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AD8001AQ Arkusz danych(PDF) 10 Page - Analog Devices |
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AD8001AQ Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page REV. C AD8001 –10– THEORY OF OPERATION A very simple analysis can put the operation of the AD8001, a current feedback amplifier, in familiar terms. Being a current feedback amplifier, the AD8001’s open-loop behavior is ex- pressed 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 the RIN is proportional to 1/gM, the equivalent voltage gain is just TZ × g M, where the gM in question is the trans- conductance of the input stage. This results in a low open-loop input impedance at the inverting input, a now familiar result. Using this amplifier as a follower with gain, Figure 40, basic analysis yields the following result. V V G TS TS G R R G R R Rg O IN Z ZIN IN M =× +× + =+ = ≈ () () / 1 1 1 2 150 Ω Recognizing that G × R IN << R1 for low gains, it can be seen to the first order that bandwidth for this amplifier is independent of gain (G). This simple analysis in conjunction with Figure 41 can, in fact, predict the behavior of the AD8001 over a wide range of conditions. VOUT R1 R2 RIN VIN Figure 40. 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 Pin 2 will also add phase in the feedback loop, so picking an optimum value for RF can be difficult. Fig- ure 42 illustrates this problem. Here the fine scale (0.1 dB/div) flatness is plotted vs feedback resistance. These plots were taken using an evaluation card which is available to customers so that these results may readily be duplicated (see Evaluation Board section). Achieving and maintaining gain flatness of better than 0.1 dB at frequencies above 10 MHz requires careful consideration of several issues. FREQUENCY – Hz 1M 10 100k 1M 1G 100M 10M 100 100k 10k 1k Figure 41. Transimpedance vs. Frequency 0.1 0 –0.9 1M 10M 100M –0.1 –0.2 –0.3 –0.4 –0.5 FREQUENCY – Hz –0.6 –0.7 –0.8 G = +2 RF = 649 RF = 698 RF = 750 Figure 42. 0.1 dB Flatness vs. Frequency Choice of Feedback and Gain Resistors Because of the above-mentioned relationship between the band- width and feedback resistor, 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. In addition, resistors of different construction have different associ- ated parasitic capacitance and inductance. Surface mount resis- tors were used for the bulk of the characterization for this data sheet. It is not recommended that leaded components be used with the AD8001. |
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Podobny opis - AD8001AQ |
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