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AD811AR-16-REEL7 Datasheet(PDF) 9 Page - Analog Devices |
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AD811AR-16-REEL7 Datasheet(HTML) 9 Page - Analog Devices |
9 / 15 page AD811 REV. D –9– APPLICATIONS General Design Considerations The AD811 is a current feedback amplifier optimized for use in high performance video and data acquisition applications. Since it uses a current feedback architecture, its closed-loop –3 dB bandwidth is dependent on the magnitude of the feedback resis- tor. The desired closed-loop gain and bandwidth are obtained by varying the feedback resistor (RFB) to tune the bandwidth, and varying the gain resistor (RG) to get the correct gain. Table I contains recommended resistor values for a variety of useful closed-loop gains and supply voltages. Table I. –3 dB Bandwidth vs. Closed-Loop Gain and Resistance Values VS = 15 V Closed-Loop –3 dB BW Gain RFB RG (MHz) +1 750 Ω 140 +2 649 Ω 649 Ω 120 +10 511 Ω 56.2 Ω 100 –1 590 Ω 590 Ω 115 –10 511 Ω 51.1 Ω 95 VS = 5 V Closed-Loop –3 dB BW Gain RFB RG (MHz) +1 619 Ω 80 +2 562 Ω 562 Ω 80 +10 442 Ω 48.7 Ω 65 –1 562 Ω 562 Ω 75 –10 442 Ω 44.2 Ω 65 VS = 10 V Closed-Loop –3 dB BW Gain RFB RG (MHz) +1 649 Ω 105 +2 590 Ω 590 Ω 105 +10 499 Ω 49.9 Ω 80 –1 590 Ω 590 Ω 105 –10 499 Ω 49.9 Ω 80 Figures 11 and 12 illustrate the relationship between the feed- back resistor and the frequency and time domain response char- acteristics for a closed-loop gain of +2. (The response at other gains will be similar.) The 3 dB bandwidth is somewhat dependent on the power supply voltage. As the supply voltage is decreased for example, the magnitude of internal junction capacitances is increased, causing a reduction in closed-loop bandwidth. To compensate for this, smaller values of feedback resistor are used at lower supply voltages. Achieving the Flattest Gain Response at High Frequency Achieving and maintaining gain flatness of better than 0.1 dB at frequencies above 10 MHz requires careful consideration of several issues. Choice of Feedback and Gain Resistors Because of the above-mentioned relationship between the 3 dB bandwidth and the feedback resistor, the fine scale gain flatness will, to some extent, vary with feedback resistor tolerance. It is, therefore, recommended that resistors with a 1% tolerance be used if it is desired to maintain flatness over a wide range of production lots. In addition, resistors of different construction have different associated parasitic capacitance and inductance. Metal-film resistors were used for the bulk of the characteriza- tion for this data sheet. It is possible that values other than those indicated will be optimal for other resistor types. Printed Circuit Board Layout Considerations As to be expected for a wideband amplifier, PC board parasitics can affect the overall closed loop performance. Of concern are stray capacitances at the output and the inverting input nodes. If a ground plane is to be used on the same side of the board as the signal traces, a space (3/16" is plenty) should be left around the signal lines to minimize coupling. Additionally, 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 Coaxial 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 the coax was ideal, then the resulting flatness would not be affected by the length of the cable. While outstanding results can be achieved using inex- pensive cables, it should be noted that some variation in flatness due to varying cable lengths may be experienced. Power Supply Bypassing Adequate power supply bypassing can be critical when optimiz- ing the performance of a high frequency circuit. Inductance in the power supply leads can form 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 many applications, more elaborate bypassing (such as using two paral- leled capacitors) may be required in some cases. |
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