LMH2100 Datasheet, PDF (25/32 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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FIGURE 8. Elimination of the Systematic Component from the Temperature Drift
The mean drift error represents the reproducible - systematic
- part of the error, while the mean ± 3 sigma limits represent
the combined systematic plus random error component. Ob-
viously the drift error must be zero at calibration temperature
T₀. If the systematic component of the drift error is included
in the estimator, the total drift error becomes equal to only the
random component, as illustrated in the graph at the right of
Figure 8. A significant reduction of the temperature drift error
can be achieved in this way only if:
• The systematic component is significantly larger than the
random error component (otherwise the difference is
negligible).
• The operating temperature is measured with sufficient
accuracy.
It is essential for the effectiveness of the temperature com-
pensation to assign the appropriate value to the temperature
sensitivity S₁. Two different approaches can be followed to
determine this parameter:
• Determination of a single value to be used over the entire
operating temperature range.
• Division of the operating temperature range in segments
and use of separate values for each of the segments.
Also for the first method, the accuracy of the extracted tem-
perature sensitivity increases when the number of measure-
ment temperatures increases. Linear regression to tempera-
ture can then be used to determine the two parameters of the
linear model for the temperature drift error: the first order tem-
perature sensitivity S₁ and the best-fit (room temperature)
value for the power estimate at T₀: F_DET[V_OUT(T),T₀]. Note that
to achieve an overall - over all temperatures - minimum error,
the room temperature drift error in the model can be non-zero
at the calibration temperature (which is not in agreement with
the strict definition).
The second method does not have this drawback but is more
complex. In fact, segmentation of the temperature range is a
form of higher-order temperature compensation using only a
first-order model for the different segments: one for temper-
atures below 25°C, and one for temperatures above 25°C.
The mean (or typical) temperature sensitivity is the value to
be used for compensation of the systematic drift error com-
ponent. Figure 9 shows the temperature drift error without and
with temperature compensation using two segments. With
compensation the systematic component is completely elim-
inated; the remaining random error component is centered
around zero. Note that the random component is slightly larg-
er at −40°C than at 85°C.
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FIGURE 9. Temperature Drift Error without and with Temperature Compensation
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