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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD8390ARC-REEL Fiches technique(PDF) 11 Page - Analog Devices |
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AD8390ARC-REEL Fiches technique(HTML) 11 Page - Analog Devices |
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11 / 16 page  AD8390 Rev. C | Page 11 of 16 RM RL VOUT,DM + – –OUT +OUT RM R3 R2 R1 R1 R3 0.1 µF RADJ R2 1:N 10 µF 0.1 µF VCC +IN VOCM –IN 10 µF 0.1 µF 0.1 µF VEE Figure 24. ADSL/ADSL2+ Application Circuit Referring to Figure 24, the following describes how to calculate the resistor values necessary to obtain the desired input imped- ance, gain, and output impedance. The differential input impedance to the circuit is simply 2R1. As such, R1 is chosen by the designer to yield the desired input impedance. When synthesizing the output impedance, a factor k is introduced, which is used to express the ratio of the negative feedback resistor to the positive feedback resistor by R2 R3 k = − 1 (5) Along with the turns ratio N, k is also used to define the value of the back termination resistors RM. Commonly used values for k are 0.1 to 0.25. A k value of 0.1 would result in back termination resistors that are only 1/10 as large as those in the simplest case described above. Lower values of k result in greater amounts of positive feedback. Therefore, values much lower than 0.1 can lead to instability and are generally not recommended. 2 2 N R k R L M × × = (6) This factor (k), along with R1, RM, and the desired gain (AV), is then used to calculate the necessary values for R3 and R2. ( ) M M V V V R k R k R1 A R1 A k R1 A R3 × − + × × × × + × × = 2 (7) The usually small value for RM allows a simplified approximation for R3. V A k R1 R3 × × × ≅ 2 (8) k R3 R2 − = 1 (9) Once RM, R3, and R2 are computed, the closest 1% resistors can be chosen and the gain rechecked with the following equation: () R1 R3 R2 R2 k R R3 R2 A M V × − + × + × = (10) Table 6 shows a comparison of the results using the exact values, the simplified approximation, and the closest 1% resistor values. In this example, R1, AV, and k were chosen to be 1.0 kΩ, 10 kΩ, and 0.1 kΩ, respectively. It should be noted that decreasing the value of the back termi- nation resistors attenuates the receive signal by approximately 1/k. However, advances in low noise receive amplifiers permit k values as small as 0.1 to be commonly used. The line impedance, turns ratio, and k factor specify the output voltage and current requirements from the AD8390. To accom- modate higher crest factors or lower supply rails, the turns ratio, N, may have to be increased. Since 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 visited. Table 6. Resistor Selection Component Exact Values Approximate Calculation Standard 1% Resistor Values 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 (Eq. 5) 0.1 0.1 0.095 MULTITONE POWER RATIO (MTPR) Multitone power ratio is a commonly used figure of merit that xDSL designers use to help describe system performance. MTPR is the measured delta between the peak of a filled frequency bin and the harmonic products that appear in an intentionally empty frequency bin. Figure 25 illustrates this principle. The plots in Figure 10 and Figure 13 show MTPR performance in various power modes. All data were taken with a circuit with a k factor of 0.1, a 1:1 turns ratio transformer, and a waveform with a 5.4 peak-to-average ratio, also known as the crest factor (CF). –70dBc 10dB/DIV CENTER 431.25kHz SPAN 10kHz 1kHz/DIV Figure 25. MTPR Measurement |
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Description similaire - AD8390ARC-REEL |
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