Simplify High-Resolution Video Designs with Fixed-Gain Triple Multiplexers

Simplify High-Resolution Video Designs with Fixed-Gain Triple Multiplexers


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Jon Munson


The LT6555 and LT6556 triple video multiplexers offer up to 750MHz performance in compact packages, requiring no external gain-setting resistors to establish a gain of two or unity. A single integrated circuit, in a choice of either 24-lead SSOP or 24-contact QFN (4mm × 4mm), performs fast switching between a pair of three-channel video sources, such as RGB or component HDTV.

The LT6555 provides a built-in gain of two that is ideal for driving back-terminated cables in playback or signal routing equipment. The LT6556 provides a unity-gain function, in the same footprints, that is ideal as an input selector in high-performance video displays and projectors.

The three video channels exhibit excellent isolation between themselves (50dB typical at 100MHz) and the inactive inputs (70dB typical at 100MHz) for the highest quality video transmission. Excellent channel-to-channel gain-matching preserves high fidelity color balance.

The increasing popularity of the UXGA professional graphics format (1600 × 1200), which generates a whopping 200-megapixel-per-second flow, has put exceptional demands on the frequency response of video amplifiers. For instance, pulse-amplitude waveforms like those of RGB baseband video, generally require reproduction of high-frequency content to at least the 5th harmonic of the fundamental frequency component, which is 2.5 times the video pixel rate, accounting for the 2-pixels-per-fundamental-cycle relationship. This means that UXGA requires a flat frequency response to beyond 0.5GHz! The wide bandwidth performance of the LT6555 and LT6556 makes them ideally suited to such high performance video applications.

Easy Solution for Multi-Channel Video Applications

Baseband video generated at these higher rates is processed in either native red, green and blue (RGB) domain or encoded into component luma plus blue and red chroma channels (YPbPr); three channels of information in either case. With frequency response requirements extending to beyond 500MHz, amplifier layouts that require external resistors for gain setting tend to be real-estate inefficient, and frequency response and crosstalk anomalies can plague the circuit development process. The LT6555 and LT6556 conveniently solve these problems by providing internal factory-matched resistors and an efficient 3-channel, 2-input group, flow-through layout arrangement.

Figure 1 shows the typical RGB cable driver application of an LT6555, and its excellent frequency and time response plots are shown in Figures 2 and 3 (as implemented on demo circuit 892A-A). Frequency markers in Figure 2 show the small-signal –0.5dB response beyond 500MHz and –3dB response above 600MHz. The LT6556, when used to drive high impedances, provides bandwidth to 750MHz, though the LT6556 demo circuit 892A-B uses 75Ω back termination (rather than 1kΩ), resulting in performance similar to the LT6555.

Figure 1. The LT6555 in an RGB cable driving multiplexer circuit.

Figure 2. Wide frequency response of circuit in Figure 1.

Figure 3. Fast pulse response of circuit in Figure 1.

Taking a Look at the Internal Details

The LT6555 and LT6556 integrate three independent sections of circuitry that form classic current-feedback amplifier (CFA) gain blocks, but with switchable input sections, all implemented on a very high-speed fabrication process. The diagram in Figure 4 shows the equivalent internal circuitry (one LT6555 section shown).

Figure 4. Simplified internal circuit functionality of the LT6555 and LT6556.

Feedback resistors are provided on-chip to set the closed-loop gain to either unity or two, depending on the part. The nominal feedback resistances are chosen to optimize flat frequency response. The LT6555 is intended to drive back-terminated 50Ω or 75Ω cables (for effective loading of 100Ω to 150Ω respectively), while the LT6556 is designed to drive ADCs or other high impedance loads (characterized with 1kΩ as a reference loading condition).

Common to all three CFAs in each part is a bias control section with a power-down command input. The input select logic steers bias current to the appropriate input circuitry, enabling the input function of the selected signal. The shutdown function includes an internal on-chip pull-up resistance to provide a default disable command, which when invoked, reduces typical power consumption to less than 125µA for an entire three-channel part. During shutdown mode the amplifier outputs become high impedance, though in the case of the LT6555, the feedback resistor string to AGND is still present. The parts come into full-power operation when the enable input voltage is brought within 1.3V above the DGND pin. The typical on-state supply current of about 9mA per amplifier provides for ample cable-drive capacity (>40mA) and ultra-fast 2.2V per nanosecond slew rate performance.

Expanding MUX Input Selection

The power-down feature of the LT6555 and LT6556 may be used to control multiple ICs in a configuration that provides additional input selections. Figure 5 shows a simple 4-input RGB selecting cable driver using two LT6555 devices with the enable pins driven by complementary logic signals. The shared-output connections between the devices need to be kept as short as possible to minimize printed-circuit parasitics that might affect frequency response. This circuit would be ideal in an A/V control-unit for driving the component-video output, for example. The same basic expansion concept applied to an LT6556 pair would be ideal at the input section of a four-source HD video display.

Figure 5. A 4-to-1 video multiplixer using the shutdown feature for expansion.

Operating with the Right Power Supplies

The LT6555 and LT6556 require a total power supply of at least 4.5V, but depending on the input and output swings required, may need more to avoid clipping the signal. The LT6556, having unity gain, makes the analysis simple—the maximum output swing is (V+ – V – 2.6)VP–P and governed only by the output saturation voltages. This means a total supply of 5V is adequate for standard video (1VP–P). For the LT6555, extra allowance is required for load-driving, so the output swing is (V+ – V – 3.8)V. This means a total supply of about 6V is required for the output to swing 2VP–P, as when driving cables. For best dynamic range along with reasonable power consumption, a good choice of supplies would be ±3V for the LT6556 and +5V/–3V for the LT6555.

Since many systems today lack a negative supply rail, a small LTC1983-3 solution can be used to generate a simple –3V rail for local use, as shown in Figure 6. The LTC1983-3 solution is more cost effective and performs at high frequencies better than AC-coupling and resistor network biasing techniques that might otherwise be employed. For example, Figure 7 shows the typical AC-coupling networks used when operating from a single supply. With six input networks and three large output capacitors required, the AC-coupled method uses more board space and adds parasitics to the signal path that can degrade frequency response.

Figure 6. Generating a local –3V supply with four tiny components.

Figure 7. AC-coupling techniques for single-supply operation.

Demonstration Circuits Available

The LT6555 and LT6556 have Demo Boards available that make evaluation of these parts a simple plug-and-play operation. To evaluate the LT6555 ask for DC858A (SSOP-24 package) or DC892A-A (QFN package). To evaluate the LT6556 ask for DC892A-B (in QFN package). All three of these demo circuits have high-quality 75Ω BNC connections for best performance and illustrate high-frequency layout practices that are important to obtaining the best performance from these super-fast amplifiers.