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AD8390AACPZ-R7 Scheda tecnica(PDF) 11 Page - Analog Devices |
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AD8390AACPZ-R7 Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 12 page Data Sheet AD8390A Rev. B | Page 11 of 12 ADSL AND ADSL2+ APPLICATIONS In a typical ADSL/ADSL2+ application, a differential line driver drives the signal from the analog front end (AFE) onto the twisted pair telephone line. Referring to the typical circuit representation in Figure 17, the differential input appears at VIN+ and VIN− from the AFE. 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. In ADSL/ADSL2+ applications, it is common practice to conserve power by using positive feedback (R3 in Figure 17) to synthesize the output resistance, lowering the required value of the line matching resistors, RM. RM RL VOUT, DM + – –OUT +OUT RM R3 R2 R1 R1 R3 0.1µF RADJ R2 1:N 10µF 0.1µF VCC +IN VCOM –IN 10µF 0.1µF 0.1µF VEE Figure 17. ADSL/ADSL2+ Application Circuit The differential input impedance to the circuit is 2 × R1. R1 is chosen by the designer to match system requirements. The synthesized value of the back termination resistor is given by the following equation. 2 2 N R k R L M × × = where RL is the line impedance, and N is the turns ratio of the transformer. The factor k defines the relationship between the negative and positive feedback resistors and is given by 2 1 R R3 k − = Commonly used values for k are between 0.1 and 0.25. Values less than 0.1 can lead to instability and are not recommended. Assuming low values for back termination resistor RM, R3 is approximated as V A k R1 R3 × × × ≅ 2 where AV is the voltage gain. R2 is given by k R3 R2 − = 1 With RM, R3, and R2 calculated, the closest 1% resistors are chosen and the gain rechecked with the following equation: ( ) [ ] R3 k R2 R R1 R3 R2 A M V − + + × = 1 Table 6 compares the results of the exact values, the simplified approximation, and the closest 1% resistor value calculations. In this example, R1 = 1.0 kΩ, AV = 10, and k = 0.1. Note that decreasing the value of the back termination resistors attenuates the receive signal by approximately 1/k. Advances in low noise receive amplifiers permit the use of k values as small as 0.1. The line impedance, turns ratio, and k factor specify the output voltage and current required from the AD8390A. To accom- modate higher crest factors or lower supply rails, the turns ratio, N, may need to be increased. Because higher turns ratios and smaller k factors both attenuate the receive signal, a large increase in N may require an increase in k to maintain the desired noise performance. Any particular design process requires that these trade-offs be addressed. Table 6. Resistor Selection Component Exact Value Approximate Calculation Standard 1% Resistor Value R1 (Ω) 1000 1000 1000 R2 (Ω) 2246.95 2222.22 2210 R3 (Ω) 2022.25 2000 2000 RM (Ω) 5 5 4.99 Actual AV 10.000 9.889 10.138 Actual k 0.1 0.1 0.095 LIGHTNING AND AC POWER FAULT When the AD8390A is an ADSL/ADSL2+ line driver, it is transformer-coupled to the twisted pair telephone line. In this environment, the AD8390A is subject to large line transients resulting from events such as lightning strikes or downed power lines. Additional circuitry is required to protect the AD8390A from damage due to these events. |
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