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AD8143 Datasheet(PDF) 22 Page - Analog Devices |
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AD8143 Datasheet(HTML) 22 Page - Analog Devices |
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22 / 24 page  AD8143 Rev. 0 | Page 22 of 24 DRIVING A CAPACITIVE LOAD The AD8143 typically drives either high impedance loads, such as crosspoint switch inputs, or doubly terminated coaxial cables. A gain of 1 is commonly used in the high impedance case because the 6 dB transmission line termination loss is not incurred. A gain of 2 is required when driving cables to compensate for the 6 dB termination loss. In all cases, the output must drive the parasitic capacitance of the feedback loop, conservatively estimated to be 2 pF, in addition to the capacitance presented by the actual load. When driving a high impedance input, it is recommended that a small series resistor be used to buffer the input capacitance of the device being driven. Clearly, the resistor value must be small enough to preserve the required bandwidth. In the ideal doubly terminated cable case, the AD8143 output sees a purely resistive load. In reality, there is some residual capacitance, and this is buffered by the series termination resistor. Figure 46 illustrates the high impedance case, and Figure 47 illustrates the cable- driving case. RS CIN RG RF + – 0.01 μF 0.01 μF +5V –5V FB REF VIN Figure 46. Buffering the Input Capacitance of a High-Z Load RS CS RG RF + – 0.01 μF 0.01 μF +5V –5V FB REF VIN RL Figure 47. Driving a Doubly Terminated Cable Small and large signal frequency responses for the High-Z case with a 40 Ω series resistor and 10 pF load capacitance are shown in Figure 10 and Figure 13; transient responses for the same conditions are shown in Figure 18 and Figure 21. In the cable driving case shown in Figure 47, CS << 2 pF for a well-designed circuit; therefore, the feedback loop capacitance is the dominant capacitive load. The feedback loop capacitance is present for all cases, and its effect is included in the data presented in the Typical Performance Characteristics and Specifications tables. POWER-DOWN The power-down feature is intended to be used to reduce power consumption when a particular device is not in use, and does not place the output in a High-Z state when asserted. The power-down feature is asserted when the voltage applied to the power-down pin drops to approximately 2 V below the positive supply. The AD8143 is enabled by pulling the power-down pin to the positive supply. COMPARATORS In addition to general-purpose applications, the two on-chip comparators can be used to receive differential digital information or to decode video sync pulses from received common-mode voltages. Built-in hysteresis helps to eliminate false triggers from noise. The comparator outputs are not designed to drive transmission lines. When the signals detected by the comparators are driven over cables or controlled impedance printed circuit board traces, the comparator outputs must be fed to a spare logic gate, FPGA, or other device that is capable of driving signals over transmission lines. An internal linear voltage regulator derives power for the comparators from the positive supply; therefore, the AD8143 must always have a minimum positive supply voltage of 4.5 V. SYNC PULSE EXTRACTION USING COMPARATORS The AD8143 is particularly useful in keyboard video mouse (KVM) applications. KVM networks transmit and receive computer video signals, which are typically comprised of red, green, and blue (RGB) video signals and separate horizontal and vertical sync signals. Because the sync signals are separate and not embedded in the color signals, it is advantageous to transmit them using a simple scheme that encodes them among the three common-mode voltages of the RGB signals. The AD8134 triple differential driver is a natural complement to the AD8143 and performs the sync pulse encoding with the necessary circuitry on-chip. |
Similar Part No. - AD8143_15 |
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Similar Description - AD8143_15 |
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