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LMH6720MFX Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMH6720MFX Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 15 page  Application Section FEEDBACK RESISTOR SELECTION One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency re- sponse independent of gain by using appropriate values for the feedback resistor (R F). The Electrical Characteristics and Typical Performance plots specify an R F of 300 Ω, a gain of +2V/V and ±5V power supplies (unless otherwise specified). Generally, lowering R F from it’s recommended value will peak the frequency response and extend the bandwidth while increasing the value of R F will cause the frequency response to roll off faster. Reducing the value of R F too far below it’s recommended value will cause overshoot, ringing and, eventually, oscillation. The plot labeled "Frequency Response vs. R F" shows the LMH6714/6720/6722’s frequency response as R F is varied (R L = 100 Ω,A V = +2). This plot shows that an RF of 147 Ω results in peaking. An R F of 300 Ω gives near maximal band- width and gain flatness with good stability. An R F of 400 Ω gives excellent stability with only a small bandwidth penalty. Since all applications are slightly different it is worth some experimentation to find the optimal R F for a given circuit. Note that it is not possible to use a current feedback amplifier with the output shorted directly to the inverting input. The buffer configuration of the LMH6714/6720/6722 requires a 600 Ω feedback resistor for stable operation. For more information see Application Note OA-13 which describes the relationship between R F and closed-loop fre- quency response for current feedback operational amplifiers. The value for the inverting input impedance for the LMH6714/6720/6722 is approximately 180 Ω. The LMH6714/ 6720/6722 is designed for optimum performance at gains of +1 to +6 V/V and −1 to −5V/V. When using gains of ±7V/V or more the low values of R G required will make inverting input impedances very low. When configuring the LMH6714/6720/6722 for gains other than +2V/V, it is usually necessary to adjust the value of the feedback resistor. The two plots labeled “R F vs. Non- inverting Gain” and “R F vs. Inverting Gain” provide recom- mended feedback resistor values for a number of gain se- lections. In the “R F vs. Non-Inverting Gain” and the “RF vs. Inverting Gain” charts the recommended value of R F is depicted by the solid line, which starts high, decreases to 200 Ω and begins increasing again. The reason that a higher R F is required at higher gains is the need to keep R G from de- creasing too far below the output impedance of the input buffer. For the LMH6714/6720/6722 the output resistance of the input buffer is approximately 180 Ω and 50Ω is a practical lower limit for R G. Due to the limitations on RG the LMH6714/ 6720/6722 begins to operate in a gain bandwidth limited fashion for gains of ±5V/V or greater. ACTIVE FILTERS When using any current feedback Operational Amplifier as an active filter it is important to be very careful when using reactive components in the feedback loop. Anything that reduces the impedance of the negative feedback, especially at higher frequencies, will almost certainly cause stability problems. Likewise capacitance on the inverting input needs 20056512 FIGURE 1. Frequency Response vs. R F 20056515 FIGURE 2. R F vs. Non-Inverting Gain 20056514 FIGURE 3. R F vs. Inverting Gain www.national.com 9 |
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