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LMV722MMX Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMV722MMX Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 15 page Typical Performance characteristics (Continued) Application Notes 1.0 Benefits of the LMV721/722 Size. The small footprints of the LMV721/722 packages save space on printed circuit boards, and enable the design of smaller electronic products, such as cellular phones, pagers, or other portable systems. The low profile of the LMV721/722 make them possible to use in PCMCIA type III cards. Signal Integrity. Signals can pick up noise between the sig- nal source and the amplifier. By using a physically smaller amplifier package, the LMV721/722 can be placed closer to the signal source, reducing noise pickup and increasing sig- nal integrity. Simplified Board Layout. These products help you to avoid using long pc traces in your pc board layout. This means that no additional components, such as capacitors and resistors, are needed to filter out the unwanted signals due to the inter- ference between the long pc traces. Low Supply Current. These devices will help you to maxi- mize battery life. They are ideal for battery powered sys- tems. Low Supply Voltage. National provides guaranteed perfor- mance at 2.2V and 5V. These guarantees ensure operation throughout the battery lifetime. Rail-to-Rail Output. Rail-to-rail output swing provides maxi- mum possible dynamic range at the output. This is particu- larly important when operating on low supply voltages. Input Includes Ground. Allows direct sensing near GND in single supply operation. Protection should be provided to prevent the input voltages from going negative more than −0.3V (at 25˚C). An input clamp diode with a resistor to the IC input terminal can be used. 2.0 Capacitive Load Tolerance The LMV721/722 can directly drive 4700pF in unity-gain without oscillation. The unity-gain follower is the most sensi- tive configuration to capacitive loading. Direct capacitive loading reduces the phase margin of amplifiers. The combi- nation of the amplifier’s output impedance and the capacitive load induces phase lag. This results in either an under- damped pulse response or oscillation. To drive a heavier ca- pacitive load, circuit in Figure 1 can be used. In Figure 1, the isolation resistor R ISO and the load capacitor C L form a pole to increase stability by adding more phase margin to the overall system. the desired performance de- pends on the value of R ISO. The bigger the RISO resistor value, the more stable V OUT will be. Figure 2 is an output waveform of Figure 1 using 100k Ω for R ISO and 2000µF for C L. The circuit in Figure 3 is an improvement to the one in Figure 1 because it provides DC accuracy as well as AC stability. If there were a load resistor in Figure 1, the output would be voltage divided by R ISO and the load resistor. Instead, in Fig- ure 3,R F provides the DC accuracy by using feed-forward techniques to connect V IN to RL. Caution is needed in choos- ing the value of R F due to the input bias current of the LMV721/722. C F and RISO serve to counteract the loss of phase margin by feeding the high frequency component of the output signal back to the amplifier’s inverting input, thereby preserving phase margin in the overall feedback Gain and Phase Margin vs. Frequency (V S = 5V, RL 600Ω) DS100922-16 Slew Rate vs. Supply Voltage DS100922-17 THD vs. Frequency DS100922-42 DS100922-18 FIGURE 1. Indirectly Driving A capacitive Load Using Resistive Isolation DS100922-31 FIGURE 2. Pulse Response of the LMV721 Circuit in Figure 1 www.national.com 8 |
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