LMH2100 Datasheet, PDF (23/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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7 Detailed Description
LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
7.1 Overview
The LMH2100 is a versatile logarithmic RF power detector suitable for use in power measurement systems. The
LMH2100 is particularly well suited for CDMA and UMTS applications. It produces a DC voltage that is a
measure for the applied RF power.
The core of the LMH2100 is a progressive compression LOG detector consisting of four gain stages. Each of
these saturating stages has a gain of approximately 10 dB and therefore realizes about 10 dB of the detector
dynamic range. The five diode cells perform the actual detection and convert the RF signal to a DC current. This
DC current is subsequently supplied to the transimpedance amplifier at the output, that converts it into an output
voltage. In addition, the amplifier provides buffering of and applies filtering to the detector output signal. To
prevent discharge of filtering capacitors between OUT and GND in shutdown, a switch is inserted at the amplifier
input that opens in shutdown to realize a high impedance output of the device.
7.2 Functional Block Diagram
VDD _A1
EN C2
en
B2 REF
RTRANS
en
I/I
-
A2 OUT
VREF
+
-
+
en
RFIN _B1
GN
D
C1
V-V
RIN
10 dB 10 dB 10 dB 10 dB
7.3 Feature Description
7.3.1 Characteristics of the LMH2100
The LMH2100 is a logarithmic RF power detector with approximately 40-dB dynamic range. This dynamic range
plus its logarithmic behavior make the LMH2100 ideal for various applications such as wireless transmit power
control for CDMA and UMTS applications. The frequency range of the LMH2100 is from 50 MHz to 4 GHz, which
makes it suitable for various applications.
The LMH2100 transfer function is accurately temperature compensated. This makes the measurement accurate
for a wide temperature range. Furthermore, the LMH2100 can easily be connected to a directional coupler
because of its 50-Ω input termination. The output range is adjustable to fit the ADC input range. The detector can
be switched into a power saving shutdown mode for use in pulsed conditions.
7.3.2 Accurate Power Measurement
The power measurement accuracy achieved with a power detector is not only determined by the accuracy of the
detector itself, but also by the way it is integrated into the application. In many applications some form of
calibration is employed to improve the accuracy of the overall system beyond the intrinsic accuracy provided by
the power detector. For example, for LOG-detectors calibration can be used to eliminate part to part spread of
the LOG-slope and LOG-intercept from the overall power measurement system, thereby improving its power
measurement accuracy.
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