Tiny RGB Video Multiplexer Switches Pixels at 100MHz Design Note 205 Frank Cox and John Wright Introduction The LT®1675, 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 Supply Current Supply Current Disabled 05/99/205_conv 20mV All Three Channels Active 30mA 1μA RED 1 V+ +1 75Ω 75Ω CABLE +2 VOUT RED +1 GREEN 1 75Ω 75Ω 75Ω +1 BLUE 1 CABLE +2 VOUT GREEN 75Ω 75Ω +1 RED 2 75Ω CABLE VOUT BLUE +2 75Ω GREEN 2 75Ω +1 V– +1 ENABLE 75Ω SELECT RGB1/RGB2 BLUE 2 75Ω DN205 F02 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. 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 L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 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. 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 R1 R3 AV = 2 AV = 2 75Ω 75Ω RED OUT 75Ω R2 LT1675 #1 R4 ENABLE ENABLE LT1675 #2 DN205 F03 CHIP SELECT 74HC04 Figure 3. Two LT1675s Make a 4-Input RGB Router LT1675 RED V+ +1 75Ω CABLE 113Ω +2 VIDEO IN VOUT RED +1 GREEN 75Ω 75Ω CABLE 113Ω +2 +1 BLUE VOUT GREEN 75Ω 75Ω CABLE 113Ω +1 VOUT BLUE +2 75Ω SELECT A 0 0 1 1 SELECT B 0 1 0 1 V– +1 OUTPUT NO VIDEO, 100% WHITE VIDEO PLUS 66% WHITE VIDEO PLUS 33% WHITE VIDEO, NO WHITE SELECT ENABLE +1 A SELECT A SELECT B B LT1675 V+ +1 226Ω +2 +1 226Ω +1 +2 +1 +2 226Ω 5V +1 10k 1.25V LT1634 10k V– SELECT 0.714V +1 ENABLE DN205 F04 Figure 4. Logo Inserter Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn205f_conv LT/TP 0599 370K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1999