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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD829AR-EBZ Fiches technique(PDF) 12 Page - Analog Devices |
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AD829AR-EBZ Fiches technique(HTML) 12 Page - Analog Devices |
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12 / 20 page  AD829 Data Sheet Rev. I | Page 12 of 20 THEORY OF OPERATION The AD829 is fabricated on the Analog Devices, Inc., proprietary complementary bipolar (CB) process, which provides PNP and NPN transistors with similar fTs of 600 MHz. As shown in Figure 35, the AD829 input stage consists of an NPN differential pair in which each transistor operates at a 600 µA collector current. This gives the input devices a high transconductance, which in turn gives the AD829 a low noise figure of 2 nV/√Hz at 1 kHz. Figure 35. Simplified Schematic The input stage drives a folded cascode that consists of a fast pair of PNP transistors. These PNPs drive a current mirror that provides a differential-input-to-single-ended-output conversion. The high speed PNPs are also used in the current-amplifying output stage, which provides a high current gain of 40,000. Even under heavy loading conditions, the high fTs of the NPN and PNPs, produced using the CB process, permit cascading two stages of emitter followers while maintaining 60 phase margin at closed-loop bandwidths greater than 50 MHz. Two stages of complementary emitter followers also effectively buffer the high impedance compensation node (at the CCOMP pin) from the output so that the AD829 can maintain a high dc open- loop gain, even into low load impedances (92 dB into a 150 Ω load and 100 dB into a 1 kΩ load). Laser trimming and PTAT biasing ensure low offset voltage and low offset voltage drift, enabling the user to eliminate ac coupling in many applications. For added flexibility, the AD829 provides access to the internal frequency compensation node. This allows users to customize the frequency response characteristics for a particular application. Unity-gain stability requires a compensation capacitance of 68 pF (Pin 5 to ground), which yields a small signal bandwidth of 66 MHz and slew rate of 16 V/µs. The slew rate and gain bandwidth product varies inversely with compensation capacitance. Table 4 and Figure 37 show the optimum compensation capacitance and the resulting slew rate for a desired noise gain. For gains between 1 and 20, choose CCOMP to keep the small signal bandwidth relatively constant. The minimum gain that will still provide stability depends on the value of the external compensation capacitance. An RC network in the output stage (see Figure 35) completely removes the effect of capacitive loading when the amplifier compensates for closed-loop gains of 10 or higher. At low frequencies, and with low capacitive loads, the gain from the compensation node to the output is very close to unity. In this case, C is bootstrapped and does not contribute to the compensation capacitance of the device. As the capacitive load increases, a pole forms with the output impedance of the output stage, which reduces the gain, and subsequently, C is incompletely bootstrapped. Therefore, some fraction of C contributes to the compensation capacitance, and the unity-gain bandwidth falls. As the load capacitance is further increased, the bandwidth continues to fall, and the amplifier remains stable. EXTERNALLY COMPENSATING THE AD829 The AD829 is stable with no external compensation for noise gains greater than 20. For lower gains, two different methods of frequency compensating the amplifier can be used to achieve closed-loop stability: shunt and current feedback compensation. SHUNT COMPENSATION Figure 36 and Figure 37 show that shunt compensation has an external compensation capacitor, CCOMP, connected between the compensation pin and ground. This external capacitor is tied in parallel with approximately 3 pF of internal capacitance at the compensation node. In addition, a small capacitance, CLEAD, in parallel with resistor R2, compensates for the capacitance at the inverting input of the amplifier. Figure 36. Inverting Amplifier Connection Using External Shunt Compensation Figure 37. Noninverting Amplifier Connection Using External Shunt Compensation Table 4 gives the recommended CCOMP and CLEAD values, as well as the corresponding slew rates and bandwidth. The capacitor values were selected to provide a small signal frequency response with <1 dB of peaking and <10% overshoot. For Table 4, ±15 V –IN +IN 1.2mA OFFSET NULL C 12.5pF R 500 Ω OUTPUT +VS –VS CCOMP 15 Ω 15 Ω 2 3 7 6 4 – + AD829 +VS –VS 0.1 µF 0.1 µF 5 CCOMP 50 Ω 50 Ω COAX CABLE R1 CLEAD R2 VOUT 1k Ω VIN 3 2 7 6 4 + – AD829 +VS –VS 0.1 µF 0.1 µF CLEAD 5 CCOMP 50 Ω 50 Ω CABLE VOUT R2 R1 1k Ω VIN |
Numéro de pièce similaire - AD829AR-EBZ |
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Description similaire - AD829AR-EBZ |
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