LMH2110 Datasheet, PDF (19/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
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SNWS022C – JANUARY 2010 – REVISED MARCH 2013
RMS detectors are in particular suited for the newer communication standards like W-CDMA and LTE that exhibit
large peak-to-average ratios and different modulation schemes (signal shapes). This is a key advantage
compared to other types of detectors in applications that employ signals with high peak-to-average power
variations or different modulation schemes. For example, the RMS detector response to a 0 dBm modulated W-
CDMA signal and a 0 dBm unmodulated carrier is essentially equal. This eliminates the need for long calibration
procedures and large calibration tables in the baseband due to different applied modulation schemes.
LMH2110 RF Power Detector
For optimal performance of the LMH2110, it needs to be configured correctly in the application. The detector will
be discussed by means of its block diagram (Figure 49). Subsequently, the details of the electrical interfacing are
separately discussed for each pin.
A1
V_DD
LDO
Internal Supply
B1 ^RFIN
C2
EN
V/I
EXP
i₁
i_OUT
A
i₂
V/I
EXP
OUT A2
V_OUT
GND
B2, C1
Figure 49. Block Diagram
For measuring the RMS (power) level of a signal, the time average of the squared signal needs to be measured
as described in section Accurate Power Measurement. This is implemented in the LMH2110 by means of a
multiplier and a low-pass filter in a negative-feedback loop. A simplified block diagram of the LMH2110 is
depicted in Figure 49. The core of the loop is a multiplier. The two inputs of the multiplier are fed by (i₁, i₂):
i₁ = i_LF + i_RF
(4)
i₂ = i_LF - i_RF
(5)
in which i_LF is a current depending on the DC output voltage of the RF detector and i_RF is a current depending on
the RF input signal. The output of the multiplier (i_OUT) is the product of these two current and equals:
i_LF2 i_RF2
i_out =
I₀
(6)
in which I₀ is a normalizing current. By a low-pass filter at the output of the multiplier the DC term of this current
is isolated and integrated. The input of the amplifier A acts as the nulling point of the negative feedback loop,
yielding:
³ ³ iLF2dt = iRF2dt
(7)
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