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AD8072JN データシート(PDF) 9 Page - Analog Devices |
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AD8072JN データシート(HTML) 9 Page - Analog Devices |
9 / 11 page REV. A AD8072/AD8073 –9– APPLICATIONS Overdrive Recovery Overdrive of an amplifier occurs when the output and/or input range are exceeded. The amplifier must recover from this overdrive condition and resume normal operation. As shown in Figure 28, the AD8072 and AD8073 recover within 75 ns from positive overdrive and 30 ns from negative overdrive. 25ns 1V VIN VOUT Figure 28. Overload Recovery; VS = ±5 V, VIN = 8 V p-p, RF = 1 k Ω, RL = 150 Ω, G = +2 Bandwidth vs. Feedback Resistor Value The closed-loop frequency response of a current feedback am- plifier is a function of the feedback resistor. A smaller feedback resistor will produce a wider bandwidth response. However, if the feedback resistance becomes too small, the gain flatness can be affected. As a practical consideration, the minimum value of feedback resistance for the AD8072/AD8073 was found to be 649 Ω. For resistances below this value, the gain flatness will be affected and more significant lot to lot variations in device per- formance will be noticed. Figure 29 shows a plot of the frequency response of an AD8072/AD8073 at a gain of two with both feed- back and gain resistors equal to 649 Ω. On the other hand, the bandwidth of a current feedback ampli- fier can be decreased by increasing the feedback resistance. This can sometimes be useful where it is desired to reduce the noise bandwidth of a system. As a practical matter, the maximum value of feedback resistor was found to be 2 k Ω. Figure 29 shows the frequency response of an AD8072/AD8073 at a gain of two with both feedback and gain resistors equal to 2 k Ω. FREQUENCY – MHz 6.1 6.0 0.1 500 1 10 100 5.9 5.8 5.4 5.7 5.6 5.5 RF = 2k RF = 649 1 dB DIV 0.1 dB DIV 6 7 5 4 3 2 1 0 VS = 5V AV = 2 RL = 150 VO = 0.2V p-p Figure 29. Frequency Response vs. RF Capacitive Load Drive When an op amp output drives a capacitive load, extra phase shift due to the pole formed by the op amp’s output impedance and the capacitor can cause peaking or even oscillation. The top trace of Figure 30, RS = 0 Ω, shows the output of one of the am- plifiers of the AD8072/AD8073 when driving a 50 pF capacitor as shown in the schematic of Figure 31. The amount of peaking can be significantly reduced by adding a resistor in series with the capacitor. The lower trace of Fig- ure 30 shows the same capacitor being driven with a 25 Ω resis- tor in series with it. In general, the resistor value will have to be experimentally determined, but from 10 Ω to 50 Ω is a practical range of values to experiment with for capacitive loads of up to a few hundred pF. 20ns 50mV RS = 0Ω RS = 25Ω Figure 30. Capacitive Low Drive 1k 1k 50 VIN = 100mV p-p RL 1k CL 50pF RS Figure 31. Capacitive Load Drive Circuit |
同様の部品番号 - AD8072JN |
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