LMH2100 Datasheet, PDF (36/49 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
VDD
RFIN
2
1
OUT ^RS
6
LMH2100
+
C_S
ADC
EN
4
3
5 ^REF
-
GND
VDD
RFIN
21
OUT
6
LMH2100
RP
EN
REF
435
GND
www.ti.com
+
CP
ADC
-
Figure 81. Low Pass Output Filter
Figure 82. Low Pass Feedback Filter
Depending on the system requirements one of the these filtering techniques can be selected. The low pass
output filter has the advantage that it preserves the output voltage when the LMH2100 is brought into shutdown.
This is elaborated in Output Behavior in Shutdown. In the feedback filter, resistor R_P discharges capacitor C_P in
shutdown and therefore changes the output voltage of the device.
A disadvantage of the low pass output filter is that the series resistor R_S limits the output drive capability. This
may cause inaccuracies in the voltage read by an ADC when the ADC input impedance is not significantly larger
than R_S. In that case, the current flowing through the ADC input induces an error voltage across filter resistor R_S.
The low pass feedback filter doesn’t have this disadvantage.
Note that adding an external resistor between OUT and REF reduces the transfer gain (LOG-slope and LOG-
intercept) of the device. The internal feedback resistor sets the gain of the transimpedance amplifier.
The filtering of the low pass output filter is realized by resistor R_S and capacitor C_S. The −3 dB bandwidth of this
filter can then be calculated by: ƒ_{−3 dB} = 1 / 2πR_SC_S. The bandwidth of the low pass feedback filter is determined
by external resistor R_P in parallel with the internal resistor R_TRANS, and external capacitor C_P in parallel with
internal capacitor C_TRANS (see Figure 85). The −3 dB bandwidth of the feedback filter can be calculated by ƒ_{−3 dB}
= 1 / 2π (R_P//R_TRANS) (C_P + C_TRANS). The bandwidth set by the internal resistor and capacitor (when no external
components are connected between OUT and REF) equals ƒ_{−3 dB} = 1 / 2π R_TRANS C_TRANS = 450 kHz.
8.2.1.2.1.4 Interface to the ADC
The LMH2100 can be connected to the ADC with a single-ended or a differential topology. The single ended
topology connects the output of the LMH2100 to the input of the ADC and the reference pin is not connected. In
a differential topology, both the output and the reference pins of the LMH2100 are connected to the ADC. The
topologies are depicted in Figure 83 and Figure 84.
VDD
RFIN
21
OUT
6
LMH2100
RP
EN
REF
435
GND
+
CP
ADC
-
VDD
RFIN
21
OUT
6
LMH2100
R_P
EN
4
3
5 ^REF
GND
+
C_P ADC
-
Figure 83. Single-Ended
Figure 84. Differential Application
The differential topology has the advantage that it is compensated for temperature drift of the internal reference
voltage. This can be explained by looking at the transimpedance amplifier of the LMH2100 (Figure 85).
36
Submit Documentation Feedback
Product Folder Links: LMH2100
Copyright © 2007–2015, Texas Instruments Incorporated