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AD815ARB-24 Arkusz danych(PDF) 10 Page - Analog Devices |
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AD815ARB-24 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page AD815 REV. B –10– Choice of Feedback and Gain Resistors The fine scale gain flatness will, to some extent, vary with feedback resistance. It therefore is recommended that once optimum resistor values have been determined, 1% tolerance values should be used if it is desired to maintain flatness over a wide range of production lots. Table I shows optimum values for several useful configurations. These should be used as starting point in any application. Table I. Resistor Values RF ( )RG ( ) G = +1 562 –1 499 499 +2 499 499 +5 499 125 +10 1 k 110 PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS As to be expected for a wideband amplifier, PC board parasitics can affect the overall closed-loop performance. Of concern are stray capacitances at the output and the inverting input nodes. 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. POWER SUPPLY BYPASSING Adequate power supply bypassing can be critical when optimizing 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 10.0 µF and 0.1 µF is recommended. Under some low frequency applications, a bypass capacitance of greater than 10 µF may be necessary. Due to the large load currents delivered by the AD815, special consideration must be given to careful bypassing. The ground returns on both supply bypass capacitors as well as signal common must be “star” connected as shown in Figure 41. RF RG (OPTIONAL) RF +VS +OUT –OUT –VS +IN –IN Figure 41. Signal Ground Connected in “Star” Configuration 1V 20ns SIDE B SIDE A G = –1 RF = 562 RL = 100 Figure 39. 4 V Step Response, G = –1 THEORY OF OPERATION The AD815 is a dual current feedback amplifier with high (500 mA) output current capability. Being a current feedback amplifier, the AD815’s open-loop behavior is expressed as transimpedance, ∆V O/ ∆I –IN , or TZ. The open-loop transimpedance behaves just as the open-loop voltage gain of a voltage feedback amplifier, that is, it has a large dc value and decreases at roughly 6 dB/octave in frequency. Since RIN is proportional to 1/gM, the equivalent voltage gain is just TZ × g M, where the gM in question is the transconductance of the input stage. Using this amplifier as a follower with gain, Figure 40, basic analysis yields the following result: V O V IN = G × T Z S () T Z S ()+G × R IN + RF where: G R R F G =+ 1 RIN = 1/gM ≈ 25 Ω RIN VIN RF VOUT RG RN Figure 40. Current Feedback Amplifier Operation Recognizing that G × R IN << RF for low gains, it can be seen to the first order that bandwidth for this amplifier is independent of gain (G). Considering that additional poles contribute excess phase at high frequencies, there is a minimum feedback resistance below which peaking or oscillation may result. This fact is used to determine the optimum feedback resistance, RF. In practice parasitic capacitance at the inverting input terminal will also add phase in the feedback loop, so picking an optimum value for RF can be difficult. Achieving and maintaining gain flatness of better than 0.1 dB at frequencies above 10 MHz requires careful consideration of several issues. |
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