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AD45048ARZ-REEL7 Fiches technique(PDF) 7 Page - Analog Devices |
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AD45048ARZ-REEL7 Fiches technique(HTML) 7 Page - Analog Devices |
7 / 8 page AD45048 Rev. A | Page 7 of 8 GENERAL DESCRIPTION The AD45048 is a voltage feedback, rail-to-rail output amplifier with high output current capability. Fabricated on Analog Devices’ proprietary high speed eXtra fast complementary bipolar high voltage process (XFCB-HV), the high bandwidth and fast slew rate of the AD45048 keep distortion to a minimum while dissipating minimum power. The XFCB-HV, silicon-on-insulator (SOI) process prevents latch-up problems and enables the construction of high frequency, low distortion amplifiers, such as the AD45048. POWER SUPPLY AND DECOUPLING The AD45048 can be powered with a good quality, well regulated, low noise supply anywhere in the range from +3 V to ±12.6 V. In order to optimize the AD45048 in standard ADSL CPE line driver applications (see Figure 8), power the amplifier with a well regulated 12 V supply. Careful attention should be paid to decoupling the power supply. High quality capacitors with low equivalent series resistance (ESR), such as multilayer ceramic capacitors (MLCCs), should be used to minimize the supply voltage ripple and power dissipation. A 0.1 μF MLCC decoupling capacitor(s) should be located no more than 1/8-inch away from the power supply pin(s). A large, usually tantalum, 10 μF to 47 μF capacitor is recommended to provide good decoupling for lower frequency signals and to supply current for fast, large signal changes at the AD45048 outputs. LAYOUT CONSIDERATIONS As is the case with all high speed applications, careful attention to printed circuit board layout details prevents associated board parasitics from becoming problematic. Proper RF layout and printed circuit board design techniques are strongly recommended. The PCB should have a low impedance return path (or ground) to the supply. Removing the ground plane from all layers in the immediate area of the amplifier reduces stray capacitances. The signal routing should be short and direct in order to minimize the parasitic inductance and capacitance associated with these traces. Termination resistors and loads should be located as close as possible to their respective inputs and outputs. Input traces should be kept as far apart as possible from the output traces to minimize coupling (crosstalk) though the board. Wherever there are complementary signals, a symmetrical layout should be provided to the extent possible in order to maximize balanced performance. When running differential signals over a long distance, the traces on the PCB should be close together or any differential wiring should be twisted together to minimize the area of the loop that is formed. This reduces the radiated energy and makes the circuit less susceptible to RF interference. Adherence to stripline design techniques for long signal traces (greater than about 1 inch) is recommended. CPE ADSL APPLICATION The low cost, high output current dual AD45048 xDSL line driver amplifiers have been specifically designed to drive high fidelity xDSL signals to within 0.5 V of the power rails on a single 12 V supply. The AD45048 can be used in transformer- coupled bridge hybrid circuits designed to drive modulated signals, including discrete multitone (DMT), upstream to the central office. TRANSFORMER SELECTION Customer premise ADSL applications require the transmission of a 13 dBm DMT signal (20 mW into 100 Ω). DMT signals can have a crest factor (V peak/V rms ratio) as high as 5.3, requiring the line driver to provide a peak power of 560 mW. The line driver is required to drive a 7.5 V peak onto the 100 Ω telephone line while maintaining about −65 dBc to −70 dBc of MTPR. Since the maximum low distortion output swing available from the AD45048 line driver is approximately 11.5 V on a 12 V supply (depending on the load), and taking into account the power lost in the transformer and termination resistors, a step-up transformer with a minimum turns ratio of 1.5 or greater is needed. In the simplified differential driver circuit shown in Figure 8, the AD45048 is driving a 25 Ω impedance reflected by 1:2 step-up transformer. R3 and R6 are 12.5 Ω each and are back-termination or load-matching resistors whose values can be calculated by (100 Ω/(N2))/2 where 100 Ω is the approximate phone line impedance and N is the transformer turns ratio. In Figure 8, the total differential load including the termination resistors is 50 Ω, and under these conditions, the AD45048 is capable of driving low distortion signals to within 0.5 V of the power rails. RECEIVE CHANNEL CONSIDERATIONS A step-up transformer of N turns used at the output of the differential line driver increases the differential output voltage to the line (see Figure 8). However, the inverse effect is seen in the receive channel as the amplitude of signal on the driver side of the transformer is divided by N turns. The decision to use a particular transformer turns ratio may be impacted by the ability of the receive circuitry to resolve low level signals in the noisy twisted pair telephone plant. Higher turns ratio transformers reduce the effective receive channel SNR (signal- to-noise ratio) due to the reduction in the received signal strength. An amplifier with low RTI noise, such as the AD8022 (2.5 nV/√Hz), is recommended for the receive channel. For a complete selection of amplifiers and other related components, see www.analog.com. |
Numéro de pièce similaire - AD45048ARZ-REEL7 |
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Description similaire - AD45048ARZ-REEL7 |
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