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AD711BQ bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD711BQ bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 16 page REV. E AD711 –11– DRIVING A LARGE CAPACITIVE LOAD The circuit in Figure 13 employs a 100 W isolation resistor which enables the amplifier to drive capacitive loads exceeding 1500 pF; the resistor effectively isolates the high frequency feedback from the load and stabilizes the circuit. Low frequency feedback is returned to the amplifier summing junction via the low pass filter formed by the 100 W series resistor and the load capaci- tance, CL. Figure 14 shows a typical transient response for this connection. RL INPUT +VS –VS 0.1 F CL 0.1 F TYPICAL CAPACITANCE LIMIT FOR VARIOUS LOAD RESISTORS RL CL UP TO 2k 1500pF 10k 1500pF 20k 1000pF 4.99k AD711 100 30pF 4.99k OUTPUT Figure 13. Circuit for Driving a Large Capacitive Load Figure 14. Transient Response RL = 2 k W, CL = 500 pF ACTIVE FILTER APPLICATIONS In active filter applications using op amps, the dc accuracy of the amplifier is critical to optimal filter performance. The amplifier’s offset voltage and bias current contribute to output error. Offset voltage will be passed by the filter and may be amplified to produce excessive output offset. For low frequency applications requiring large value input resistors, bias currents flowing through these resistors will also generate an offset voltage. In addition, at higher frequencies, an op amp’s dynamics must be carefully considered. Here, slew rate, bandwidth, and open-loop gain play a major role in op amp selection. The slew rate must be fast as well as symmetrical to minimize distortion. The amplifier’s bandwidth in conjunction with the filter’s gain will dictate the frequency response of the filter. The use of a high performance amplifier such a s the AD711 will minimize both dc and ac errors in all active filter applica- tions. SECOND ORDER LOW PASS FILTER Figure 15 depicts the AD711 configured as a second order Butterworth low pass filter. With the values as shown, the corner frequency will be 20 kHz; however, the wide bandwidth of the AD711 permits a corner frequency as high as several hundred kilohertz. Equations for component selection are shown below. R1 = R2 = user selected (typical values: 10 k W – 100 kW) (4) C1 = 1.414 (2 p)( fcutoff)(R1) , C2 = 0.707 (2 p)( fcutoff)(R1) (5) Where: C1 and C2 are in farads. VOUT +15V –15V 0.1 F 0.1 F AD711 VIN C2 280pF R2 20k R1 20k C1 560pF Figure 15. Second Order Low Pass Filter An important property of filters is their out-of-band rejection. The simple 20 kHz low pass filter shown in Figure 15, might be used to condition a signal contaminated with clock pulses or sampling glitches which have considerable energy content at high frequencies. The low output impedance and high bandwidth of the AD711 minimize high frequency feedthrough as shown in Figure 16. The upper trace is that of another low-cost BiFET op amp showing 17 dB more feedthrough at 5 MHz. Figure 16. voltage. If the A/D conversion speed is not excessive and the bandwidth of the amplifier is sufficient, the amplifier’s output will return to the nominal value before the converter makes its comparison. However, many amplifiers have relatively narrow bandwidth yielding slow recovery from output transients. The AD711 is ideally suited to drive high speed A/D converters since it offers both wide bandwidth and high open-loop gain. a. Source Current = 2 mA b. Sink Current = 1 mA Figure 12. ADC Input Unity Gain Buffer Recovery Times |
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Mô tả tương tự - AD711BQ |
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