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AD711KR-REEL Datasheet(PDF) 11 Page - Analog Devices |
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AD711KR-REEL Datasheet(HTML) 11 Page - Analog Devices |
11 / 12 page AD711 REV. A –11– loading can introduce errors in instantaneous input 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. DRIVING A LARGE CAPACITIVE LOAD The circuit in Figure 36 employs a 100 Ω isolation resistor which enables the amplifier to drive capacitive loads exceeding 1500 pF; the resistor effectively isolates the high frequency feed- back 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 Ω series resistor and the load capacitance, CL. Figure 37 shows a typical transient response for this connection. Figure 36. Circuit for Driving a Large Capacitive Load Figure 37. Transient Response RL = 2 kΩ, 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 am- plified 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 as the AD711 will minimize both dc and ac errors in all active filter applications. SECOND ORDER LOW PASS FILTER Figure 38 depicts the AD711 configured as a second order Butterworth low pass filter. With the values as shown, the cor- ner frequency will be 20 kHz; however, the wide bandwidth of the AD711 permits a corner frequency as high as several hun- dred kilohertz. Equations for component selection are shown below. R1 = R2 = user selected (typical values: 10 k Ω – 100 kΩ) C1 = 1.414 (2 π)( f cutoff )( R1) , C2 = 0.707 (2 π)( f cutoff )( R1) Where C1 and C2 are in farads. Figure 38. 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 38, 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 39. The upper trace is that of another low-cost BiFET op amp showing 17 dB more feedthrough at 5 MHz. Figure 39. 9-POLE CHEBYCHEV FILTER Figure 40 shows the AD711 and its dual counterpart, the AD712, as a 9-pole Chebychev filter using active frequency de- pendent negative resistors (FDNR). With a cutoff frequency of 50 kHz and better than 90 dB rejection, it may be used as an anti-aliasing filter for a 12-bit Data Acquisition System with 100 kHz throughput. As shown in Figure 40, the filter is comprised of four FDNRs (A, B, C, D) having values of 4.9395 10 –15 and 5.9276 10 –15 farad-seconds. Each FDNR active network provides a two-pole response; for a total of 8 poles. The 9th pole consists of a 0.001 µF capacitor and a 124 kΩ resistor at Pin 3 of ampli- fier A2. Figure 41 depicts the circuits for each FDNR with the |
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