LMH2110
LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range
Literature Number: SNWS022B
LMH2110
October 13, 2010
8 GHz Logarithmic RMS Power Detector with 45 dB
dynamic range
General Description
The LMH2110 is a 45 dB Logarithmic RMS power detector
particularly suited for accurate power measurement of mod-
ulated RF signals that exhibit large peak-to-average ratios,
i.e. large variations of the signal envelope. Such signals are
encountered in W-CDMA and LTE cell phones. The RMS
measurement topology inherently ensures a modulation in-
sensitive measurement.
The device has an RF frequency range from 50 MHz to 8 GHz.
It provides an accurate, temperature and supply insensitive,
output voltage that relates linearly to the RF input power in
dBm. The LMH2110's excellent conformance to a logarithmic
response enables an easy integration by using slope and in-
tercept only, reducing calibration effort significantly. The de-
vice operates with a single supply from 2.7V to 5V. The
LMH2110 has an RF power detection range from -40 dBm to
5 dBm and is ideally suited for use in combination with a di-
rectional coupler. Alternatively a resistive divider can be used
as well.
The device is active for EN = High, otherwise it is in a low
power consumption shutdown mode. To save power and pre-
vent discharge of an external filter capacitance, the output
(OUT) is high-impedance during shutdown.
The LMH2110 power detector is offered in a tiny 6-bump mi-
croSMD package.
Features
■Logarithmic root mean square response
■45 dB linear-in-dB power detection range
■Multi-band operation from 50 MHz to 8 GHz
■LOG conformance better than ±0.5 dB
■Highly temperature insensitive, ±0.25 dB
■Modulation independent response, 0.08 dB
■Minimal Slope and Intercept variation
■Shutdown functionality
■Wide supply range from 2.7V to 5V
■Tiny 6-bump microSMD package
Applications
■Multi Mode, Multi band RF power control
—GSM/EDGE
—CDMA/CDMA2000
—W-CDMA
—OFDMA
—LTE
■Infrastructure RF Power Control
Typical Application
30064804
Output Voltage and Log Conformance Error vs.
RF Input Power at 1900 MHz
30064838
© 2010 National Semiconductor Corporation 300648 www.national.com
LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
VBAT - GND 5.5V
RF Input
Input power 12 dBm
DC Voltage 1V
Enable Input Voltage GND-0.4V <VEN and
VEN< Min (VDD-0.4, 3.6V)
ESD Tolerance (Note 2)
Human Body Model 2000V
Machine Model 200V
Charge Device Model 1000V
Storage Temperature
Range −65°C to 150°C
Junction Temperature
(Note 3) 150°C
Maximum Lead Temperature
(Soldering,10 sec) 260°C
Operating Ratings (Note 1)
Supply Voltage 2.7V to 5V
Temperature Range −40°C to +85°C
RF Frequency Range 50 MHz to 8 GHz
RF Input Power Range −40 dBm to 5 dBm
Package Thermal Resistance θJA
(Note 3) 166.7°C/W
2.7V and 4.5V DC and AC Electrical Characteristics
Unless otherwise specified: all limits are guaranteed to; TA = 25°C, VBAT = 2.7V and 4.5V (worst of the 2 is specified),
RFIN = 1900 MHz CW (Continuous Wave, unmodulated). Boldface limits apply at the temperature extremes (Note 4).
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)Units
Supply Interface
IBAT Supply Current Active mode: EN = High, no signal
present at RFIN.
3.7
2.9 4.8 5.5
5.9 mA
Shutdown: EN = Low, no
signal present at RFIN.
VBAT= 2.7V 3.7 4.7
5μA
VBAT= 4.5V 4.6 5.7
6.1
EN = Low, RFIN = 0 dBm,
1900 MHz
VBAT= 2.7V 3.5 4.7
5μA
VBAT= 4.5V 4.6 5.7
6.1
PSRR Power Supply Rejection Ratio
(Note 8)
RFIN = −10 dBm, 1900 MHz,
2.7V<VBAT<5V 45 56 dB
Logic Enable Interface
VLOW EN Logic Low Input Level
(Shutdown mode)
0.6 V
VHIGH EN Logic High Input Level 1.1 V
IEN Current into EN Pin 50 nA
Input / Output Interface
RIN Input Resistance 44 50 56 Ω
VOUT Minimum Output Voltage
(Pedestal)
No input Signal 01.5 8mV
ROUT Output Impedance EN = High, RFIN = -10 dBm, 1900 MHz,
ILOAD = 1 mA, DC measurement 0.2 2
3Ω
IOUT Output Short Circuit Current Sinking, RFIN = -10 dBm, OUT
connected to 2.5V
37
32 42
mA
Sourcing, RFIN = -10 dBm, OUT
connected to GND
40
34 46
IOUT,SD Output Leakage Current in
Shutdown mode
EN = Low, OUT connected to 2V 50 nA
enOutput Referred Noise
(Note 8)
RFIN = −10 dBm, 1900 MHz, output
spectrum at 10 kHz 3 µV/√Hz
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LMH2110
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)Units
vNIntegrated Output Referred Noise
(Note 8)
Integrated over frequency band
1 kHz - 6.5 kHz, RFIN = -10 dBm,
1900 MHz
210 µVRMS
Timing Characteristics
tON Turn-on Time from shutdown RFIN = -10 dBm, 1900 MHz, EN Low-
High transition to OUT at 90% 15 19 µs
tRRise Time
(Note 8)
Signal at RFIN from -20 dBm to 0 dBm,
10% to 90%, 1900 MHz 2.2 µs
tFFall Time
(Note 8)
Signal at RFIN from 0 dBm to -20 dBm,
90% to 10%, 1900 MHz 31 µs
RF Detector Transfer
RFIN = 50 MHz, fit range -20 dBm to -10 dBm (Note 7)
PMIN Minimum Power level, bottom
end of dynamic range
Log Conformance Error within ±1 dB -39
dBm
PMAX Maximum Power level, top end of
dynamic range 7
VMIN Minimum Output Voltage At PMIN 3 mV
VMAX Maximum Output Voltage At PMAX 1.96 V
KSLOPE Logarithmic Slope 42.2 44.3 46.4 mV/dB
PINT Logarithmic Intercept -38.6 -38.3 -38.0 dBm
DR Dynamic Range for specified
accuracy
±1 dB Log Conformance Error (ELC) 46
45
dB
±3 dB Log Conformance Error (ELC) 51
50
±0.5 dB Input referred Variation over
Temperature (EVOT), from PMIN
42
RFIN = 900 MHz, fit range -20 dBm to -10 dBm (Note 7)
PMIN Minimum Power level, bottom
end of dynamic range
Log Conformance Error within ±1 dB -38
dBm
PMAX Maximum Power level, top end of
dynamic range 0
VMIN Minimum Output Voltage At PMIN 3 mV
VMAX Maximum Output Voltage At PMAX 1.58 V
KSLOPE Logarithmic Slope 41.8 43.9 46.0 mV/dB
PINT Logarithmic Intercept -37.4 -37.0 -36.7 dBm
DR Dynamic Range for specified
accuracy
±1 dB Log Conformance Error (ELC) 38
37
dB
±3 dB Log Conformance Error (ELC) 45
44
±0.5 dB Input referred Variation over
Temperature (EVOT), from PMIN
44
±0.3 dB Error for a 1dB Step (E1dB
STEP) 41
38
±1 dB Error for a 10dB Step (E10dB
STEP) 32
EMOD Input referred Variation due to
Modulation
W-CDMA Release 6/7/8,
-38 dBm<RFIN<-5 dBm 0.08 dB
LTE, -38 dBm<RFIN<-5 dBm 0.19
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LMH2110
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)Units
RFIN = 1900 MHz, fit range -20 dBm to -10 dBm (Note 7)
PMIN Minimum Power level, bottom
end of dynamic range
Log Conformance Error within ±1 dB -36
dBm
PMAX Maximum Power level, top end of
dynamic range 0
VMIN Minimum Output Voltage At PMIN 3 mV
VMAX Maximum Output Voltage At PMAX 1.5 V
KSLOPE Logarithmic Slope 41.8 43.9 46.1 mV/dB
PINT Logarithmic Intercept -35.5 -35.1 -34.7 dBm
DR Dynamic Range for specified
accuracy
±1 dB Log Conformance Error (ELC) 36
36
dB
±3 dB Log Conformance Error (ELC) 45
43
±0.5 dB Input referred Variation over
Temperature (EVOT), from PMIN
41
±0.3 dB Error for a 1dB Step (E1dB
STEP) 40
38
±1 dB Error for a 10dB Step (E10dB
STEP) 30
EMOD Input referred Variation due to
Modulation
W-CDMA Release 6/7/8,
-38 dBm<RFIN<-5 dBm 0.09 dB
LTE, -38 dBm<RFIN<-5 dBm 0.18
RFIN = 3500 MHz, fit range -15 dBm to -5 dBm (Note 7)
PMIN Minimum Power level, bottom
end of dynamic range
Log Conformance Error within ±1 dB -31
dBm
PMAX Maximum Power level, top end of
dynamic range 6
VMIN Minimum Output Voltage At PMIN 2 mV
VMAX Maximum Output Voltage At PMAX 1.52 V
KSLOPE Logarithmic Slope 41.8 44.0 46.1 mV/dB
PINT Logarithmic Intercept -30.5 -29.7 -28.8 dBm
DR Dynamic Range for specified
accuracy
±1 dB Log Conformance Error (ELC) 37
36
dB
±3 dB Log Conformance Error (ELC) 44
42
±0.5 dB Input referred Variation over
Temperature (EVOT), from PMIN
39
RFIN = 5800 MHz, fit range -20 dBm to 3 dBm (Note 7)
PMIN Minimum Power level, bottom
end of dynamic range
Log Conformance Error within ±1 dB -22
dBm
PMAX Maximum Power level, top end of
dynamic range 10
VMIN Minimum Output Voltage At PMIN 3 mV
VMAX Maximum Output Voltage At PMAX 1.34 V
KSLOPE Logarithmic Slope 42.5 44.8 47.1 mV/dB
PINT Logarithmic Intercept -22.0 -21.0 -19.9 dBm
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LMH2110
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)Units
DR Dynamic Range for specified
accuracy
±1 dB Log Conformance Error (ELC) 32
31
dB
±3 dB Log Conformance Error (ELC) 39
37
±0.5 dB Input referred Variation over
Temperature (EVOT), from PMIN
33
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics.
Note 2: Human body model, applicable std. MIL-STD-883, Method 3015.7. Machine model, applicable std. JESD22–A115–A (ESD MM std of JEDEC). Field-
Induced Charge-Device Model, applicable std. JESD22–C101–C. (ESD FICDM std. of JEDEC)
Note 3: The maximum power dissipation is a function of TJ(MAX) , θJA. The maximum allowable power dissipation at any ambient temperature is
PD = (TJ(MAX) - TA)/θJA. All numbers apply for packages soldered directly into a PC board.
Note 4: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating
of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where
TJ > TA.
Note 5: All limits are guaranteed by test or statistical analysis.
Note 6: Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will
also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.
Note 7: All limits are guaranteed by design and measurements which are performed on a limited number of samples. Limits represent the mean ±3–sigma values.
The typical value represents the statistical mean value.
Note 8: This parameter is guaranteed by design and/or characterization and is not tested in production.
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LMH2110
Connection Diagram
6-Bump mircoSMD
30064805
Top View
6-Bump microSMD Marking
30064890
Top View
X = Date Code
T = Die Traceability
P = LMH2110TM
Pin Descriptions
microSMD Name Description
Power Supply A1 VDD Positive Supply Voltage.
C1 GND Power Ground.
B2 GND Power Ground. May be left floating in case grounding is not feasible.
Logic Input C2 EN The device is enabled for EN = High, and in shutdown mode for EN = Low. EN should be
<2.5V for having low IEN. For EN >2.5V, IEN increases slightly, while device is still
functional. Absolute maximum rating for EN = 3.6V.
Analog Input B1 RFIN RF input signal to the detector, internally terminated with 50Ω.
Output A2 OUT Ground referenced detector output voltage.
Ordering Information
Package Part Number Package
Marking
Transport Media NSC Drawing Status
6–Bump microSMD LMH2110TM P250 Units Tape and Reel TMD06BBA Released
LMH2110TMX 3k Units Tape and Reel
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LMH2110
Block Diagram
30064806
LMH2110
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LMH2110
Typical Performance Characteristics Unless otherwise specified: TA = 25°C, VBAT = 2.7V,
RFin = 1900 MHz CW (Continuous Wave, unmodulated). Specified errors are input referred.
Supply Current vs. Supply Voltage (Active)
30064811
Supply Current vs. Supply Voltage (Shutdown)
30064812
Supply Current vs. Enable Voltage (EN)
30064814
Supply Current vs. RF Input Power
30064813
Sourcing Output Current vs. RF Input Power
30064861
Sinking Output Current vs. RF Input Power
30064862
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LMH2110
RF Input Impedance vs. Frequency,
Resistance (R) and Reactance (X)
30064865
Power Supply Rejection Ratio vs. Frequency
30064863
Output Voltage Noise vs. Frequency
30064864
Output Voltage vs. RF Input Power
30064817
Output Voltage vs. Frequency
30064818
Log Slope vs. Frequency
30064815
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LMH2110
Log Intercept vs. Frequency
30064816
Output Voltage and Log Conformance Error vs.
RF Input Power at 50 MHz
30064819
Log Conformance Error (50 units) vs.
RF Input Power at 50 MHz
30064820
Temperature Variation vs.
RF Input Power at 50 MHz
30064821
Temperature Variation (50 units) vs.
RF Input Power at 50 MHz
30064822
Output Voltage and Log Conformance Error vs.
RF Input Power at 900 MHz
30064827
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LMH2110
Log Conformance Error (50 units) vs.
RF Input Power at 900 MHz
30064828
Temperature Variation vs.
RF Input Power at 900 MHz
30064829
Temperature Variation (50 units) vs.
RF Input Power at 900 MHz
30064830
1 dB Power Step Error vs.
RF Input Power at 900 MHz
30064831
10 dB Power Step Error vs.
RF Input Power at 900 MHz
30064832
W-CDMA Variation vs.
RF Input Power at 900 MHz
30064833
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LMH2110
LTE Variation vs.
RF Input Power at 900 MHz
30064834
Output Voltage and Log Conformance Error vs.
RF Input Power at 1900 MHz
30064838
Log Conformance Error (50 units) vs.
RF Input Power at 1900 MHz
30064839
Temperature Variation vs.
RF Input Power at 1900 MHz
30064840
Temperature Variation (50 units) vs.
RF Input Power at 1900 MHz
30064841
1 dB Power Step Error vs.
RF Input Power at 1900 MHz
30064842
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LMH2110
10 dB Power Step Error vs.
RF Input Power at 1900 MHz
30064843
W-CDMA Variation vs.
RF Input Power at 1900 MHz
30064844
LTE Input referred Variation vs.
RF Input Power at 1900 MHz
30064845
Output Voltage and Log Conformance Error vs.
RF Input Power at 3500 MHz
30064849
Log Conformance Error (50 units) vs.
RF Input Power at 3500 MHz
30064850
Temperature Variation vs.
RF Input Power at 3500 MHz
30064851
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LMH2110
Temperature Variation (50 units) vs.
RF Input Power at 3500 MHz
30064852
Output Voltage and Log Conformance Error vs.
RF Input Power at 5800 MHz
30064857
Log Conformance Error (50 units) vs.
RF Input Power at 5800 MHz
30064858
Temperature Variation vs.
RF Input Power at 5800 MHz
30064859
Temperature Variation (50 units) vs.
RF Input Power at 5800 MHz
30064860
Output Voltage and Log Conformance Error vs.
RF Input Power at 8000 MHz
30064866
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LMH2110
Temperature Variation vs.
RF Input Power at 8000 MHz
30064867
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LMH2110
Application Information
The LMH2110 is a 45 dB Logarithmic RMS power detector
particularly suited for accurate power measurements of mod-
ulated RF signals that exhibit large peak-to-average ratios
(PAR’s). The RMS detector implements the exact definition
of power resulting in a power measurement insensitive to high
PAR’s. Such signals are encountered e.g. in LTE and W-CD-
MA applications. The LMH2110 has an RF frequency range
from 50 MHz to 8 GHz. It provides an output voltage that re-
lates linearly to the RF input power in dBm. Its output voltage
is highly insensitive to temperature and supply variations.
TYPICAL APPLICATION
The LMH2110 can be used in a wide variety of applications
like LTE, W-CDMA, CDMA, GSM. This section discusses the
LMH2110 in a typical transmit power control loop for such
applications.
Transmit-power-control-loop circuits make the transmit power
level insensitive to power amplifier (PA) inaccuracy. This is
desired, since power amplifiers are non-linear devices and
temperature dependent, making it hard to estimate the exact
transmit power level. If a control loop is used, the inaccuracy
of the PA is eliminated from the overall accuracy of the trans-
mit power level. The accuracy of the transmit power level now
depends on the RF detector accuracy instead. The LMH2110
is especially suited for transmit power control applications,
since it accurately measures transmit power and is insensitive
to temperature, supply voltage and modulation variations.
Figure 1 shows a simplified schematic of a typical transmit
power control system. The output power of the PA is mea-
sured by the LMH2110 through a directional coupler. The
measured output voltage of the LMH2110 is digitized by the
ADC inside the baseband chip. Accordingly, the baseband
controls the PA output power level by changing the gain con-
trol signal of the RF VGA. Although the output ripple of the
LMH2110 is typically low enough, an optional low-pass filter
can be placed in between the LMH2110 and the ADC to fur-
ther reduce the ripple.
30064870
FIGURE 1. Transmit Power Control System
ACCURATE POWER MEASUREMENT
Detectors have evolved over the years along with the com-
munication standards. Newer communication standards like
LTE and W-CDMA raise the need for more advanced accu-
rate power detectors. To be able to distinguish the various
detector types it is important to understand what the ideal
power measurement should look like and how a power mea-
surement is implemented.
Power is a metric for the average energy content of a signal.
By definition it is not a function of the signal shape over time.
In other words, the power content of a 0 dBm sine wave is
identical to the power content of a 0 dBm square wave or a 0
dBm W-CDMA signal; all these signals have the same aver-
age power content.
The average power can be described by the following formu-
la:
(1)
Where, T is the time interval over which is averaged, v(t) is
the instantaneous voltage at time t, R is the resistance in
which the power is dissipated, and VRMS is the equivalent
RMS voltage.
According to aforementioned formula for power, an exact
power measurement can be done via measuring the RMS
voltage (VRMS) of a signal. The RMS voltage is described by:
(2)
Implementing the exact formula for RMS can be difficult
though. A simplification can be made in determining the av-
erage power when information about the waveform is avail-
able. If the signal shape is known, the relationship between
RMS value and, for instance, the peak value of the RF signal
is also known. It thus enables a measurement based on mea-
suring peak voltage rather than measuring the RMS voltage.
To calculate the RMS value (and therewith the average pow-
er), the measured peak voltage is translated into an RMS
voltage based on the waveform characteristics. A few exam-
ples:
•Sine wave: VRMS = VPEAK/ √2
•Square wave: VRMS = VPEAK
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LMH2110
•Saw-tooth wave: VRMS = VPEAK/ √3
For more complex waveforms it is not always easy to deter-
mine the exact relationship between RMS value and peak
value. A peak measurement can then become impractical. An
approximation can be used for the VRMS to VPEAK relationship
but it can result in a less accurate average power estimate.
Depending on the detection mechanism, power detectors
may produce a slightly different output signal in response to
the earlier mentioned waveforms, even though the average
power level of these signals are the same. This error is due
to the fact that not all power detectors strictly implement the
definition for signal power, being the root mean square (RMS)
of the signal. To cover for the systematic error in the output
response of a detector, calibration can be used. After calibra-
tion a look-up table corrects for the error. Multiple look-up
tables can be created for different modulation schemes.
TYPES OF RF DETECTORS
This section provides an overview of detectors based on their
detection principle. Detectors that will be discussed are:
•Peak detectors
•LOG Amp detectors
•RMS detectors
Peak Detectors
A peak detector is one of the simplest types of detectors. Ac-
cording to the naming, the peak detector “stores” the highest
value arising in a certain time window. However, usually a
peak detector is used with a relative long holding time when
compared to the carrier frequency and a relative short holding
time with respect to the envelope frequency. In this way a
peak detector is used as AM demodulator or envelope tracker
(Figure 2).
30064880
FIGURE 2. Peak detection vs. envelope tracking
A peak detector usually has a linear response. An example of
this is a diode detector (Figure 3). The diode rectifies the RF
input voltage and subsequently the RC filter determines the
averaging (holding) time. The selection of the holding time
configures the diode detector for its particular application. For
envelope tracking a relatively small RC time constant is cho-
sen, such that the output voltage tracks the envelope nicely.
A configuration with a relatively large time constant can be
used for supply regulation of the power amplifier (PA). Con-
trolling the supply voltage of the PA can reduce power con-
sumption significantly. The optimal mode of operation is to set
the supply voltage such that it is just above the maximum
output voltage of the PA. A diode detector with relative large
RC time constant measures this maximum (peak) voltage.
30064888
FIGURE 3. Diode Detector
Since peak detectors measure a peak voltage, their response
is inherently depended on the signal shape or modulation
form as discussed in the previous section. Knowledge about
the signal shape is required to determine an RMS value. For
complex systems having various modulation schemes, the
amount of calibration and look-up tables can become unman-
ageable.
LOG Amp Detectors
LOG Amp detectors are widely used RF power detectors for
GSM and the early W-CDMA systems. The transfer function
of a LOG amp detector has a linear-in-dB response, which
means that the output in volts changes linearly with the RF
power in dBm. This is convenient since most communication
standards specify transmit power levels in dBm as well. LOG
amp detectors implement the logarithmic function by a piece-
wise linear approximation. Consequently, the LOG amp de-
tector does not implement an exact power measurement,
which implies a dependency on the signal shape. In systems
using various modulation schemes calibration and lookup ta-
bles might be required.
RMS Detectors
An RMS detector has a response that is insensitive to the
signal shape and modulation form. This is because its oper-
ation is based on exact determination of the average power,
i.e. it implements:
(3)
RMS detectors are in particular suited for the newer commu-
nication 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 con-
figured correctly in the application. The detector will be dis-
cussed by means of its block diagram (Figure 4). Subse-
quently, the details of the electrical interfacing are separately
discussed for each pin.
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LMH2110
30064871
FIGURE 4. Block Diagram
For measuring the RMS (power) level of a signal, the time
average of the squared signal needs to be measured as de-
scribed 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 4. The core of
the loop is a multiplier. The two inputs of the multiplier are fed
by (i1, i2):
i1 = iLF + iRF (4)
i2 = iLF - iRF (5)
in which iLF is a current depending on the DC output voltage
of the RF detector and iRF is a current depending on the RF
input signal. The output of the multiplier (iOUT) is the product
of these two current and equals:
(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:
(7)
which implies that the average power content of the current
related to the output voltage of the LMH2110 is made equal
to the average power content of the current related to the RF
input signal.
For a negative-feedback system, the transfer function is given
by the inverse function of the feedback block. Therefore, to
have a logarithmic transfer for this RF detector, the feedback
network implements an exponential function resulting in an
overall transfer function for the LMH2110 of:
(8)
in which V0 and VX are normalizing voltages. Note that as a
result of the feedback loop also a square-root is implemented
yielding the RMS function.
Given this architecture for the RF detector, the high-perfor-
mance of the LMH2110 can be understood. In theory the
accuracy of the logarithmic transfer is set by:
•The exponential feedback network, which basically needs
to process a DC signal only.
•A high loop gain for the feedback loop, which is
guaranteed by the amplifier gain A.
The RMS functionality is inherent to the feedback loop and
the use of a multiplier. So, a very accurate LOG-RMS RF
power detector is obtained.
To guarantee a low dependency on the supply voltage, the
internal detector circuitry is supplied via a low drop-out (LDO)
regulator. This enables the usage of a wide range of supply
voltage (2.7V to 5V) in combination with a low sensitivity of
the output signal for the external supply voltage.
RF Input
RF systems typically use a characteristic impedance of 50Ω.
The LMH2110 is no exception to this. The RF input pin of the
LMH2110 has an input impedance of 50Ω. It enables an easy,
direct connection to a directional coupler without the need for
additional components (Figure 1). For an accurate power
measurement the input power range of the LMH2110 needs
to be aligned with the output power range of the power am-
plifier. This can be done by selecting a directional coupler with
the correct coupling factor.
Since the LMH2110 has a constant input impedance, a re-
sistive divider can also be used in stead of a directional
coupler (Figure 5).
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LMH2110
30064881
FIGURE 5. Application with Resistive Divider
Resistor R1 implements an attenuator together with the de-
tector input impedance to match the output range of the PA
to the input range of the LMH2110. The attenuation (AdB) re-
alized by R1 and the effective input impedance of the
LMH2110 equals:
(9)
Solving this expression for R1 yields:
(10)
Suppose the desired attenuation is 30 dB with a given
LMH2110 input impedance of 50Ω, the resistor R1 needs to
be 1531Ω. A practical value is 1.5 kΩ. Although this is a
cheaper solution than the application with directional coupler,
it also comes with a disadvantage. After calculating the re-
sistor value it is possible that the realized attenuation is less
then expected. This is because of the parasitic capacitance
of resistor R1 which results in a lower actual realized attenu-
ation. Whether the attenuation will be reduced depends on
the frequency of the RF signal and the parasitic capacitance
of resistor R1. Since the parasitic capacitance varies from re-
sistor to resistor, exact determination of the realized attenu-
ation can be difficult. A way to reduce the parasitic
capacitance of resistor R1 is to realize it as a series connec-
tion of several separate resistors.
Enable
To save power, the LMH2110 can be brought into a low-power
shutdown mode by means of the enable pin (EN). The device
is active for EN = HIGH (VEN>1.1V) and in the low-power
shutdown mode for EN = LOW (VEN < 0.6V). In this state the
output of the LMH2110 is switched to a high impedance
mode. This high impedance mode prevents the discharge of
the optional low-pass filter which is good for the power effi-
ciency. Using the shutdown function, care must be taken not
to exceed the absolute maximum ratings. Since the device
has an internal operating voltage of 2.5V, the voltage level on
the enable should not be higher than 3V to prevent damage
to the device. Also enable voltage levels lower than 400 mV
below GND should be prevented. In both cases the ESD de-
vices start to conduct when the enable voltage range is ex-
ceeded and excessive current will be drawn. A correct
operation is not guaranteed then. The absolute maximum rat-
ings are also exceeded when the enable (EN) is switched to
HIGH (from shutdown to active mode) while the supply volt-
age is switched off. This situation should be prevented at all
times. A possible solution to protect the device is to add a
resistor of 1 kΩ in series with the enable input to limit the cur-
rent.
Output
The output of the LMH2110 provides a DC voltage that is a
measure for the applied RF power to the input pin. The output
voltage has a linear-in-dB response for an applied RF signal.
RF power detectors can have some residual ripple on the
output due to the modulation of the applied RF signal. The
residual ripple on the LMH2110’s output is small though and
therefore additional filtering is usually not needed. This is be-
cause its internal averaging mechanism reduces the ripple
significantly. For some modulation types however, having
very high peak-to-average ratios, additional filtering might be
useful.
Filtering can be applied by an external low-pass filter. It should
be realized that filtering reduces not only the ripple, but also
increases the response time. In other words, it takes longer
before the output reaches its final value. A trade-off should be
made between allowed ripple and allowed response time. The
filtering technique is depicted in Figure 6. The filtering of the
low pass output filter is realized by resistor RS and capacitor
CS. The -3 dB bandwidth of this filter can be calculated by:
f−3 dB = 1 / (2πRSCS) (11)
30064872
FIGURE 6. Low-Pass Output Filter for Residual Ripple
Reduction
The output impedance of the LMH2110 is HIGH in shutdown.
This is especially beneficial in pulsed mode systems. It en-
sures a fast settling time when the device returns from shut-
down into active mode and reduces power consumption.
In pulse mode systems, the device is active only during a
fraction of the time. During the remaining time the device is in
low-power shutdown. Pulsed mode system applications usu-
ally require that the output value is available at all times. This
can be realized by a capacitor connected between the output
and GND that “stores” the output voltage level. To apply this
principle it should be ensured that discharging of the capacitor
is minimized in shutdown mode. The connected ADC input
should thus have a high input impedance to prevent a possi-
ble discharge path through the ADC. When an additional filter
is applied at the output, the capacitor of the RC-filter can be
used to store the output value. An LMH2110 with a high
impedance shutdown mode save power in pulse mode sys-
tems. This is because the capacitor CS doesn’t need to be
fully re-charged each cycle.
19 www.national.com
LMH2110
Supply
The LMH2110 has an internal LDO to handle supply voltages
between 2.7V to 5V. This enables a direct connection to the
battery in cell phone applications. The high PSRR of the
LMH2110 ensures that the performance is constant over its
power supply range.
SPECIFYING DETECTOR PERFORMANCE
The performance of the LMH2110 can be expressed by a va-
riety of parameters. This section discusses the key parame-
ters.
Dynamic Range
The LMH2110 is designed to have a predictable and accurate
response over a certain input power range. This is called the
dynamic range (DR) of a detector. For determining the dy-
namic range a couple of different criteria can be used. The
most commonly used ones are:
•Log conformance error, ELC
•Variation over temperature error, EVOT
•1 dB step error, E1 dB
•10 dB step error, E10 dB
•Variation due to modulation, EMOD
The specified dynamic range is the range in which the spec-
ified error metric is within a predefined window. An explana-
tion of these errors is given in the following paragraphs.
Log Conformance error
The LMH2110 implements a logarithmic function. In order to
describe how close the transfer is to an ideal logarithmic func-
tion the log conformance error is used. To calculate the log
conformance error the detector transfer function is modeled
as a linear-in-dB relationship between the input power and the
output voltage.
The ideal linear-in-dB transfer is modeled by 2 parameters:
•Slope
•Intercept
and is described by:
VOUT = KSLOPE (PIN – PINT) (12)
Where KSLOPE is the slope of the line in mV/dB, PIN the input
power level and PINT is the power level in dBm at which the
line intercepts VOUT = 0V (See Figure 7).
30064873
FIGURE 7. Ideal Logarithmic Response
To determine the log conformance error two steps are re-
quired:
1. Determine the best fitted line at 25°C.
2. Determine the difference between the actual data and
the best fitted line.
The best fit can be determined by standard routines. A careful
selection of the fit range is important. The fit range should be
within the normal range of operation of the device. Outcome
of the fit is KSLOPE and PINT.
Subsequently, the difference between the actual data and the
best fitted line is determined. The log conformance is speci-
fied as an input referred error. The output referred error is
therefore divided by the KSLOPE to obtain the input referred
error. The log conformance error is calculated by the following
equation:
(13)
Where VOUT is the measured output voltage at a power level
at PIN at a temperature. KSLOPE 25°C (mV/dB) and PINT 25°C
(dBm) are the parameters of the best fitted line of the 25°C
transfer.
In Figure 8 it can be seen that both the error with respect to
the ideal LOG response as well as the error due to tempera-
ture variation are included in this error metric. This is because
the measured data for all temperatures is compared to the
fitted line at 25°C. The measurement result of a typical
LMH2110 in Figure 8 shows a dynamic range of 36 dB for
ELC= ±1dB.
30064838
FIGURE 8. VOUT and ELC vs. RF input Power at 1900 MHz
Variation over Temperature Error
In contrast to the log conformance error, the variation over
temperature error (EVOT) 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 KSLOPE at 25°C. The equation for variation over
temperature is given by:
EVOT = (VOUT_TEMP – VOUT 25°C) / KSLOPE 25°C (14)
The variation over temperature is shown in Figure 9, where a
dynamic range of 41 dB is obtained (from PMIN = -36 dBm) for
EVOT = ±0.5 dB.
www.national.com 20
LMH2110
30064840
FIGURE 9. EVOT 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. Often, this condition is used to define a
useful dynamic range of the detector.
The 1 dB step error is calculated in 3 steps:
1. Determine the maximum sensitivity.
2. Determine average sensitivity.
3. Calculate the 1 dB step error.
First the maximum sensitivity (SMAX) is calculated per tem-
perature by determining the maximum difference between
two output voltages for a 1 dB step within the power range:
SMAX = VOUT P+1 – VOUT P (15)
For calculating the 1 dB step error an average sensitivity
(SAVG) is used which is the average of the maximum sensi-
tivity and an allowed minimum sensitivity (SMIN). The allowed
minimum sensitivity is determined by the application. In this
datasheet SMIN = 30 mV/dB is used. Subsequently, the aver-
age sensitivity can be calculated by:
SAVG = (SMAX + SMIN) / 2 (16)
The 1dB error is than calculated by:
E1 dB = (SACTUAL - SAVG) / SAVG (17)
Where, SACTUAL (actual sensitivity) is the difference between
two output voltages for a 1 dB step at a given power level.
Figure 10 shows the typical 1 dB step error at 1900 MHz,
where a dynamic range of 38 dB over temperature is obtained
for E1dB = ±0.3 dB.
30064842
FIGURE 10. 1 dB Step Error vs. RF Input Power at 1900
MHz
10 dB step error
This error is defined in a different manner than the 1 dB step
error. This parameter shows the input power error over tem-
perature when a 10 dB power step is made. The 10 dB step
at 25°C is taken as a reference.
To determine the 10 dB step error first the output voltage lev-
els (V1 and V2) for power levels “P” and “P+10dB” at the 25°
C are determined (Figure 11). Subsequently these 2 output
voltages are used to determine the corresponding power lev-
els at temperature T (PT and PT+X). The difference between
those two power levels subtracted by 10 results in the 10 dB
step error.
30064887
FIGURE 11. Graphical Representation of 10 dB Step
calculations
Figure 12 shows the typical 10 dB step error at 1900 MHz,
where a dynamic range of 30 dB is obtained for E10dB = ±1
dB.
21 www.national.com
LMH2110
30064843
FIGURE 12. 10 dB Step Error vs. RF Input Power at 1900
MHz
Variation due to Modulation
The response of an RF detector may vary due to different
modulation schemes. How much it will vary depends on the
modulation form and the type of detector. Modulation forms
with high peak-to-average ratios (PAR) can cause significant
variation, especially with traditional RF detectors. This is be-
cause the measurement is not an actual RMS measurement
and is therefore waveform dependent.
To calculate the variation due to modulation (EMOD), the mea-
surement result for an un-modulated RF carrier is subtracted
from the measurement result of a modulated RF carrier. The
calculations are similar to those for variation over tempera-
ture. The variation due to modulation can be calculated by:
EMOD = (VOUT_MOD – VOUT_CW) / KSLOPE (18)
Where VOUT_MOD is the measured output voltage for an ap-
plied power level of a modulated signal, VOUT_CW is the output
voltage as a result of an applied un-modulated signal having
the same power level.
Figure 13 shows the variation due to modulation for W-CDMA,
where a EMOD of 0.09 dB in obtained for a dynamic range from
-38 dBm to –5 dBm.
30064844
FIGURE 13. Variation due to Modulation for W-CDMA
LAYOUT RECOMMENDATIONS
As with any other RF device, careful attention must me paid
to the board layout. If the board layout isn’t properly designed,
performance might be less then can be expected for the ap-
plication.
The LMH2110 is designed to be used in RF applications,
having a characteristic impedance of 50Ω. To achieve this
impedance, the input of the LMH2110 needs to be connected
via a 50Ω transmission line. Transmission lines can be cre-
ated on PCBs using microstrip or (grounded) coplanar waveg-
uide (GCPW) configurations.
In order to minimize injection of RF interference into the
LMH2110 through the supply lines, the PCB traces for VDD
and GND should be minimized for RF signals. This can be
done by placing a small decoupling capacitor between the
VDD and GND. It should be placed as close as possible to the
VDD and GND pins of the LMH2110.
www.national.com 22
LMH2110
Physical Dimensions inches (millimeters) unless otherwise noted
6-Bump microSMD
NS Package Number TMD06BBA
X1 = 0.840 ± 0.030 mm, X2 = 1.240 ± 0.030 mm, X3 = 0.600 ± 0.075 mm
23 www.national.com
LMH2110
Notes
LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range
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