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AD9621AQ Fiches technique(PDF) 4 Page - Analog Devices |
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AD9621AQ Fiches technique(HTML) 4 Page - Analog Devices |
4 / 6 page AD9621 REV. 0 –4– phase margin (55 °), low noise current (3.6 pA/ √Hz), and slew rate (1200 V/ µs) give higher performance capabilities to these applications over previous voltage feedback designs. With a settling time of 11 ns to 0.01% and 7 ns to 0.1%, the de- vice is an excellent choice for DAC I/V conversion. The same characteristics, along with low harmonic distortion, make it a good choice for ADC buffering/amplification. With its superb linearity at relatively high signal frequencies, it is an ideal driver for ADCs up to 14 bits. Layout Considerations As with all wide bandwidth components, printed circuit layout is critical to obtain best dynamic performance with the AD9621. The ground plane in the area of the amplifier and its associated components should cover as much of the component side of the board as possible (or first interior layer of a multi layer surface mount board). The ground plane should be removed in the area of the inputs and RF and RG to minimize stray capacitance at the input. The same precaution should be used for CB, if used. Each power supply trace should be decoupled close to the package with a 0.1 µF ceramic capacitor, plus a 6.8 µF tantalum nearby. All lead lengths for input, output, and feedback resistor should be kept as short as possible. All gain setting resistors should be chosen for low values of parasitic capacitance and inductance, i.e., microwave resistors and/or carbon resistors. Microstrip techniques should be used for lead lengths in excess of one inch. Sockets should be avoided if at all possible because of their high series inductance. If sockets are necessary, indi- vidual pin sockets such as AMP p/n 6-330808-3 should be used. These contribute far less stray reactance than molded socket assemblies. An evaluation board is available from Analog Devices for a nominal charge. Pulse Response Unlike a traditional voltage feedback amplifier in which slew speed is dictated by its front end dc quiescent current and gain bandwidth product, the AD9621 provides “on demand” trans- conductance current that increases proportionally to the input “step” signal amplitude. This results in slew speeds (1200 V/ µs) comparable to wideband current feedback designs. This, com- bined with relatively low input noise current (3.6 pA/ √Hz), gives the AD9621 the best attributes of both voltage and current feed- back amplifiers. Bootstrap Capacitor (CB) In most applications, the CB capacitor will not be required. Under certain conditions, it can be used to further enhance set- tling time performance. The CB capacitor (0.001 µF) connects to the internal high im- pedance nodes of the amplifier. Using this capacitor will reduce the large signal (4 V) step output settling time by 3 to 5 ns for 0.05% or greater accuracy. For settling accuracy less than 0.05% or for smaller step sizes, its effect will be less apparent. Under heavy slew conditions, this capacitor forces the internal signal (initial step) amplitude to be controlled by the “on” (slewed) transistor, preventing its complement from completely turning off. This allows for faster settling time of these internal nodes and also the output. In the frequency domain, total (high frequency) distortion will be approximately the same with or without CB. Typically, the 3rd harmonic will be greater than the 2nd without CB. This will be reversed with CB in place. APPLICATIONS The AD9621 is a voltage feedback amplifier and is well suited for such applications as photo-detector preamp, active filters, and log amplifiers. The device’s wide bandwidth (350 MHz), R F C F C I V OUT Figure 1. Transimpedance Configuration 2 3 4 7 6 0.1 µF 0.1 µF 1 8 6.8 µF R G V IN 6.8 µF +V S –V S C B (OPTIONAL) C F V OUT R F 500 Ω A V = 1+ R F R G Figure 3. Noninverting Gain Connection Diagram 2 3 4 7 6 0.1 µF 0.1 µF 1 8 6.8 µF R F R G R G V IN 6.8 µF +V S –V S C B (OPTIONAL) C F V OUT R F 500 Ω A V = –R F R G Figure 2. Inverting Gain Connection Diagram |
Numéro de pièce similaire - AD9621AQ |
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Description similaire - AD9621AQ |
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