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AD811AR-20-REEL Datasheet(PDF) 10 Page - Analog Devices |
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AD811AR-20-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 15 page AD811 REV. D –10– Driving Capacitive Loads The feedback and gain resistor values in Table I will result in very flat closed-loop responses in applications where the load capacitances are below 10 pF. Capacitances greater than this will result in increased peaking and overshoot, although not necessarily in a sustained oscillation. There are at least two very effective ways to compensate for this effect. One way is to increase the magnitude of the feedback resistor, which lowers the 3 dB frequency. The other method is to include a small resistor in series with the output of the ampli- fier to isolate it from the load capacitance. The results of these two techniques are illustrated in Figure 32. Using a 1.5 k Ω feedback resistor, the output ripple is less than 0.5 dB when driv- ing 100 pF. The main disadvantage of this method is that it sacrifices a little bit of gain flatness for increased capacitive load drive capability. With the second method, using a series resistor, the loss of flatness does not occur. VIN RG –VS +VS RFB 0.1 F 0.1 F RL AD811 RT CL RS (OPTIONAL) VOUT Figure 31. Recommended Connection for Driving a Large Capacitive Load 12 –6 3 –3 0 1M 9 6 10M 100M FREQUENCY – Hz G = +2 VS = 15V RL = 10k CL = 100pF RFB = 1.5k RS = 0 RFB = 649 RS = 30 Figure 32. Performance Comparison of Two Methods for Driving a Capacitive Load 100 0 1000 30 10 100 20 10 60 40 50 70 80 90 LOAD CAPACITANCE – pF G = +2 V = 15V R VALUE SPECIFIED IS FOR FLATTEST FREQUENCY RESPONSE S S Figure 33. Recommended Value of Series Resistor vs. the Amount of Capacitive Load Figure 33 shows recommended resistor values for different load capacitances. Refer again to Figure 32 for an example of the results of this method. Note that it may be necessary to adjust the gain setting resistor, RG, to correct for the attenuation which results due to the divider formed by the series resistor, RS, and the load resistance. Applications which require driving a large load capacitance at a high slew rate are often limited by the output current available from the driving amplifier. For example, an amplifier limited to 25 mA output current cannot drive a 500 pF load at a slew rate greater than 50 V/ µs. However, because of the AD811’s 100 mA output current, a slew rate of 200 V/ µs is achievable when driv- ing this same 500 pF capacitor (see Figure 34). 10 90 100 0% 5V 2V 100ns VIN VOUT Figure 34. Output Waveform of an AD811 Driving a 500 pF Load. Gain = +2, RFB = 649 Ω, RS = 15 Ω, RS = 10 k Ω |
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