LMH2100 Datasheet, PDF (37/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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C_TRANS
I_DET
V_{REF +}
-
R_TRANS
-
+
LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
REF
OUT
Figure 85. Output Stage of the LMH2100
It can be seen that the output of the amplifier is set by the detection current I_DET multiplied by the resistor R_TRANS
plus the reference voltage V_REF:
V_OUT = I_DET R_TRANS + V_REF
(14)
I_DET represents the detector current that is proportional to the RF input power. The equation shows that
temperature variations in V_REF are also present in the output V_OUT. In case of a single ended topology the output
is the only pin that is connected to the ADC. The ADC voltage for single ended is thus:
V_ADC = I_DET R_TRANS + V_REF
(15)
A differential topology also connects the reference pin, which is the value of reference voltage V_REF. The ADC
reads V_OUT – V_REF:
V_ADC = V_OUT – V_REF = I_DET R_TRANS
(16)
Equation 16 does not contain the reference voltage V_REF anymore. Temperature variations in this reference
voltage are therefore not measured by the ADC.
8.2.1.3 Application Curves
2.0
2.0
1855 MHz
1.6
1.6
900 MHz
11..22
50 MHz
0.8
0.8
2500 MHz
0.4
3000 MHz
0.4
0.0
4000 MHz
0.0
-60 -50 -40 -30 -20 -10 0 10
RF INPUT POWER (dBm)
2.0
1.6
1.2
0.8
0.4
0.0
10M
RF_IN = - 5 dBm
RF_IN = -15 dBm
RF_IN = -25 dBm
RF_IN = -35 dBm
RF_IN = -45 dBm
100M
1G
10G
FREQUENCY (Hz)
Figure 86. Output Voltage vs RF Input Power
Figure 87. Output Voltage vs Frequency
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