lmv228tlx National Semiconductor Corporation, lmv228tlx Datasheet - Page 25

• Manufacturer Part Number: lmv228tlx
• Description: Rf Power Detector For Cdma And Wcdma
• Manufacturer: National Semiconductor Corporation

Application Notes
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:
where,
n = number of gain cells
A = gain per gaincell
Figure 9 shows a logarithmic function on a linear scale and
the piecewise approximation of the logarithmic function.
FIGURE 9. Log-Function on Lin Scale
FIGURE 8. Gain Cell
20 · n · log (A)
(Continued)
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Figure 10 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.
LAYOUT CONSIDERATIONS
For a proper functioning part a good board layout is neces-
sary. Special care should be taken for the series resistance
R
sistor values the parasitic capacitance of the resistor may
significantly impact the realized attenuation. The effective
attenuation will be lower than intended. To reduce the para-
sitic capacitance across resistor R
composed of several components in series instead of using
a single component.
1
(Figure 1) that determines the attenuation. For high re-
FIGURE 10. Log-Function on Log Scale
1
, this resistor can be
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