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AD9985KSTZ-110 Arkusz danych(PDF) 11 Page - Analog Devices |
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AD9985KSTZ-110 Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 32 page AD9985 Rev. 0 | Page 11 of 32 DESIGN GUIDE GENERAL DESCRIPTION The AD9985 is a fully integrated solution for capturing analog RGB signals and digitizing them for display on flat-panel monitors or projectors. The circuit is ideal for providing a computer interface for HDTV monitors or as the front end to high performance video scan converters. Implemented in a high performance CMOS process, the interface can capture signals with pixel rates up to 110 MHz. The AD9985 includes all necessary input buffering, signal dc restoration (clamping), offset and gain (brightness and contrast) adjustment, pixel clock generation, sampling phase control, and output data formatting. All controls are programmable via a 2-wire serial interface. Full integration of these sensitive analog functions makes system design straightforward and less sensitive to the physical and electrical environment. With a typical power dissipation of only 500 mW and an operating temperature range of 0°C to 70°C, the device requires no special environmental considerations. DIGITAL INPUTS All digital inputs on the AD9985 operate to 3.3 V CMOS levels. However, all digital inputs are 5 V tolerant. Applying 5 V to them will not cause any damage. INPUT SIGNAL HANDLING The AD9985 has three high impedance analog input pins for the Red, Green, and Blue channels. They will accommodate signals ranging from 0.5 V to 1.0 V p-p. Signals are typically brought onto the interface board via a DVI-I connector, a 15-pin D connector, or via BNC connectors. The AD9985 should be located as close as practical to the input connector. Signals should be routed via matched-impedance traces (normally 75 Ω) to the IC input pins. At that point the signal should be resistively terminated (75 Ω to the signal ground return) and capacitively coupled to the AD9985 inputs through 47 nF capacitors. These capacitors form part of the dc restoration circuit. In an ideal world of perfectly matched impedances, the best performance can be obtained with the widest possible signal bandwidth. The ultrawide bandwidth inputs of the AD9985 (300 MHz) can track the input signal continuously as it moves from one pixel level to the next, and digitize the pixel during a long, flat pixel time. In many systems, however, there are mismatches, reflections, and noise, which can result in excessive ringing and distortion of the input waveform. This makes it more difficult to establish a sampling phase that provides good image quality. It has been shown that a small inductor in series with the input is effective in rolling off the input bandwidth slightly and providing a high quality signal over a wider range of conditions. Using a Fair-Rite #2508051217Z0 High Speed Signal Chip Bead inductor in the circuit of Figure 3 gives good results in most applications. RGB INPUT RAIN GAIN BAIN 47nF 75 Ω Figure 3. Analog Input Interface Circuit HSYNC, VSYNC INPUTS The interface also takes a horizontal sync signal, which is used to generate the pixel clock and clamp timing. This can be either a sync signal directly from the graphics source, or a preproc- essed TTL or CMOS level signal. The Hsync input includes a Schmitt trigger buffer for immunity to noise and signals with long rise times. In typical PC-based graphic systems, the sync signals are simply TTL-level drivers feeding unshielded wires in the monitor cable. As such, no termination is required. SERIAL CONTROL PORT The serial control port is designed for 3.3 V logic. If there are 5 V drivers on the bus, these pins should be protected with 150 Ω series resistors placed between the pull-up resistors and the input pins. OUTPUT SIGNAL HANDLING The digital outputs are designed and specified to operate from a 3.3 V power supply (VDD). They can also work with a VDD as low as 2.5 V for compatibility with other 2.5 V logic. CLAMPING RGB Clamping To properly digitize the incoming signal, the dc offset of the input must be adjusted to fit the range of the on-board A/D converters. Most graphics systems produce RGB signals with black at ground and white at approximately 0.75 V. However, if sync signals are embedded in the graphics, the sync tip is often at ground and black is at 300 mV. Then white is at approximately 1.0 V. Some common RGB line amplifier boxes use emitter- follower buffers to split signals and increase drive capability. This introduces a 700 mV dc offset to the signal, which must be removed for proper capture by the AD9985. The key to clamping is to identify a portion (time) of the signal when the graphic system is known to be producing black. An offset is then introduced which results in the A/D converters producing a black output (code 00h) when the known black |
Podobny numer części - AD9985KSTZ-110 |
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Podobny opis - AD9985KSTZ-110 |
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