Datasheet AD711 (Analog Devices) - 11

AD711
voltage. If the A/D conversion speed is not excessive and the large value input resistors, bias currents flowing through these bandwidth of the amplifier is sufficient, the amplifier’s output resistors will also generate an offset voltage. will return to the nominal value before the converter makes its In addition, at higher frequencies, an op amp’s dynamics must comparison. However, many amplifiers have relatively narrow be carefully considered. Here, slew rate, bandwidth, and open-loop bandwidth yielding slow recovery from output transients. The gain play a major role in op amp selection. The slew rate must AD711 is ideally suited to drive high speed A/D converters since be fast as well as symmetrical to minimize distortion. The amplifier’s it offers both wide bandwidth and high open-loop gain. 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 a. Source Current = 2 mA b. Sink Current = 1 mA kilohertz. Equations for component selection are shown below. Figure 12. ADC Input Unity Gain Buffer Recovery Times R1 = R2 = user selected (typical values: 10 kW – 100 kW) (4)
DRIVING A LARGE CAPACITIVE LOAD
The circuit in Figure 13 employs a 100 W isolation resistor which C1 = 1.414 , C2 = 0.707 (5) (2 p)( f cutoff )(R1) (2 p)( f cutoff )(R1) enables the amplifier to drive capacitive loads exceeding 1500 pF; the resistor effectively isolates the high frequency feedback from Where: the load and stabilizes the circuit. Low frequency feedback is C1 and C2 are in farads. returned to the amplifier summing junction via the low pass filter formed by the 100 W series resistor and the load capaci-
C1
tance, C
560pF
L. Figure 14 shows a typical transient response for this connection.
+15V 0.1 F R1 R2 4.99k 20k 20k 30pF C2 AD711 VOUT VIN +V 280pF S 0.1 F 0.1 F INPUT 4.99k 100 AD711 OUTPUT –15V 0.1 F TYPICAL CAPACITANCE CL RL LIMIT FOR VARIOUS
Figure 15. Second Order Low Pass Filter
LOAD RESISTORS –VS RL CL UP TO
An important property of filters is their out-of-band rejection.
2k 1500pF 10k 1500pF
The simple 20 kHz low pass filter shown in Figure 15, might be
20k 1000pF
used to condition a signal contaminated with clock pulses or Figure 13. Circuit for Driving a Large Capacitive Load 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 14. Transient Response RL = 2 kW, 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 Figure 16. REV. E –11– Document Outline FEATURES PRODUCT DESCRIPTION CONNECTION DIAGRAMS PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE Typical Performance Characteristics OPTIMIZING SETTLING TIME OP AMP SETTLING TIME—A MATHEMATICAL MODEL GUARDING D/A CONVERTER APPLICATIONS NOISE CHARACTERISTICS DRIVING THE ANALOG INPUT OF AN A/D CONVERTER DRIVING A LARGE CAPACITIVE LOAD ACTIVE FILTER APPLICATIONS SECOND ORDER LOW PASS FILTER 9-POLE CHEBYCHEV FILTER OUTLINE DIMENSIONS Revision History