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AD828AR Datasheet(PDF) 10 Page - Analog Devices |
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AD828AR Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD828 REV. B –10– THEORY OF OPERATION The AD828 is a low cost, dual video operational amplifier designed to excel in high performance, high output current video applications. The AD828 (Figure 40) consists of a degenerated NPN differen- tial pair driving matched PNPs in a folded-cascode gain stage. The output buffer stage employs emitter followers in a class AB amplifier that delivers the necessary current to the load while maintaining low levels of distortion. The AD828 will drive terminated cables and capacitive loads of 10 pF or less. As the closed-loop gain is increased, the AD828 will drive heavier cap loads without oscillating. –IN +IN OUTPUT +VS –VS Figure 40. AD828 Simplified Schematic INPUT CONSIDERATIONS An input protection resistor (RIN in Figure 34) is required in cir- cuits where the input to the AD828 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 “bal- ancing” 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 AD828 The AD828 is a breakthrough dual amp that delivers precision and speed at low cost with low power consumption. The AD828 offers excellent static and dynamic matching characteristics, combined with the ability to drive heavy resistive loads. As with all high frequency circuits, care should be taken to main- tain 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 interamp 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–5 pF) may be paralleled with Rf to neutralize this effect. Finally, sock- ets should be avoided, because of their tendency to increase interlead capacitance. Power Supply Bypassing Proper power supply decoupling is critical to preserve the integ- rity 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 mini- mum. These measures greatly reduce undesired inductive effects on the amplifier’s response. Though two 0.1 µF capacitors will typically be effective in de- coupling the supplies, several capacitors of different values can be paralleled to cover a wider frequency range. PARALLEL AMPS PROVIDE 100 mA TO LOAD By taking advantage of the superior matching characteristics of the AD828, enhanced performance can easily be achieved by employing the circuit in Figure 41. Here, two identical cells are paralleled to obtain even higher load driving capability than that of a single amplifier (100 mA min guaranteed). R1 and R2 are included to limit current flow between amplifier outputs that would arise in the presence of any residual mismatch. 2 +VS VIN VOUT 3 8 1k R2 5 –VS RL 1/2 AD828 1/2 AD828 1 F 0.1 F 7 5 6 1 1 F 0.1 F 4 R1 5 1k 1k 1k Figure 41. Parallel Amp Configuration |
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