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LMV227SD Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV227SD Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 12 page Application Notes (Continued) PRINCIPLE OF OPERATION The logarithmic response of the LMV227 is implemented by a de-modulating logarithmic amplifier as shown in Figure 6. The logarithmic amplifier consists of a number of cascaded linear gain cells. With these gain cells, a piecewise approxi- mation of the logarithmic function is constructed. Every gain cell has a response according to Figure 7.At a certain threshold (E K), the gain cell starts to saturate, which means that the gain drops to zero. The output of gain cell 1 is connected to the input of gain cell 2 and so on. All gain cell outputs are AM-demodulated with a peak detec- tor and summed together. This results in a logarithmic func- tion. The logarithmic range is about: 20·n·log (A) where, n = number of gain cells A = gain per gaincell Figure 8 shows a logarithmic function on a linear scale and the piecewise approximation of the logarithmic function. Figure 9 shows a logarithmic function on a logarithmic scale and the piecewise approximation of the logarithmic function. The maximum error for this approximation occurs at the geometric mean of a gain section, which is e.g. for the third segment: The size of the error increases with distance between the thresholds. 20118125 FIGURE 5. Output Ripple vs. RF Input Power 20118119 FIGURE 6. Logarithmic Amplifier 20118120 FIGURE 7. Gain Cell 20118121 FIGURE 8. Log-Function on Lin Scale 20118122 FIGURE 9. Log-Function on Log Scale www.national.com 10 |
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Similar Description - LMV227SD |
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