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AD8079AR-REEL7 Datasheet(PDF) 7 Page - Analog Devices |
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AD8079AR-REEL7 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page 9 REV. A AD8079 –7– FREQUENCY – Hz 10k 1G 100k 1M 10M 100M 100 10 1 0.1 0.01 VS = ±5.0V POWER = 0dBm (223.6mV rms) RbT = 50Ω RbT = 0Ω Figure 21. Output Resistance vs. Frequency –44.0 –66.5 125 –61.5 –64.0 –35 –55 –59.0 –56.5 –54.0 –51.5 –49.0 –46.5 105 85 65 45 25 5 –15 JUNCTION TEMPERATURE – °C –69.0 –PSRR +PSRR 2V SPAN CURVES ARE FOR WORST CASE CONDITION WHERE ONE SUPPLY IS VARIED WHILE THE OTHER IS HELD CONSTANT. Figure 22. PSRR vs. Temperature FREQUENCY – Hz 0 –4 –84 30k 500M 100k 1M 10M 100M –14 –24 –64 –34 –44 –54 –74 VIN = 200mV –PSRR +PSRR Figure 23. PSRR vs. Frequency THEORY OF OPERATION The AD8079, a dual current feedback amplifier, is internally configured for a gain of either +2 (AD8079A) or +2.2 (AD8079B). The internal gain-setting resistors effectively elimi- nate any parasitic capacitance associated with the inverting in- put pin, accounting for the AD8079’s excellent gain flatness response. The carefully chosen pinout greatly reduces the cross- talk between each amplifier. Up to four back-terminated 75 Ω video loads can be driven by each amplifier, with a typical dif- ferential gain and phase performance of 0.01%/0.17 °, respec- tively. The AD8079B, with a gain of +2.2, can be employed as a single gain-trimming element in a video signal chain. Finally, the AD8079A/B used in conjunction with our AD8116 cross- point matrix, provides a complete turn-key solution to video distribution. Printed Circuit Board Layout Considerations As to be expected for a wideband amplifier, PC board parasitics can affect the overall closed-loop performance. If a ground plane is to be used on the same side of the board as the signal traces, a space (5 mm min) should be left around the signal lines to minimize coupling. Line lengths on the order of less than 5 mm are recommended. If long runs of coaxial cable are being driven, dispersion and loss must be considered. Power Supply Bypassing Adequate power supply bypassing can be critical when optimiz- ing the performance of a high frequency circuit. Inductance in the power supply leads can form resonant circuits that produce peaking in the amplifier’s response. In addition, if large current transients must be delivered to the load, then bypass capacitors (typically greater than 1 µF) will be required to provide the best settling time and lowest distortion. A parallel combination of 4.7 µF and 0.1 µF is recommended. Some brands of electrolytic capacitors will require a small series damping resistor ≈ 4.7 Ω for optimum results. DC Errors and Noise There are three major noise and offset terms to consider in a current feedback amplifier. For offset errors refer to the equa- tion below. For noise error the terms are root-sum-squared to give a net output error. In the circuit below (Figure 24) they are input offset (VIO) which appears at the output multiplied by the noise gain of the circuit (1 + RF/RI), noninverting input current (IBN × R N) also multiplied by the noise gain, and the inverting input current, which when divided between RF and RI and sub- sequently multiplied by the noise gain always appears at the out- put as IBN × R F. The input voltage noise of the AD8079 is a low 2 nV/ √Hz. At low gains though the inverting input current noise times RF is the dominant noise source. Careful layout and de- vice matching contribute to better offset and drift specifications for the AD8079 compared to many other current feedback am- plifiers. The typical performance curves in conjunction with the equations below can be used to predict the performance of the AD8079 in any application. V OUT =V IO × 1+ R F R I ± IBN × RN × 1+ R F R I ± IBI × RF where: RF = RI = 750 Ω for AD8079A RF = 750 Ω, R I = 625 Ω for AD8079B |
Similar Part No. - AD8079AR-REEL7 |
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Similar Description - AD8079AR-REEL7 |
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