250MHz RGB Video Multiplexer in Space-Saving Package Drives Cables, Switches Pixels at 100MHz

Introduction

One of the first products from LTC’s new proprietary high speed bipolar process is a 250MHz RGB (red, green, blue) multiplexer that is optimized for switching speed and makes excellent use of the new complementary 6GHz transistors. This new MUX, the LT1675, is designed for pixel switching in video graphics and for RGB routing. It is configured with three SPDT (single pole, double throw) RGB video switches and three current feedback amplifiers for direct driving of cables.

The new RGB MUX is similar to the LT1203/LT1205 video switches combined with the LT1260 triple CFA, but with greatly enhanced performance in far less space. The boost over the older configuration is a factor of five in switching speed and a factor of 2.5 in bandwidth, while the PCB footprint is reduced by more than five. This “juiced” performance is accomplished with one-third less supply current than required by the equivalent multichip design.

Dense Process Yields Big Performance from Tiny PC Board Space

One advantage of the dense, high speed bipolar process is that it results in a reduced die size for the LT1675, even though it has well over 300 active devices. The benefit to the user is that the LT1675 comes in a small 16-pin SSOP package, which is the same size as an SO-8. To enhance the small PC board theme, the LT1675 is configured for a fixed gain of two, eliminating six external gain setting resistors. The fixed gain of two in the CFA is ideal for driving double terminated 50Ω or 75Ω cables. Additionally, stray PCB capacitance on the sensitive feedback node is no longer a problem. Figure 1 shows a typical application switching between two RGB sources and driving 75Ω cables. In contrast, some competitive solutions are housed in bulky 24-pin, wide-SO packages and draw significantly more supply current.

Figure 1. LT1675 typical application: switching between two RGB sources and driving three cables.

The LT1675’s internal switches change state in less than 1ns but the output of the MUX switches in 2.5ns. This increased time is due to the finite bandwidth of the current feedback amplifier that drives the cable. To toggle at 100MHz, as shown in Figure 2, implies a pixel width of 5ns; accomplishing this requires a slew rate in excess of 1000V/µs. In Figure 2, the Select pin (pin 10) is driven from a sine wave generator, since only crossings of the logic threshold are required.

Figure 2. Select pin switches inputs at 100MHz. RED 1 = 0V, RED 2 = 1V, RL = 100Ω, 10pF scope probe; measured between 50Ω back termination and 50Ω load.

The fast current steering break-before-make SPDT tee switches minimize switching glitches. The switching transients of Figure 3, measured between the 75Ω back termination and the 75Ω load, show what the monitor receives. The glitch is only 50mVp-p, the duration is only 5ns and nature of this transient is small and fast enough to not be visible even on quality graphics terminals. Additionally, the break-before-make SPDT switch is open before the alternate channel is connected, which means there is no input feedthrough or crosstalk during switching.

Figure 3. Input-referred switching transient. RL = 150Ω, 10pF scope probe.

Expanding Inputs Does Not Increase Power Dissipation

In video routing applications, where the ultimate in speed is not mandatory, as it is in pixel switching, it is possible to expand the number of MUX inputs by shorting the LT1675 outputs together and switching with the ENABLE pins. This technique does not increase the power dissipation because LT1675s draw virtually zero current when disabled. The internal gain-set resistors have a nominal value of 750Ω and cause a 1500Ω shunt across the 75Ω cable termination. Figure 4 shows schematically the effect of expanding the number of inputs. The effect of this loading is to cause a gain error that can be calculated by the following formula:

equation1

where n is the total number of LT1675s.

Figure 4. Each off channel loads the cable termination with the 1575Ω.

For example, using ten LT1675s (20 red, 20 green, 20 blue) the gain error is only −1.7dB per channel.

Figure 5 shows a 4-input RGB router. The response from red 1 input to red output is shown in Figure 6, for a 25MHz square wave with Chip Select = 0V. In this example, the gain error is just −0.23dB. The response to toggling between IC1 and IC2 with Chip Select is shown in Figure 7. In this case red 1 input is connected to 0V, and red 3 is connected to an uncorrelated sine wave.

Figure 5. Two LT1675s build a 4-input RGB router.

Figure 6. Square wave response: chip select = 0V, IC 2 disabled.

Figure 7. Toggling the 4-input router: Red 1 input = 0V; Red 3 input = uncorrelated sine wave.

Performance

Table 1 summarizes the major performance specifications of the LT1675; Figure 8 shows a graph of crosstalk.

Figure 8. LT1675 crosstalk rejection vs frequency.

Table 1. LT1675 performance, VS = ±5V
Parameter Conditions Typical Values
−3dB Bandwidth RL = 150Ω 250MHz
0.1dB Gain Flatness RL = 150Ω 70MHz
Crosstalk Between Active Channels at 10MHz −60dB
Slew Rate RL = 150Ω 1100V/µs
Differential Gain RL = 150Ω 0.07%
Differential Phase RL = 150Ω 0.05˚
Channel Select Time RL = 150Ω, VIN = 1V 2.5ns
Enable Time RL = 150Ω 10ns
Output Voltage Swing RL = 150Ω ±3V
Gain Error RL = 150Ω, VIN = ±1V 4%
Output Offset Voltage 20mV
Supply Current All Three Channels Active 30mA
Supply Current Disabled 1µA

Conclusion

By taking full advantage of LTC’s new complementary high speed bipolar process, the LT1675 RGB multiplexer dramatically raises the level of performance while saving PC board space. A channel-to-channel toggle rate of 100MHz makes the LT1675 perfect for pixel switching and the simple expansion feature using the ENABLE pin is ideal for RGB routing. A fixed gain of two for driving double terminated cables simplifies PC board layout and boosts performance. These high performance multiplexers complement the large number of video products offered by LTC.


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John Wright

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Frank Cox