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LMH6574 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LMH6574 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 14 page  Application Notes (Continued) The LMH6574 has a current feedback output buffer with gain determined by external feedback (R F) and gain set (RG) resistors. With current feedback amplifiers, the closed loop frequency response is a function of R F. For a gain of 2 V/V, the recommended value of R F is 575 Ω. For other gains see the chart “Suggested R F vs Gain”. Generally, lowering RF from the recommended value will peak the frequency re- sponse 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 the recommended value will cause overshoot, ringing and, eventually, oscilla- tion. Since all applications are slightly different it is worth some experimentation to find the optimal R F for a given circuit. For more information see Application Note OA-13 which de- scribes the relationship between R F and closed-loop fre- quency response for current feedback operational amplifiers. The impedance looking into pin 12 is approximately 20 Ω. This allows for good bandwidth at gains up to 10 V/V. When used with gains over 10 V/V, the LMH6574 will exhibit a “gain bandwidth product” similar to a typical voltage feedback amplifier. For gains of over 10 V/V consider selecting a high performance video amplifier like the LMH6720 to provide additional gain. SD vs. EN The LMH6574 has both shutdown and disable capability. The shutdown feature affects the entire chip, whereas the disable function only affects the output buffer. When in shut- down mode, minimal power is consumed. The shutdown function is very fast, but causes a very brief spike of about 400 mV to appear on the output. When in shutdown mode the LMH6574 consumes only 1.8 mA of supply current. For maximum input to output isolation use the shutdown func- tion. The EN pin only disables the output buffer which results in a substantially reduced output glitch of only 50 mV. While disabled the chip consumes 4.7 mA, considerably more than when shutdown. This is because the input buffers are still active. For minimal output glitch use the EN pin. Also, care should be taken to ensure that, while in the disabled state, the voltage differential between the active input buffer (the one selected by pins A0 and A1) and the output pin stays less than 2V. As the voltage differential increases, input to output isolation decreases. Normally this is not an issue. See the section on MULTIPLEXER EXPANSION for further de- tails. To reduce the output glitch when using the SD pin, switch the EN pin at least 10 ns before switching the SD pin. This can be accomplished by using an RC delay circuit between the two pins if only one control signal is available. Logic inputs "SD" and "EN" will revert to the "High", while "A 0" and "A1" will revert to the "Low" state when left floating. MULITPLEXER EXPANSION Figure 3 shows an 8:1 MUX using two LMH6574’s. 20119718 FIGURE 3. 8:1 MUX USING TWO LMH6574’s www.national.com 11 |
Similar Part No. - LMH6574_05 |
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Similar Description - LMH6574_05 |
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