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AD829JN Datasheet(PDF) 9 Page - Analog Devices |
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AD829JN Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page AD829 REV. E –9– Table I. Component Selection for Shunt Compensation Slew –3 dB Follower Inverter R1 R2 CL CCOMP Rate Small Signal Gain Gain pF pF V/ s Bandwidth – MHz 1 Open 100 0 68 16 66 2 –1 1k 1k 5 25 38 71 5 –4 511 2.0k 1 7 90 76 10 –9 226 2.05k 0 3 130 65 20 –19 105 2k 0 0 230 55 25 –24 105 2.49 0 0 230 39 100 –99 20 2k 0 0 230 7.5 Table I gives recommended CCOMP and CLEAD values along with the corresponding slew rates and bandwidth. The capacitor values given were selected to provide a small signal frequency response with less than 1 dB of peaking and less than 10% over- shoot. For this table, supply voltages of ±15 volts should be used. Figure 28 is a graphical extension of the table which shows the slew rate/gain trade-off for lower closed-loop gains, when using the shunt compensation scheme. NOISE GAIN 100 1 1 100 10 10 10 100 1k SLEW RATE VS = 15V CCOMP Figure 28. Value of CCOMP & Slew Rate vs. Noise Gain Current Feedback Compensation Bipolar nondegenerated amplifiers which are single pole and internally compensated have their bandwidths defined as: fT = 1 2 π r e CCOMP = I 2 π kT q CCOMP where: fT is the unity gain bandwidth of the amplifier I is the collector current of the input transistor CCOMP is the compensation capacitance re is the inverse of the transconductance of the input transistors kT/q is approximately equal to 26 mV @ 27 °C. Since both fT and slew rate are functions of the same variables, the dynamic behavior of an amplifier is limited. Since: Slew Rate = 2I CCOMP then: Slew Rate fT = 4 π kT q This shows that the slew rate will be only 0.314 V/ µs for every MHz of bandwidth. The only way to increase slew rate is to increase the fT and that is difficult, due to process limitations. Unfortunately, an amplifier with a bandwidth of 10 MHz can only slew at 3.1 V/ µs, which is barely enough to provide a full power bandwidth of 50 kHz. The AD829 is especially suited to a new form of compensation which allows for the enhancement of both the full power band- width and slew rate of the amplifier. The voltage gain from the inverting input pin to the compensation pin is large; therefore, if a capacitance is inserted between these pins, the amplifier’s bandwidth becomes a function of its feedback resistor and this capacitance. The slew rate of the amplifier is now a function of its internal bias (2I) and this compensation capacitance. Since the closed-loop bandwidth is a function of RF and CCOMP (Figure 29), it is independent of the amplifier closed-loop gain, as shown in Figure 31. To preserve stability, the time constant of RF and CCOMP needs to provide a bandwidth of less than 65 MHz. For example, with CCOMP = 15 pF and RF = 1 k Ω, the small signal bandwidth of the AD829 is 10 MHz, while Figure 30 shows that the slew rate is in excess of 60 V/ µs. As can be seen in Figure 31, the closed-loop bandwidth is constant for gains of –1 to –4, a property of current feedback amplifiers. FOR C1 OF CCOMP CCOMP SHOULD NEVER EXCEED 15pF FOR THIS CONNECTION 0pF 15pF <7pF 7pF 50 50 COAX CABLE –VS 0.1 F 0.1 F AD829 RF VOUT R1 +VS VIN RL 1k IN4148 CCOMP C1* *RECOMMENDED VALUE Figure 29. Inverting Amplifier Connection Using Current Feedback Compensation |
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