LMH2100 Datasheet, PDF (25/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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Feature Description (continued)
LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
2.0
1.8
1.6
2.0
11.4.8
1.6
11.2.4
1.2
11.0.0
0.8
00.8.6
0.4
000.6..20
0.4
0.2
0.0
-55
-40°C
25°C
85°C
85°C
25°C
-40°C
-45 -35 -25 -15 -5
RF INPUT POWER (dBm)
2.5
2.0
1.5
2.5
21..00
1.5
10..05
0.5
00..00
-0.5
--01..50
-1.5
---221...050
-1.5
-2.0
-2.5
5
Figure 71. LOG-Conformance Error and LOG-Detector Transfer Function
In the center of the detector's dynamic range, the LOG-conformance error is small, especially at room
temperature; in this region the transfer function closely follows the linear-in-dB relationship while K_SLOPE and
P_INTERCEPT are determined based on room temperature measurements. At the temperature extremes the error in
the center of the range is slightly larger due to the temperature drift of the detector transfer function. The error
rapidly increases toward the top and bottom end of the detector's dynamic range; here the detector saturates and
its transfer function starts to deviate significantly from the ideal LOG-linear model. The detector dynamic range is
usually defined as the power range for which the LOG conformance error is smaller than a specified amount.
Often an error of ±1 dB is used as a criterion.
7.3.2.2 Temperature Drift Error
A more accurate power measurement system can be obtained if the first error contribution, due to the deviation
from the ideal LOG-linear model, is eliminated. This is achieved if the actual measured detector transfer function
at room temperature is used as a model for the detector, instead of the ideal LOG-linear transfer function used in
the previous section.
The formula used for such a detector is:
V_OUT,MOD = F_DET(P_IN,T_O)
where
• T_O represents the temperature during calibration (room temperature).
(4)
The transfer function of the corresponding estimator is thus the inverse of this:
P_EST = F_D-1ET[V_OUT(T),T₀]
(5)
In this expression V_OUT(T) represents the measured detector output voltage at the operating temperature T.
The resulting measurement error is only due to drift of the detector transfer function over temperature, and can
be expressed as:
E_DRIFT (T,T₀) = P_EST - P_IN = F^-D1ET[V_OUT(T),T₀] - P_IN
=
-1
F_DET[V_OUT(T),T₀]
-
F_D-E1T[V_OUT(T),T)]
(6)
Unfortunately, the (numeric) inverse of the detector transfer function at different temperatures makes this
expression rather impractical. However, since the drift error is usually small V_OUT(T) is only slightly different from
V_OUT(T_O). This means that we can apply the following approximation:
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