LMH2120 Datasheet, PDF (24/34 Pages) National Semiconductor (TI) – Linear RMS power detector particularly suited for accurate

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
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Variation over Temperature Error
In contrast to the linear conformance error, the variation over temperature error (E_VOT) purely measures the error
due to temperature variation. The measured output voltage at 25°C is subtracted from the output voltage at
another temperature. Subsequently, it is translated into an input referred error by dividing it by K_SLOPE at 25°C.
The equation for variation over temperature is given by:
E_VOT = (V_{OUT_TEMP} - V_{OUT 25°C}) / K_SLOPE
(14)
The variation over temperature is shown in Figure 62, where a dynamic range of 40 dB is obtained for E_VOT =
±0.5 dB.
2.0
1.5
1.0
85°C
0.5
0.0
-0.5
-40°C
-1.0
-1.5
-2.0
-50 -40 -30 -20 -10 0 10
RF INPUT POWER (dBm)
Figure 62. E_VOT vs. RF Input Power at 1900 MHz
1 dB Step Error
This parameter is a measure for the error for an 1 dB power step. According to a 3GPP specification, the error
should be less than ±0.3 dB. This condition is often used to define a useful dynamic range of the detector.
The 1 dB step error is calculated in 2 steps:
1. Determine the maximum sensitivity.
2. Calculate the 1 dB step error.
First the maximum sensitivity (S_MAX) is calculated per temperature. It is defined as the maximum difference
between two output voltages for a 1 dB step within the power range:
S_MAX = V_{OUT P+1} - V_{OUT P}
(15)
The 1dB error is than calculated by:
E_{1 dB} = (S_ACTUAL - S_MAX) / S_MAX
(16)
where S_ACTUAL (actual sensitivity) is the difference between two output voltages for a 1 dB step at a given power
level. Figure 63 shows the typical 1 dB step error at 1900 MHz, where a dynamic range of 36 dB over
temperature is obtained for E_1dB = ±0.3 dB.
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