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AD8001 Datasheet(PDF) 11 Page - Analog Devices |
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AD8001 Datasheet(HTML) 11 Page - Analog Devices |
11 / 17 page REV. D 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 expressed as transimpedance, ∆VO/∆I–IN, or TZ. The open-loop transimped- ance 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 4, 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 VOUT R1 R2 RIN VIN Figure 4. Follower with Gain 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 5 can, in fact, predict the behavior of the AD8001 over a wide range of conditions. FREQUENCY – Hz 1M 10 100k 1M 1G 100M 10M 100 100k 10k 1k Figure 5. Transimpedance vs. Frequency 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. Figure 6 illustrates this problem. Here the fine scale (0.1 dB/ div) flatness is plotted versus feedback resistance. These plots were taken using an evaluation card which is available to cus- tomers so that these results may readily be duplicated. Achieving and maintaining gain flatness of better than 0.1 dB at frequencies above 10 MHz requires careful consideration of several issues. 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 6. 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 associated parasitic capacitance and inductance. Surface-mount resistors were used for the bulk of the characterization for this data sheet. It is not recommended that leaded components be used with the AD8001. |
Similar Part No. - AD8001_17 |
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Similar Description - AD8001_17 |
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