LMH2100 Datasheet, PDF (28/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
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Feature Description (continued)
It is essential for the effectiveness of the temperature compensation 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 temperature sensitivity increases when the number of
measurement temperatures increases. Linear regression to temperature can then be used to determine the two
parameters of the linear model for the temperature drift error: the first order temperature 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 temperatures 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 component.
Figure 75 shows the temperature drift error without and with temperature compensation using two segments.
With compensation the systematic component is completely eliminated; the remaining random error component
is centered around zero. Note that the random component is slightly larger at −40°C than at 85°C.
Figure 74. Temperature Drift Error without Temperature
Compensation
Figure 75. Temperature Drift Error without with
Temperature Compensation
In a practical power measurement system, temperature compensation is usually only applied to a small power
range around the maximum power level for two reasons:
• The various communication standards require the highest accuracy in this range to limit interference.
• The temperature sensitivity itself is a function of the power level it becomes impractical to store a large
number of different temperature sensitivity values for different power levels.
The 2.7-V DC and AC Electrical Characteristics in the datasheet specifies the temperature sensitivity for the
aforementioned two segments at an input power level of −10 dBm (near the top-end of the detector dynamic
range). The typical value represents the mean which is to be used for calibration.
7.3.2.2.2 Differential Power Errors
Many third generation communication systems contain a power control loop through the base station and mobile
unit that requests both to frequently update the transmit power level by a small amount (typically 1 dB). For such
applications it is important that the actual change of the transmit power is sufficiently close to the requested
power change.
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