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AD8392AAREZ датащи(PDF) 10 Page - Analog Devices |
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AD8392AAREZ датащи(HTML) 10 Page - Analog Devices |
10 / 13 page Data Sheet AD8392A Rev. A | Page 9 of 12 TYPICAL ADSL/ADSL2+ APPLICATION In a typical ADSL/ADSL2+ application, a differential line driver is used to take the signal from the analog front end (AFE) and drive it onto the twisted pair telephone line. Referring to the typical circuit representation in Figure 18, the differential input appears at VIN+ and VIN− from the AFE, while the differential output is transformer coupled to the telephone line at tip and ring. The common-mode operating point, generally midway between the supplies, is set through VCOM. R4 R4 R3 R3 Rm R2 R1 R2 VCOM 1:N TIP RING ROUT RIN Rm VOA VOA VP VP RBIAS RBIAS VIN– VIN+ Figure 18. Typical ADSL/ADSL2+ Application Circuit In ADSL/ADSL2+ applications, it is common practice to conserve power by using positive feedback to synthesize the output resistance, thereby lowering the required ohmic value of the line matching resistors, Rm. The circuit in Figure 18 is somewhat unique in that the positive feedback introduced via R3 has the effect of synthesizing the input resistance as well. The following definitions and equations can be used to calculate the resistor values necessary to obtain the desired gain, input resistance, and output resistance for a given application. For simplicity, the following calculations assume a lossless transformer. The following values are used in the design equations and are assumed already known or chosen by the designer. Value Definition VIN Differential input voltage RIN Desired differential input resistance N Transformer turns ratio VLINE Differential output voltage at tip and ring Rm Each is typically 5% to 15% of the transformer reflected line impedance R2 Recommended in the amplifier data sheet VP Voltage at the + inputs to the amplifier, approximately ½ VIN (must be less than VIN for positive input resistance) RL Transformer reflected line impedance Additional definitions for calculating resistor values include: Value Definition VOA Voltage at the amplifier outputs K Matching resistance reduction factor AV Gain from VIN to transformer primary β Negative feedback factor α Positive feedback factor Note: R1 must be calculated before β and α. ( ) N k V V LINE OA + = 1 L m R R k 2 = IN LINE V V N V A = R2 R1 R1 2 β + = ( ) k − = 1 β α With the above known quantities and definitions, the remaining resistors can readily be calculated. P OA P V V R V R1 − = 2 2 ( ) IN P IN IN V 2 V V R R4 − = ( ) ( ) R2 R1 R R R2 R R1 R R1 R R1 R4 A R3 L L L L m V 2 α 2α α 2 + − − + = ( ) R4 R3 R4 R4 R3 RBIAS + − = α α After building the circuit with the closest 1% resistor values, the actual gain, input resistance, and output resistance can be verified with the following equations. ( ) ( ) R3 R4 R R4 R3 R4 k N GAIN BIAS LINE to VIN − + + + = 1 1 β + − = L L m V IN R R4 R R A R4 R 2 β 1 2 ( ) + + + + − = BIAS BIAS BIAS BIAS m OUT R R4 R R4 R3 R2 R1 R R4 R1 R R4 N R R 2 1 2 2 |
Аналогичный номер детали - AD8392AAREZ |
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Аналогичное описание - AD8392AAREZ |
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