LMH2100 Datasheet, PDF (27/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
Feature Description (continued)
7.3.2.2.1 Temperature Compensation
A further reduction of the power measurement error is possible if the operating temperature is measured in the
application. For this purpose, the detector model used by the estimator should be extended to cover the
temperature dependency of the detector.
Since the detector transfer function is generally a smooth function of temperature (the output voltage changes
gradually over temperature), the temperature is in most cases adequately modeled by a first-order or second-
order polynomial (see Equation 9).
V_OUT,MOD = F_DET(P_IN,T₀)[1 + (T-T₀)TC₁(P_IN)
+ (T-T₀)²TC₂(P_IN) + O(T³)]
(9)
The required temperature dependence of the estimator, to compensate for the detector temperature dependence
can be approximated similarly:
P_EST = F_D-E1T[V_OUT(T),T₀]{1 + (T-T₀)S₁[V_OUT(T)] +
+ (T-T₀)²S₂[V_OUT(T)] + O(T³)}
| F_DE-1T[V_OUT(T),T₀]{1 + (T-T₀)S₁[V_OUT(T)]}
(10)
The last approximation results from the fact that a first-order temperature compensation is usually sufficiently
accurate. The remainder of this section will therefore concentrate on first-order compensation. For second and
higher-order compensation a similar approach can be followed.
Ideally, the temperature drift could be completely eliminated if the measurement system is calibrated at various
temperatures and input power levels to determine the Temperature Sensitivity S₁. In a practical application,
however that is usually not possible due to the associated high costs. The alternative is to use the average
temperature drift in the estimator, instead of the temperature sensitivity of each device individually. In this way it
becomes possible to eliminate the systematic (reproducible) component of the temperature drift without the need
for calibration at different temperatures during manufacturing. What remains is the random temperature drift,
which differs from device to device. Figure 73 illustrates the idea. The graph at the left schematically represents
the behavior of the drift error versus temperature at a certain input power level for a large number of devices.
ERROR
ERROR
+3s
T
MEAN
-3s
+3s
-3s
T
MEAN
AFTER
TEMPERATURE
CORRECTION
Figure 73. 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. Obviously 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 73. 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.
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