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LMH6574MAX データシート(PDF) 11 Page - National Semiconductor (TI) |
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LMH6574MAX データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Notes (Continued) 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. EVALUATION BOARDS National Semiconductor provides the following evaluation boards as a guide for high frequency layout and as an aid in device testing and characterization. Many of the data sheet plots were measured with this board. Device Package Evaluation Board LMH6574 SOIC LMH730276 An evaluation board can be shipped when a sample request is placed with National Semiconductor. Samples can be ordered on the National web page. (www.national.com) MULTIPLEXER EXPANSION With the SHUTDOWN pin putting the output stage into a high impedance state, several LMH6574’s can be tied to- gether to form a larger input MUX. However, there is a loading effect on the active output caused by the unselected devices. The circuit in Figure 3 shows how to compensate for this effect. For the 16:1 MUX function shown in Figure 3 below the gain error would be about −0.8 dB, or about 9%. In the circuit in Figure 3, resistor ratios have been adjusted to compensate for this gain error. By adjusting the gain of each multiplexer circuit the error can be reduced to the tolerance of the resistors used (1% in this example). Disabling of the LMH6574 using the EN pin is not recom- mended for use when doing multiplexer expansion. While disabled, If the voltage between the selected input and the chip output exceeds approximately 2V the device will begin to enter a soft breakdown state. This will show up as reduced input to output isolation. The signal on the non-inverting input of the output driver amplifier will leak through to the inverting input, and then to the output through the feedback resistor. The worst case is a gain of 1 configuration where the non inverting input follows the active input buffer and (through the feedback resistor) the inverting input follows the voltage driving the output stage. The solution for this is to use shutdown mode for multiplexer expansion. BUILDING an 8:1 MULITPLEXER Figure 4 shows an 8:1 MUX using two LMH6574’s. 20119717 FIGURE 3. Multiplexer Gain Compensation www.national.com 11 |
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