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LT1675 Datasheet(PDF) 11 Page - Linear Technology |
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LT1675 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT6555 11 6555f APPLICATIO S I FOR ATIO This systematic gain error can be significantly reduced by lowering the value of the 75 Ωseriesterminationresistors. The compromise of this approach is an increased depen- dence on the accuracy of the 75 Ω shunt termination at the receiving end of the line. A table of values for 1% series termination resistors from n = 2 to n = 4 is shown below. NUMBER OF DEVICES (n) SERIES RT 2 63.9 3 56.2 4 49.9 Another approach that does not compromise gain accu- racy is to connect the outputs directly together before the series termination. In this case, there will be slightly increased output glitching and supply current spiking during the EN pin switching, but the additional output loading will not increase the gain error, and the series termination resistors remain at their ideal value for AC response. See Figure 4 for a scope photo showing the result of the outputs connected both before and after the series terminations, and Figure 8 for a full schematic of a 4:1 RGB multiplexer with the output pins directly con- nected together. It is imperative that the output traces be as short as possible before the series termination in order to reduce capacitance and minimize AC peaking. Figure 4. 4-Input Router Switching with Outputs Directly Connected and with Outputs Connected After 63.9 Ω Series Termination ESD Protection The LT6555 has reverse-biased ESD protection diodes on all pins. If any pins are forced a diode drop above the positive supply or a diode drop below the negative supply, large currents may flow through these diodes. If the current is kept below 10mA, no damage to the devices will occur. TIME ( µs) 0 –0.5 0 0.5 4 6555 F04 –1.0 1 2 3 0.5 1.5 2.5 3.5 1.5 1.0 0 50 IS – IS + 100 150 VS = ±5V VIN(AMP1) = –0.5V VIN(AMP2) = 0.5V RL = 150Ω OUTPUTS DIRECTLY CONNECTED SERIES 63.9 Ω AT EACH OUTPUT TYPICAL APPLICATIO RGB Multiplexer Demo Board The DC858A Demo Board illustrates optimal routing, bypassing and termination using the LT6555 as an RGB video multiplexer. The schematic is shown in Fig- ure 5. All inputs and outputs are routed to have a charac- teristic impedance of 75 Ωand75Ωinputshuntandoutput series terminations are connected as close to the part as possible. The board is fabricated with four layers with internal ground and power planes. For ideal operation, a 75 Ω load termination should be connected at the output. The LT6555’s gain of 2 will compensate for the resulting divider between the series and load termination resistors. Figures 6 and 7 show the topside and bottom side board layout and placement. |
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