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AD5539 数据表(PDF) 7 Page - Analog Devices |
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AD5539 数据表(HTML) 7 Page - Analog Devices |
7 / 16 page AD5539 –7– REV. B APPLYING THE AD5539 The AD5539 is stable for closed-loop gains of 4 or more as an inverter and at (noise) gains of 5 or greater as a voltage follower. This means that whenever the AD5539 is operated at noise gains below 5, external frequency compensation must be used to insure stable operation. The following sections outline specific compensation circuits which permit stable operation of the AD5539 down to follower (noise) gains of 3 (inverting gains of 2) with corresponding –3 dB bandwidths up to 390 MHz. External compensation is achieved by modifying the frequency response to the AD5539’s external feedback network (i.e., by adding lead-lag compensa- tion) so that the amplifier operates at a noise gain of 5 (or more) at frequencies over 44 MHz, independent of signal gain. Figure 13. Small Signal Open-Loop Gain and Phase vs. Frequency GENERAL PRINCIPLES OF LEAD AND LAG COMPENSATION The AD5539 has its first pole or breakpoint in its open-loop fre- quency response at about 10 MHz (see Figure 13). At frequen- cies beyond 100 MHz, phase shift increases such that the output lags the input by 180 °—well before the unity gain crossover fre- quency. Therefore, severe peaking (and possible oscillation) will result if the AD5539 is operated at noise gains below 5, unless external compensation is employed. Figure 14 shows the un- compensated closed-loop frequency response of the AD5539 Figure 14. AD5539 Uncompensated Response, Closed- Loop Gain = 7 when operating at a noise gain of 7. Under these conditions, ex- cess phase shift causes nearly 10 dB of peaking at 150 MHz. Figure 15 illustrates the use of both lead and lag compensation to permit stable low-gain operation. The AD5539 is shown con- nected as an inverting amplifier with the required external com- ponents added to provide stability and improve high frequency response. The stray capacitance between the amplifier summing junction and ground, CX, represents whatever capacitance is as- sociated with the particular type of op amp package used plus the stray wiring capacitance at the summing junction. Evaluating the lead capacitance first (ignoring RLAG and CLAG for now): the feedback network, consisting of R2 and CLEAD, has a pole frequency equal to: FA = 1 2 π C LEAD + CX () R1||R2 () (1) and a zero frequency equal to: FB = 1 2 π R1 × C LEAD () (2) Usually, frequency FA is made equal to FB; that is, (R1CX) = (R2 CLEAD), in a manner similar to the compensation used for an attenuator or scope probe. However, if the pole frequency, FA, will lie above the unity gain crossover frequency (440 MHz), then the optimum location of FB will be near the crossover Figure 15. Inverting Amplifier Model Showing Both Lead and Lag Compensation Figure 16. A Model of the Feedback Network of the Inverting Amplifier OBSOLETE |
类似零件编号 - AD5539_15 |
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类似说明 - AD5539_15 |
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