Rate)/ (Retrace Factor) Horiz Res = Number of Pixels/Line. Vert Res = Number of Lines/Frame. Refresh Rate = Horizontal Scan Rate. This is the rate at which the screen must be refreshed, typi- cally 60 Hz for a noninterlaced system or 30 Hz for an interlaced system. Retrace Factor = Total Blank Time Factor. This takes into account that the display is blanked for a certain fraction of the total duration of each frame (e.g., 0.8). If we, therefore, have a graphics system with a 1024 × 1024 resolution, a noninterlaced 60 Hz refresh rate and a retrace fac- tor of 0.8, then: Dot Rate = 1024 × 1024 × 60/0.8 = 78.6 MHz The required CLOCK frequency is thus 78.6 MHz. All video data and control inputs are latched into the ADV7128 on the rising edge of CLOCK, as previously described in the “Digital Inputs” section. It is recommended that the CLOCK input to the ADV7128 be driven by a TTL buffer (e.g., 74F244). 100 IRE WHITE LEVEL BLACK LEVEL 17.61 0.66 0 0 mA V I OUT NOTES 1. OUTPUTS CONNECTED TO A DOUBLY TERMINATED 75 Ω LOAD. 2. V REF = 1.235V, R SET = 560 Ω. 3. RS–343A LEVELS AND TOLERANCES ASSUMED ON ALL LEVELS. Figure 3. IOUT Video Output Waveform Table I. Video Output Truth Table for the ADV7128 Description IOUT 1 DAC Input Data WHITE LEVEL 17.62 3FF VIDEO video data VIDEO to BLACK video data BLACK LEVEL 0 00H NOTE 1Typical with full scale = 17.62 mA. V REF = 1.235 V, RSET = 560 Ω. |
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