Design Note 205: Tiny RGB Video Multiplexer Switches Pixels at 100MHz


The LT1675, a new 3-channel, 2-input 250MHz video multiplexer, is designed for pixel switching, video graphics and RGB routing. The complete circuit squeezes into a 16-lead SSOP package and uses only 0.25in2 of PC board area (Figure 1). The LT1675 features a fixed gain of 2 for driving double-terminated cables. By incorporating internal feedback resistors, the circuit simplifies PC board layout and boosts performance by eliminating stray capacitance. A single channel 2:1 MUX, the LT1675-1 is available in the small MSOP package. Table 1 summarizes the major performance specifications of this new multiplexer, and Figure 2 shows a typical application: switching between two RGB sources.

Figure 1. Board Photo Actual Size.

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
 Output Offset Voltage    20mV
 Supply Current   All Three Channels Active 30mA 
 Supply Current Disabled   1μA 

Figure 2. LT1675 Typical Application: Switching Between Two RGB Sources and Driving Three Cables.

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 between the two LT1675s with the ENABLE pins. This technique, shown in Figure 3, does not increase the power dissipation because LT1675s draw virtually zero current when disabled.

Figure 3. Two LT1675s Make a 4-Input RGB Router.

Add Your Own Logo

The circuit of Figure 4 highlights a section of picture under control of a synchronous key signal. The technique is used for adding the logo you see in the bottom corner of commercial television pictures or any type of overlay signal, such as a crosshair or a reticule. The key signal has two bits of control, so there can be four levels of highlighting: unmodified video, video plus 33% white, video plus 66% white and 100% white. Two LT1675s are configured as a 2-bit DAC, and resistors on the output set the relative bit weights. The output of the LT1675, labeled B, is one-half the weight of the A device. To properly match the 75Ω video cable, the output resistors are selected so the parallel combination of the two is 75Ω. The output will never exceed peak white, which is 0.714V for this NTSC-related RGB video. The reference white signal is adjusted to a lower level than peak white to make the effect less intrusive.

Figure 4. Logo Inserter.



Frank Cox


John Wright