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AD818AN 数据表(PDF) 11 Page - Analog Devices |
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AD818AN 数据表(HTML) 11 Page - Analog Devices |
11 / 15 page REV. AD818 –11– –IN +IN NULL 1 NULL 8 OUTPUT +VS –VS Figure 4. AD818 Simplified Schematic THEORY OF OPERATION The AD818 is a low cost video operational amplifier designed to excel in high performance, high output current video applications. The AD818 (Figure 4) 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 AD818 will drive terminated cables and capacitive loads of 10 pF or less. As the closed-loop gain is increased, the AD818 will drive heavier capacitive loads without oscillating. INPUT CONSIDERATIONS An input protection resistor (RIN in TPC 28) is required in circuits where the input to the AD818 will be subjected to tran- sients of 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. GROUNDING AND BYPASSING When designing high frequency circuits, some special precautions are in order. Circuits must be built with short interconnect leads. When wiring components, care should be taken to provide a low resistance, low inductance path to ground. Sockets should be avoided, since their increased interlead capacitance can degrade circuit bandwidth. Feedback resistors should be of low enough value ( £1 kW) to ensure that the time constant formed with the inherent stray capacitance at the amplifier’s summing junction will not limit performance. This parasitic capacitance, along with the parallel resistance of RF RIN, forms a pole in the loop transmission, which may result in peaking. A small capacitance (1 pF–5 pF) may be used in parallel with the feedback resistor to neutralize this effect. Power supply leads should be bypassed to ground as close as possible to the amplifier pins. Ceramic disc capacitors of 0.1 mF are recommended. 10k –VS VOS ADJUST +VS AD818 Figure 5. Offset Null Configuration OFFSET NULLING The input offset voltage of the AD818 is inherently very low. However, if additional nulling is required, the circuit shown in Figure 5 can be used. The null range of the AD818 in this configuration is ±10 mV. SINGLE SUPPLY OPERATION Another exciting feature of the AD818 is its ability to perform well in a single supply configuration. The AD818 is ideally suited for applications that require low power dissipation and high output current. Referring to Figure 6, careful consideration should be given to the proper selection of component values. The choices for this particular circuit are: R1 + R3 R2 combine with C1 to form a low frequency corner of approximately 10 kHz. C4 was inserted in series with R4 to maintain amplifier stability at high frequency. Combining R3 with C2 forms a low-pass filter with a corner frequency of approximately 500 Hz. This is needed to maintain amplifier PSRR, since the supply is connected to VIN through the input divider. The values for R2 and C2 were chosen to demonstrate the AD818’s exceptional output drive capability. In this configuration, the output is centered around 2.5 V. In order to eliminate the static dc current associated with this level, C3 was inserted in series with R L. R2 3.3k R1 3.3k R3 100 C2 3.3 F VIN C1 0.01 F C4 0.001 F R4 1k AD818 VOUT VS 3.3 F 0.01 F SELECT C1, R1, R2 FOR DESIRED LOW FREQUENCY CORNER. C3 0.1 F RL 150 1k Figure 6. Single-Supply Amplifier Configuration D |
类似零件编号 - AD818AN |
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类似说明 - AD818AN |
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