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LT6206IMS8 Datasheet(PDF) 11 Page - Linear Technology |
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LT6206IMS8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT6205/LT6206/LT6207 11 620567f APPLICATIO S I FOR ATIO invisible on a CRT. The “black” level of the waveform is at (or “setup” very slightly above) the upper limit of the sync information. Waveform content above the black-level is intensity information, with peak brightness represented at the maximum signal level. In the case of composite video, the modulated color subcarrier is superimposed on the waveform, but the dynamics remain inside the 1VP-P limit (a notable exception is the chroma ramp used for differen- tial-gain and differential-phase measurements, which can reach 1.15VP-P). DC-Coupled Video Amplifier Considerations Typically video amplifiers drive cables that are series terminated (“back-terminated”) at the source and load- terminated at the destination with resistances equal to the cable characteristic impedance, Z0 (usually 75Ω). This configuration forms a 2:1 resistor divider in the cabling that must be accounted for in the driver amplifier by delivering 2VP-P output into an effective 2 • Z0 load (e.g. 150 Ω). Driving the cable can require more than 13mA while the output is approaching the saturation-limits of the amplifier output. The absolute minimum supply is: VMIN = 2 + VOH +VOL. For example, the LT6206 dual operating on 3.3V as shown on the front page of this datasheet, with exceptionally low VOH ≤ 0.5V and VOL ≤ 0.35V, provides a design margin of 0.45V. The design margin must be large enough to include supply variations and DC bias accuracy for the DC-coupled video input. Handling AC-Coupled Video Signals AC-coupled video inputs are intrinsically more difficult to handle than those with DC-coupling because the average signal voltage of the video waveform is effected by the picture content, meaning that the black-level at the ampli- fier “wanders” with scene brightness. The wander is measured as 0.56V for a 1VP-P NTSC waveform changing from black-field to white-field and vice-versa, so an addi- tional 1.12V allowance must be made in the amplifier supply (assuming gain of 2, so VMIN = 3.12 + VOH +VOL). For example, an LT6205 operating on 5V has a conserva- tive design margin of 1.03V. The amplifier output (for gain of 2) must swing +1.47V to –1.65V around the DC- operating point, so the biasing circuitry needs to be designed accordingly for optimal fidelity. Clamped AC-Input Cable Driver A popular method of further minimizing supply require- ments with AC-coupling is to employ a simple clamping scheme as shown in Figure 2. In this circuit, the LT6205 operates from 3.3V by having the sync-tips control the charge on the coupling capacitor C1, thereby reducing the black-level input wander to ≈ 0.07V. The only minor drawback to this circuit is the slight sync-tip compression ( ≈ 0.025V at input) due to the diode conduction current, though the picture content remains full fidelity. This circuit has nearly the design margin of its DC-coupled counter- part, at 0.31V (for this circuit, VMIN = 2.14 + VOH +VOL). The clamp-diode anode bias is selected to set the sync-tip output voltage at or slightly above VOL. YPbPr to RGB Component-Video Converter The back-page application uses the LT6207 quad to imple- ment a minimum amplifier count topology to transcode consumer component-video into RGB. In this circuit, signals only pass through one active stage from any input to any output, with passive additions being performed by the cable back-termination resistors. The compromise in using passive output addition is that the amplifier outputs must be twice as large as that of a conventional cable driver. The Y-channel section also has the demanding requirement that it single-handedly drives all three out- puts to full brightness during times of white content, so a helper current source is used to assure unclipped video when operating from ±5V supplies. This circuit maps sync-on-Y to sync on all the RGB channels, and for best results should have input black-levels at 0V nominal to prevent clipping. |
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