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AD826ARZ-REEL7 Fiches technique(PDF) 10 Page - Analog Devices |
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AD826ARZ-REEL7 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 15 page AD826 -10- Rev. C THEORY OF OPERATION The AD826 is a low cost, wide band, high performance dual operational amplifier which can drive heavy capacitive and resistive loads. It also achieves a constant slew rate, bandwidth and settling time over its entire specified temperature range. The AD826 (Figure 35) consists of a degenerated NPN differential pair driving matched PNPs in a folded-cascode gain stage. The output buffer stage employs emitter followers in a class AB amplifier which delivers the necessary current to the load while maintaining low levels of distortion. OUTPUT CF NULL1 NULL8 –VS –IN +IN +VS Figure 35. Simplified Schematic The capacitor, CF, in the output stage mitigates the effect of capacitive loads. With low capacitive loads, the gain from the compensation node to the output is very close to unity. In this case, CF is bootstrapped and does not contribute to the overall compensation capacitance of the device. As the capacitive load is increased, a pole is formed with the output impedance of the output stage. This reduces the gain, and therefore, CF is incompletely bootstrapped. Effectively, some fraction of CF contributes to the overall compensation capacitance, reducing the unity gain bandwidth. As the load capacitance is further increased, the bandwidth continues to fall, maintaining the stability of the amplifier. INPUT CONSIDERATIONS An input protection resistor (RIN in Figure 25) is required in circuits where the input to the AD826 will be subjected to transient or continuous overload voltages exceeding the ±6 V maximum differential limit. This resistor provides protection for the input transistors by limiting their maximum base current. For high performance circuits, it is recommended that a “balancing” resistor be used to reduce the offset errors caused by bias current flowing through the input and feedback resistors. The balancing resistor equals the parallel combination of RIN and RF and thus provides a matched impedance at each input terminal. The offset voltage error will then be reduced by more than an order of magnitude. APPLYING THE AD826 The AD826 is a breakthrough dual amp that delivers precision and speed at low cost with low power consumption. The AD826 offers excellent static and dynamic matching characteristics, combined with the ability to drive heavy resistive and capacitive loads. As with all high frequency circuits, care should be taken to maintain overall device performance as well as their matching. The following items are presented as general design considerations. Circuit Board Layout Input and output runs should be laid out so as to physically isolate them from remaining runs. In addition, the feedback resistor of each amplifier should be placed away from the feedback resistor of the other amplifier, since this greatly reduces inter-amp coupling. Choosing Feedback and Gain Resistors In order to prevent the stray capacitance present at each amplifier’s summing junction from limiting its performance, the feedback resistors should be ≤1 kΩ. Since the summing junction capacitance may cause peaking, a small capacitor (1 pF–5pF) maybe paralleled with RF to neutralize this effect. Finally, sockets should be avoided, because of their tendency to increase interlead capacitance. Power Supply Considerations To ensure the proper operation of the AD826, connect the positive supply before the negative supply. Also, proper power supply decoupling is critical to preserve the integrity of high frequency signals. In carefully laid out designs, decoupling capacitors should be placed in close proximity to the supply pins, while their lead lengths should be kept to a minimum. These measures greatly reduce undesired inductive effects on the amplifier’s response. Though two 0.1 μF capacitors will typically be effective in decoupling the supplies, several capacitors of different values can be paralleled to cover a wider frequency range. |
Numéro de pièce similaire - AD826ARZ-REEL7 |
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Description similaire - AD826ARZ-REEL7 |
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