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LMV641MGX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LMV641MGX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  Application Information ADVANTAGES OF THE LMV641 Low Voltage and Low Power Operation The LMV641 has performance guaranteed at supply voltages of 2.7V and 10V. It is guaranteed to be operational at all sup- ply voltages between 2.7V and 12.0V. The LMV641 draws a low supply current of 138 µA. The LMV641 provides the low voltage and low power amplification which is essential for portable applications. Wide Bandwidth Despite drawing the very low supply current of 138 µA, the LMV641 manages to provide a wide unity gain bandwidth of 10 MHz. This is easily one of the best bandwidth to power ratios ever achieved, and allows this op amp to provide wide- band amplification while using the minimum amount of power. This makes the LMV641 ideal for low power signal processing applications such as portable media players and other ac- cessories. Low Input Referred Noise The LMV641 provides a flatband input referred voltage noise density of 14 nV/ , which is significantly better than the noise performance expected from a low power op amp. This op amp also feature exceptionally low 1/f noise, with a very low 1/f noise corner frequency of 4 Hz. Because of this the LMV641 is ideal for low power applications which require de- cent noise performance, such as PDAs and portable sensors. Ground Sensing and Rail-to-Rail Output The LMV641 has a rail-to-rail output stage, which provides the maximum possible output dynamic range. This is espe- cially important for applications requiring a large output swing. The input common mode range of this part includes the neg- ative supply rail which allows direct sensing at ground in a single supply operation. Small Size The small footprint of the packages for the LMV641 saves space on printed circuit boards, and enables the design of smaller and more compact electronic products. Long traces between the signal source and the op amp make the signal path susceptible to noise. By using a physically smaller pack- age, these op amps can be placed closer to the signal source, reducing noise pickup and enhancing signal integrity. STABILITY OF OP AMP CIRCUITS If the phase margin of the LMV641 is plotted with respect to the capacitive load (C L) at its output, and if CL is increased beyond 100 pF then the phase margin reduces significantly. This is because the op amp is designed to provide the maxi- mum bandwidth possible for a low supply current. Stabilizing the LMV641 for higher capacitive loads would have required either a drastic increase in supply current, or a large internal compensation capacitance, which would have reduced the bandwidth. Hence, if this device is to be used for driving higher capacitive loads, it will have to be externally compensated. 20203359 FIGURE 1. Gain vs. Frequency for an Op Amp An op amp, ideally, has a dominant pole close to DC which causes its gain to decay at the rate of 20 dB/decade with re- spect to frequency. If this rate of decay, also known as the rate of closure (ROC), remains the same until the op amp's unity gain bandwidth, then the op amp is stable. If, however, a large capacitance is added to the output of the op amp, it combines with the output impedance of the op amp to create another pole in its frequency response before its unity gain frequency (Figure 1). This increases the ROC to 40 dB/ decade and causes instability. In such a case, a number of techniques can be used to restore stability to the circuit. The idea behind all these schemes is to modify the frequency response such that it can be restored to an ROC of 20 dB/decade, which ensures stability. In The Loop Compensation Figure 2 illustrates a compensation technique, known as in the loop compensation, that employs an RC feedback circuit within the feedback loop to stabilize a non-inverting amplifier configuration. A small series resistance, R S, is used to isolate the amplifier output from the load capacitance, C L, and a small capacitance, C F, is inserted across the feedback resistor to bypass C L at higher frequencies. 20203358 FIGURE 2. In the Loop Compensation 11 www.national.com |
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