Datasheet AD8392A (Analog Devices) - 9

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Data Sheet AD8392A TYPICAL ADSL/ADSL2+ APPLICATION
Additional definitions for calculating resistor values include: In a typical ADSL/ADSL2+ application, a differential line driver
Value Definition
is used to take the signal from the analog front end (AFE) and V drive it onto the twisted pair telephone line. Referring to the OA Voltage at the amplifier outputs K Matching resistance reduction factor typical circuit representation in Figure 18, the differential input A appears at V V Gain from VIN to transformer primary IN+ and VIN− from the AFE, while the differential β Negative feedback factor output is transformer coupled to the telephone line at tip and ring. The common-mode operating point, generally midway α Positive feedback factor between the supplies, is set through V Note: R1 must be calculated before β and α. COM.
R3
V ( + k) 2 R V
R4
V LINE = 1 m k = LINE A =
V V
V
IN+ OA
OA
V
N R N V
P
L IN
TIP Rm
R1
R
= α = β 1−
BIAS
β ( k)
R2
R1+ 2R2
R R IN OUT VCOM 1:N R1
With the above known quantities and definitions, the remaining
R2 R
resistors can readily be calculated.
BIAS Rm RING
2 P V R2
R4 VP
R1 =
V V IN– OA
OA V − P V 024 R (V −V )
R3
06477- IN IN P R4 = Figure 18. Typical ADSL/ADSL2+ Application Circuit 2 IN V A + − − V R4 (2 R1Rm R1RL αR1RL 2αR2RL ) In ADSL/ADSL2+ applications, it is common practice to R3 = conserve power by using positive feedback to synthesize the α R + L (R1 2 R2) output resistance, thereby lowering the required ohmic value α R3R4 of the line matching resistors, Rm. The circuit in Figure 18 is RBIAS = R4 − α(R3 + R4) somewhat unique in that the positive feedback introduced via R3 has the effect of synthesizing the input resistance as well. After building the circuit with the closest 1% resistor values, The following definitions and equations can be used to calculate the actual gain, input resistance, and output resistance can be the resistor values necessary to obtain the desired gain, input verified with the fol owing equations. resistance, and output resistance for a given application. For simplicity, the following calculations assume a lossless N GAIN(V = IN to LINE ) transformer. β(   k + ) R4 R4 R4 1 1+ +  −  R3 RBIAS  R3 The fol owing values are used in the design equations and are assumed already known or chosen by the designer. 2 R = IN 1  2R + R 
Value Definition
− m L V A  β  R4 R4R V  L  IN Differential input voltage RIN Desired differential input resistance 2R 2 m N N Transformer turns ratio R = OUT   VLINE Differential output voltage at tip and ring    R4 R  R1+ 2R2 R BIAS   m Each is typically 5% to 15% of the transformer reflected 1−  R1(R4+R ) line impedance  BIAS R4 RBIAS     R3 +  R2 Recommended in the amplifier data sheet  R4 + RBIAS  VP Voltage at the + inputs to the amplifier, approximately ½ V IN (must be less than VIN for positive input resistance) RL Transformer reflected line impedance Rev. A | Page 9 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Maximum Power Dissipation ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS SUPPLIES, GROUNDING, AND LAYOUT POWER MANAGEMENT THERMAL CONSIDERATIONS TYPICAL ADSL/ADSL2+ APPLICATION MULTITONE POWER RATIO OUTLINE DIMENSIONS ORDERING GUIDE NOTES