LMH2100 Datasheet, PDF (26/49 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
Feature Description (continued)
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E_DRIFT(T,T₀) | E_DRIFT(T₀,T₀)
+
(T
-
T₀)
w
w7
{F_D-1ET[V_OUT(T),T₀]
-
F_D-1ET[V_OUT(T),T]}
(7)
This expression is easily simplified by taking the following considerations into account:
• The drift error at the calibration temperature E(T_O,T_O) equals zero (by definition).
• The estimator transfer F_DET(V_OUT,T_O) is not a function of temperature; the estimator output changes over
temperature only due to the temperature dependence of V_OUT.
• The actual detector input power P_IN is not temperature dependent (in the context of this expression).
• The derivative of the estimator transfer function to V_OUT equals approximately 1/K_SLOPE in the LOG-linear
region of the detector transfer function (the region of interest).
Using this, we arrive at:
E_DRIFT (T,T₀)
|
(T
±
T₀)
w
w7
F_D-1ET[V_OUT(T),T₀]
= (T ± T₀)
w V_OUT(T)
w7
w
wV_OUT
F_D-E1T[V_OUT(T),T₀]
| ^{VOUT(T) ± VOUT(T0)}
K_SLOPE
(8)
This expression is very similar to the expression of the LOG-conformance error determined previously. The only
difference is that instead of the output of the ideal LOG-linear model, the actual detector output voltage at the
calibration temperature is now subtracted from the detector output voltage at the operating temperature.
Figure 72 depicts an example of the drift error.
1.5
11..05
1.0
0.5
0.5
00..00
-0.5
-0.5
-1.0
--11..05
-40°C
85°C
-1.5
-55 -45 -35 -25 -15 -5 5
RF INPUT POWER (dBm)
Figure 72. Temperature Drift Error of the LMH2100 at ƒ = 1855 MHz
In agreement with the definition, the temperature drift error is zero at the calibration temperature. Further, the
main difference with the LOG-conformance error is observed at the top and bottom end of the detection range;
instead of a rapid increase the drift error settles to a small value at high and low input power levels due to the
fact that the detector saturation levels are relatively temperature independent.
In a practical application it may not be possible to use the exact inverse detector transfer function as the
algorithm for the estimator. For example it may require too much memory and/or too much factory calibration
time. However, using the ideal LOG-linear model in combination with a few extra data points at the top and
bottom end of the detection range - where the deviation is largest - can already significantly reduce the power
measurement error.
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