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AD8145 Datasheet(PDF) 16 Page - Analog Devices |
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AD8145 Datasheet(HTML) 16 Page - Analog Devices |
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16 / 24 page  AD8145 Rev. 0 | Page 16 of 24 To achieve unity gain from VREF to VOUT in this configuration, divide VREF by the same factor used in the feedback loop; the divider resistors, RD, need not be the same values used in the internal feedback loop. Figure 35 illustrates this approach. GAIN REF R R 0.01µF 0.01µF +5V –5V C VREF VOUT VIN RD RD Figure 35. Basic Gain Circuit: VOUT = 2VIN + VREF The gain equation for the circuit in Figure 35 is VOUT = 2VIN + VREF (3) Another configuration that provides the same gain equation as Equation 3 is shown in Figure 36. In this configuration, it is important to keep the source resistance of VREF much smaller than 350 Ω to avoid gain errors. GAIN REF R R 0.01µF 0.01µF +5V –5V C VREF VOUT VIN Figure 36. Basic Gain Circuit: VOUT = 2VIN + VREF For stability reasons, the inductance of the trace connected to the REF pin must be kept to less than 10 nH. The typical inductance of 50 Ω traces on the outer layers of the FR-4 boards is 7 nH/in, and on the inner layers, it is typically 9 nH/in. Vias must be accounted for as well. The inductance of a typical via in a 0.062 inch board is on the order of 1.5 nH. If longer traces are required, a 200 Ω resistor should be placed in series with the trace to reduce the Q-factor of the inductance. 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 AD8145 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 37. GAIN REF R R 0.01µF 0.01µF +5V –5V C OUT VOUT VIN 100Ω UTP 100Ω Figure 37. Differential-Mode Termination with G = 1 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 38. GAIN REF R R 0.01µF 0.01µF +5V –5V C OUT VOUT VIN 100Ω UTP 50Ω 50Ω VTERM(CM) Figure 38. Common-Mode Termination with G = 1 |
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