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AD8143 Datasheet(PDF) 19 Page - Analog Devices |
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AD8143 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 24 page  AD8143 Rev. 0 | Page 19 of 24 In many dual-supply applications, VREF can be directly connected to ground right at the device. TERMINATING THE INPUT One of the key benefits of the active-feedback architecture is the separation that exists between the differential input signal and the feedback network. Because of this separation, the differential input maintains its high CMRR and provides high differential and common-mode input impedances, making line termination a simple task. Most applications that use the AD8143 involve transmitting broadband video signals over 100 Ω UTP cable and use dc-coupled terminations. The two most common types of dc-coupled terminations are differential and common-mode. Differential termination of 100 Ω UTP is implemented by simply connecting a 100 Ω resistor across the amplifier input, as shown in Figure 40. + – +5V –5V REF FB RG RF 0.01 μF 0.01 μF + – VIN VOUT 100 Ω UTP 100 Ω Figure 40. Differential-Mode Termination Some applications require common-mode terminations for common-mode currents generated at the transmitter. In these cases, the 100 Ω termination resistor is split into two 50 Ω resistors. The required common-mode termination voltage is applied at the tap between the two resistors. In many of these applications, the common-mode tap is connected to ground (VTERM (CM) = 0). This scheme is illustrated in Figure 41. + – +5V –5V REF FB RG RF 0.01 μF 0.01 μF + – VIN VOUT VTERM(CM) 100 Ω UTP 50 Ω 50 Ω Figure 41. Common-Mode Termination INPUT CLAMPING The differential input that is assigned to receive the input signal includes clamping diodes that limit the differential input swing to approximately 5.5 V p-p at 25°C. Because of this, the input and feedback stages should never be interchanged. Figure 31 illustrates the clamping action at the signal input stage. The supply current drawn by the AD8143 has a strong dependence on input signal magnitude because the input transconductance stages operate with differential input signals that can be up to a few volts peak-to-peak. This behavior is distinctly different from that of traditional op-amps, where the differential input signal is driven to essentially 0 V by negative feedback. Figure 28 illustrates the supply current dependence on input voltage. For most applications, including receiving RGB video signals, the input signal magnitudes encountered are well within the safe operating limits of the AD8143 over its full power supply and operating temperature ranges. In some extreme applications where large differential and/or common-mode voltages can be encountered, external clamping may be necessary. Another application where external common-mode clamping is sometimes required is when an unpowered AD8143 receives a signal from an active driver. In this case, external diodes are required when the current drawn by the internal ESD diodes cannot be kept to less than 5 mA. When using ±12 V supplies, the differential input signal must be kept to less than 4 V p-p. In applications that use ±12 V supplies where the input signals are expected to reach or exceed 4 V p-p, external differential clamping at a maximum of 4 V p-p is required. Figure 42 shows a general approach to external differential- mode clamping. POSITIVE CLAMP NEGATIVE CLAMP RS RT VIN VOUT RS – + – + Figure 42. Differential-Mode Clamping The positive and negative clamps are nonlinear devices that exhibit very low impedance when the voltage across them reaches a critical threshold (clamping voltage), thereby limiting the voltage across the AD8143 input. The positive clamp has a positive threshold, and the negative clamp has a negative threshold. |
Similar Part No. - AD8143_15 |
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Similar Description - AD8143_15 |
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