LMV227SD datasheet(10/12 Pages) NSC

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.

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FIGURE 5. Output Ripple vs. RF Input Power

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FIGURE 6. Logarithmic Amplifier

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FIGURE 7. Gain Cell

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FIGURE 8. Log-Function on Lin Scale

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FIGURE 9. Log-Function on Log Scale

LMV227SD Datasheet(PDF) 10 Page - National Semiconductor (TI)