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AD810AR 数据表(PDF) 11 Page - Analog Devices |
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AD810AR 数据表(HTML) 11 Page - Analog Devices |
11 / 16 page AD810 REV. A –11– GENERAL DESIGN CONSIDERATIONS The AD810 is a current feedback amplifier optimized for use in high performance video and data acquisition systems. Since it uses a current feedback architecture, its closed-loop bandwidth depends on the value of the feedback resistor. Table I below contains recommended resistor values for some useful closed- loop gains and supply voltages. As you can see in the table, the closed-loop bandwidth is not a strong function of gain, as it would be for a voltage feedback amp. The recommended resistor values will result in maximum bandwidths with less than 0.1 dB of peaking in the gain vs. frequency response. The –3 dB bandwidth is also somewhat dependent on the power supply voltage. Lowering the supplies increases the values of internal capacitances, reducing the bandwidth. To compensate for this, smaller values of feedback resistor are sometimes used at lower supply voltages. The characteristic curves illustrate that bandwidths of over 100 MHz on 30 V total and over 50 MHz on 5 V total supplies can be achieved. Table I. –3 dB Bandwidth vs. Closed-Loop Gain and Resistance Values (RL = 150 ) VS = 15 V Closed-Loop –3 dB BW Gain RFB RG (MHz) +1 1 k Ω 80 +2 715 Ω 715 Ω 75 +10 270 Ω 30 Ω 65 –1 681 Ω 681 Ω 70 –10 249 Ω 24.9 Ω 65 VS = 5 V Closed-Loop –3 dB BW Gain RFB RG (MHz) +1 910 Ω 50 +2 715 Ω 715 Ω 50 +10 270 Ω 30 Ω 50 –1 620 Ω 620 Ω 55 –10 249 Ω 24.9 Ω 50 ACHIEVING VERY FLAT GAIN RESPONSE AT HIGH FREQUENCY Achieving and maintaining gain flatness of better than 0.1 dB above 10 MHz is not difficult if the recommended resistor values are used. The following issues should be considered to ensure consistently excellent results. CHOICE OF FEEDBACK AND GAIN RESISTOR Because the 3 dB bandwidth depends on the feedback resistor, the fine scale flatness will, to some extent, vary with feedback resistor tolerance. It is recommended that resistors with a 1% tolerance be used if it is desired to maintain exceptional flatness over a wide range of production lots. PRINTED CIRCUIT BOARD LAYOUT As with all wideband amplifiers, PC board parasitics can affect the overall closed-loop performance. Most important are stray capacitances at the output and inverting input nodes. (An added capacitance of 2 pF between the inverting input and ground will add about 0.2 dB of peaking in the gain of 2 response, and increase the bandwidth to 105 MHz.) A space (3/16" is plenty) should be left around the signal lines to minimize coupling. Also, signal lines connecting the feedback and gain resistors should be short enough so that their associated inductance does not cause high frequency gain errors. Line lengths less than 1/4" are recommended. QUALITY OF COAX CABLE Optimum flatness when driving a coax cable is possible only when the driven cable is terminated at each end with a resistor matching its characteristic impedance. If coax were ideal, then the resulting flatness would not be affected by the length of the cable. While outstanding results can be achieved using inexpensive cables, some variation in flatness due to varying cable lengths is to be expected. POWER SUPPLY BYPASSING Adequate power supply bypassing can be critical when optimizing the performance of a high frequency circuit. Inductance in the power supply leads can contribute to resonant circuits that produce peaking in the amplifier's response. In addition, if large current transients must be delivered to the load, then bypass capacitors (typically greater than 1 µF) will be required to provide the best settling time and lowest distortion. Although the recommended 0.1 µF power supply bypass capacitors will be sufficient in most applications, more elaborate bypassing (such as using two paralleled capacitors) may be required in some cases. POWER SUPPLY OPERATING RANGE The AD810 will operate with supplies from ±18 V down to about ±2.5 V. On ±2.5 V the low distortion output voltage swing will be better than 1 V peak to peak. Single supply operation can be realized with excellent results by arranging for the input common-mode voltage to be biased at the supply midpoint. OFFSET NULLING A 10 k Ω pot connected between Pins 1 and 5, with its wiper connected to V+, can be used to trim out the inverting input current (with about ±20 µA of range). For closed-loop gains above about 5, this may not be sufficient to trim the output offset voltage to zero. Tie the pot's wiper to ground through a large value resistor (50 k Ω for ±5 V supplies, 150 kΩ for ±15 V supplies) to trim the output to zero at high closed-loop gains. Applications– |
类似零件编号 - AD810AR |
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类似说明 - AD810AR |
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