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SGM9123XS Datasheet(PDF) 6 Page - SG Micro Corp |
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SGM9123XS Datasheet(HTML) 6 Page - SG Micro Corp |
6 / 9 page SGM9123 6 Applications Information Functional Description SGM9123 operates from a single +2.5V to +5V supply. In application, SGM9123 is a fully integrated solution for filtering and buffering SDTV signals in front of video decoder or behind video encoder. For example, SGM9123 can replace three passive LC filters and three amplifier drivers at CVBS and S-video output side in set-top box and DVD player, this solution can help you save PCB size and production cost, it also improves video signal performance comparing with traditional design using discrete components. SGM9123 features a DC-coupled input buffer, 3-pole low-pass filter to eliminate out-of-band noise of video encoder, and a gain of +6dB in the output amplifier to drive 75Ω load. The AC or DC-coupled input buffer eliminates sync crush, droop, and field tilt. The output of SGM9123 also can be DC-coupled or AC-coupled. Input Considerations Besides AC coupling, the SGM9123 inputs also can be DC-coupled. In DC coupling application, No input coupling capacitors are needed because the amplitude of input video signal from DAC includes ground and extends up to 1.4V, then SGM9123 can be directly connected to the output of a single-supply, current-output DAC without any external bias network. Some time, if DAC’s output level exceeds the range of 0V to 1.4V, or SGM9123 is driven by an unknown external source or a SCART switch which has its own clamping circuit, AC coupling is needed in such applications. Output Considerations The SGM9123 outputs can be DC-coupled or AC-coupled. When 0V is input, the SGM9123 output voltage is 260mV typically. In DC coupling design, one 75Ω resistor is used to Connect SGM9123’s output pin with external load directly, this serial back-termination resistor is used to match the impedance of the transmission line between SGM9123 and external load to cancel the signal reflection. The SGM9123 outputs can sink and source current allowing the device to be AC-coupled with external load, in AC coupling, 220µF at least capacitor will be used in order to eliminate field tilt. Power-Supply Bypassing and Layout Correct power supply bypassing is very important for optimizing video performance in design. One 0.1µF and one 10µF capacitors are always used to Bypass VCC pin of SGM9123, please place these two capacitors as close to the SGM9123 output pin as possible, a large ground plane is also needed to ensure optimum performance. The input and output termination resistors should be placed as close to the related pin of SGM9123 as possible to avoid performance degradation. The PCB traces at the output side should have 75Ω characteristic impedance in order to match the 75Ω characteristic impedance cable connecting external load. In design, please keep the board trace at the inputs and outputs of the SGM9123 as short as possible to minimize the parasitic stray capacitance and noise pickup. Typical Application Diagram The following schematic in Figure 3 is normally used for AC coupled output and DC-coupled input with DAC which has an output voltage range of 0V to 1.4V. AC coupled output offer slightly lower power dissipation and high ESD protection ability. The schematics in Figure 1 and Figure 2 are also popular in design. Figure 4 is a kind of special application in STB. CVBS1 Y C 75Ω 1 2 3 4 5 6 7 8 IN1 IN2 IN3 GND OUT1 OUT2 OUT3 +VS 75Ω 75Ω 75Ω SGM9123 0.1µF or 0.01µF 10µF +5V 75Ω 75Ω CVBS1 Y C S-Video S-Video Figure 1. DC Coupling Application Schematic Figure 2. AC Coupling Application Schematic 4 5 6 7 8 IN1 IN2 IN3 GND OUT1 OUT2 OUT3 +VS 75Ω 75Ω 75Ω SGM9123 0.1µF or 0.01µF 10µF +5V CVBS1 Y C 220µF 220µF 220µF S-Video CVBS1 Y C 75Ω 1 2 3 75Ω 75Ω S-Video Figure 3. Input DC Coupling and Output AC Coupling Application Schematic |
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