LMH2120
LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range
Literature Number: SNWS021B
LMH2120
October 13, 2010
6 GHz Linear RMS Power Detector with 40 dB Dynamic
Range
General Description
The LMH2120 is a 40 dB Linear RMS power detector partic-
ularly suited for accurate power measurement of modulated
RF signals that exhibit large peak-to-average ratios, i.e, large
variations of the signal envelope. Such signals are encoun-
tered in W-CDMA and LTE cell phones. The RMS measure-
ment topology inherently ensures a modulation insensitive
measurement.
The device has an RF frequency range from 50 MHz to
6 GHz. It provides an accurate, temperature and supply in-
sensitive, output voltage that relates linearly to the RF input
power in volt. The LMH2120's excellent conformance to a lin-
ear response enables an easy integration by using slope and
intercept only, reducing calibration effort significantly. The
device operates with a single supply from 2.7V to 5V. The
LMH2120 has an RF power detection range from -35 dBm to
5 dBm and is ideally suited for use in combination with a di-
rectional coupler. Alternatively, a resistive divider can be
used.
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 LMH2120 power detector is offered in a tiny 6-bump
micro SMD package.
Features
■Linear root mean square response
■40 dB linear-in-V power detection range
■Multi-band operation from 50 MHz to 6 GHz
■Lin conformance better than ±0.5 dB
■Highly temperature insensitive
■Modulation independent response
■Minimal Slope and Intercept variation
■Shutdown functionality
■Wide supply range from 2.7V to 5V
■Tiny 6-bump micro SMD package
Applications
■Multi Mode, Multi band RF power control
—GSM/EDGE
—CDMA/CDMA2000
—W-CDMA
—OFDMA
—LTE
■Infrastructure RF Power Control
Typical Application
30055701
Output Voltage vs. RF Input Power at 1900 MHz
30055733
© 2010 National Semiconductor Corporation 300557 www.national.com
LMH2120 6 GHz Linear RMS Power Detector with 40 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
VDD - GND 5.5V
RF Input
Input power 12 dBm
DC Voltage 1V
Enable (EN) 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
For soldering specifications:
See product folder at www.national.com and
www.national.com/ms/MS/MS-SOLDERING.pdf
Operating Ratings (Note 1)
Supply Voltage 2.7V to 5V
Temperature Range −40°C to +85°C
RF Frequency Range 50 MHz to 6 GHz
RF Input Power Range −35 dBm to 5 dBm
Package Thermal Resistance θJA
(Note 3) 166.7°C/W
2.7 V and 4.5V DC and AC Electrical Characteristics
Unless otherwise specified, all limits are guaranteed to TA = 25°C, VDD = 2.7V and 4.5V (worst case 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
IDD Supply Current Active mode: EN = High, no signal
present at RFIN. 2.9 3.5
4.0 mA
Shutdown: EN = LOW,
no signal present at
RFIN
VBAT = 2.7V 3.8 4.7
5.0 µA
VBAT = 4.5V 4.7 5.7
6.1
EN = LOW, RFIN =
0 dBm, 1900 MHz VBAT = 2.7V 3.8 4.7
5.0 µA
VBAT = 4.5V 4.7 5.7
6.1
PSRR Power Supply Rejection Ratio RFIN = -10 dBm, 1900 MHz,
2.7V < VBAT < 5V 50 60 dB
Logic Enable Interface
VLOW EN logic LOW input level
(Shutdown mode)
0.6 V
VHIGH EN logic HIGH input level 1.1
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 18 29
33 mV
ROUT Output Resistance EN = HIGH, RFIN = -10 dBm, 1900 MHz,
ILOAD = 1 mA, DC measurement 1 2
3Ω
IOUT Output Sinking Current RFIN = -10 dBm, 1900 MHz, OUT
connected to 2.5V
30
25 42
mA
Output Sourcing Current RFIN = -10 dBm, 1900 MHz, OUT
connected to GND
36
31 45
IOUT, SD Output Leakage Current in
Shutdown Mode
EN = LOW, OUT connected to 2V 80 nA
enOutput Referred Noise (Note 8) RFIN = -10 dBm, 1900 MHz, output
spectrum at 10 kHz 5 µV/√Hz
www.national.com 2
LMH2120
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)
Units
vnOutput Referred Noise Integrated
(Note 8)
Integrated over frequency band 1 kHz -
6.5 kHz, RFIN = -10 dBm, 1900 MHz
390 µVRMS
Timing Characteristics
tON Turn-on Time from shutdown RFIN = -10 dBm, 1900 MHz, EN LOW-
to-HIGH transition to OUT at 90%
13 18 µs
tRRise Time Signal at RFIN from -20 dBm to 0 dBm,
10% to 90%, 1900 MHz 7 µs
tFFall Time Signal at RFIN from 0 dBm to -20 dBm,
90% to 10%, 1900 MHz 18 µs
RF Detector Transfer, fit range -15 dBm to -5 dBm for Linear Slope and Intercept
RFIN = 50 MHz (Note 7)
PMIN Minimum Power Level, bottom
end of Dynamic Range
Log Conformance Error within ±1 dB -37
dBm
PMAX Maximum Power Level, top end
of Dynamic Range 4
VMIN Minimum Output Voltage At PMIN 31 mV
VMAX Maximum Output Voltage At PMAX 2.6 V
KSLOPE Linear Slope 1 dB/dB
PINT Linear Intercept VOUT = 0 dBV -5.7 -5.5 -5.3 dBm
DR Dynamic Range for specified
Accuracy
±1 dB Lin Conformance Error (ELC) 37
36
41
40
dB
±3 dB Lin Conformance Error (ELC) 44
43
48
47
±0.5 dB Variation over Temperature
(EVOT)41 45
RFIN = 900 MHz (Note 7)
PMIN Minimum Power Level, bottom
end of Dynamic Range
Lin Conformance Error within ±1 dB -35
dBm
PMAX Maximum Power Level, top end
of Dynamic Range 5
VMIN Minimum Output Voltage At PMIN 33 mV
VMAX Maximum Output Voltage At PMAX 2.5 V
KSLOPE Linear Slope 1 dB/dB
PINT Linear Intercept VOUT = 0 dBV -4.2 -4.0 -3.8 dBm
DR Dynamic Range for specified
Accuracy
±1 dB Lin Conformance Error (ELC) 36
33
40
37
dB
±3 dB Lin Conformance Error (ELC) 45
44
48
47
±0.5 dB Variation over Temperature
(EVOT)41 44
±0.3 dB Error for a 1dB Power Step
(E1dB) 41
40
±1 dB Error for a 10dB Power Step
(E10dB) 45
EMOD Input referred Variation due to
Modulation
W-CDMA Release 6/7/8,
-35 dBm<RFIN<-3 dBm 0.15 dB
LTE, -35 dBm<RFIN<-3 dBm 0.29
3 www.national.com
LMH2120
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)
Units
RFIN = 1900 MHz (Note 7)
PMIN Minimum Power Level, bottom
end of Dynamic Range
Lin Conformance Error within ±1 dB -34
dBm
PMAX Maximum Power Level, top end
of Dynamic Range 4
VMIN Minimum Output Voltage At PMIN 30 mV
VMAX Maximum Output Voltage At PMAX 1.7 V
KSLOPE Linear Slope 1 dB/dB
PINT Linear Intercept VOUT = 0 dBV -2.2 -1.8 -1.4 dBm
DR Dynamic Range for specified
Accuracy
±1 dB Lin Conformance Error (ELC) 35
31
38
35
dB
±3 dB Lin Conformance Error (ELC) 44
41
48
45
±0.5 dB Variation over Temperature
(EVOT)35 40
±0.3 dB Error for a 1dB Power Step
(E1dB) 39
36
±1 dB Error for a 10dB Power Step
(E10dB) 35
EMOD Input referred Variation due to
Modulation
W-CDMA Release 6/7/8,
-34 dBm<RFIN<-2 dBm 0.16 dB
LTE, -34 dBm<RFIN<-2 dBm 0.24
RFIN = 2600 MHz (Note 7)
PMIN Minimum Power Level, bottom
end of Dynamic Range
Lin Conformance Error within ±1 dB -30
dBm
PMAX Maximum Power Level, top end
of Dynamic Range 6
VMIN Minimum Output Voltage At PMIN 31 mV
VMAX Maximum Output Voltage At PMAX 1.5 V
KSLOPE Linear Slope 1 dB/dB
PINT Linear Intercept VOUT = 0 dBV 0.8 1.7 2.6 dBm
DR Dynamic Range for specified
Accuracy
±1 dB Lin Conformance Error (ELC) 32
29
36
33
dB
±3 dB Lin Conformance Error (ELC) 43
40
45
42
±0.5 dB Variation over Temperature
(EVOT)34 39
RFIN = 3500 MHz (Note 7)
PMIN Minimum Power Level, bottom
end of Dynamic Range
Lin Conformance Error within ±1 dB -26
dBm
PMAX Maximum Power Level, top end
of Dynamic Range 7
VMIN Minimum Output Voltage At PMIN 32 mV
VMAX Maximum Output Voltage At PMAX 1.1 V
KSLOPE Linear Slope 1 dB/dB
PINT Linear Intercept VOUT = 0 dBV 4.4 5.5 6.7 dBm
www.national.com 4
LMH2120
Symbol Parameter Condition Min
(Note 5)
Typ
(Note 6)
Max
(Note 5)
Units
DR Dynamic Range for specified
Accuracy
±1 dB Lin Conformance Error (ELC) 30
27
33
30
dB
±3 dB Lin Conformance Error (ELC) 39
36
42
40
±0.5 dB Variation over Temperature
(EVOT)27 35
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: Limits are guaranteed by design and measurements which are performed on a limited number of samples.
Note 8: This parameter is guaranteed by design and/or characterization and is not tested in production.
5 www.national.com
LMH2120
Connection Diagram
6-Bump micro SMD
30055702
Top View
6–Bump microSMD Marking
30055792
Top View
X = Date Code
T = Die Traceability
R = LMH2120UM
Pin Descriptions
microSMD Name Description
Power Supply A1 VDD Positive Supply Voltage.
C1 GND Ground. Both C1 and B2 need to be connected to GND.
B2
Logic Input C2 EN The device is enabled for EN = High, and in shutdown mode for EN = LOW. EN should
be <2.5V when IEN is LOW. For EN >2.5V, IEN increases slightly while the device is still
functional. Absolute maximum rating for EN = 3.6V.
Analog Input B1 RFIN RF input signal to the detector, internally terminated with 50 Ω.
Output A2 OUT Ground referenced detector output voltage.
Ordering Information
Package Part Number Package
Marking
Transport Media NSC Drawing Status
6-Bump micro SMD LMH2120UM R250 Units Tape and Reel UMD06AAA Released
LMH2120UMX 3k Units Tape and Reel
www.national.com 6
LMH2120
Block Diagram
30055703
LMH2120
7 www.national.com
LMH2120
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)
30055710
Supply Current vs. Supply Voltage (Shutdown)
30055711
Supply Current vs. Enable Voltage (EN)
30055712
Supply Current vs. RF Input Power
30055713
Output Sourcing Current vs. RF Input Power
30055761
Output Sinking Current vs. RF Input Power
30055762
www.national.com 8
LMH2120
RF Input Impedance vs. Frequency,
Resistance (R) and Reactance (X)
30055765
Power Supply Rejection Ratio vs. Frequency
30055763
Output Voltage Noise vs. Frequency
30055764
Lin Slope vs. Frequency
30055715
Lin Intercept vs. Frequency
30055716
Output Voltage vs. RF Input Power
30055717
9 www.national.com
LMH2120
Output Voltage vs. Frequency
30055718
Output Voltage vs. RF Input Power at 50 MHz
30055719
Lin Conformance vs. RF Input Power at 50 MHz
30055720
Lin Conformance (50 units) vs.
RF Input Power at 50 MHz
30055721
Temperature Variation vs. RF Input Power at 50 MHz
30055722
Temperature Variation (50 units) vs.
RF Input Power at 50 MHz
30055723
www.national.com 10
LMH2120
Output Voltage vs. RF Input Power at 900 MHz
30055724
Lin Conformance vs. RF Input Power at 900 MHz
30055725
Lin Conformance (50 units) vs.
RF Input Power at 900 MHz
30055726
Temperature Variation vs. RF Input Power at 900 MHz
30055727
Temperature Variation (50 units) vs.
RF Input Power at 900 MHz
30055728
1 dB Power Step Error vs.
RF Input Power at 900 MHz
30055729
11 www.national.com
LMH2120
10 dB Power Step Error vs.
RF Input Power at 900 MHz
30055730
W-CDMA variation vs. RF Input Power at 900 MHz
30055731
LTE variation vs. RF Input Power at 900 MHz
30055732
Output Voltage vs. RF Input Power at 1900 MHz
30055733
Lin Conformance vs. RF Input Power at 1900 MHz
30055734
Lin Conformance (50 units) vs.
RF Input Power at 1900 MHz
30055735
www.national.com 12
LMH2120
Temperature Variation vs. RF Input Power at 1900 MHz
30055736
Temperature Variation (50 units) vs.
RF Input Power at 1900 MHz
30055737
1 dB Power Step Error vs.
RF Input Power at 1900 MHz
30055738
10 dB Power Step Error vs.
RF Input Power at 1900 MHz
30055739
W-CDMA variation vs. RF Input Power at 1900 MHz
30055740
LTE variation vs. RF Input Power at 1900 MHz
30055741
13 www.national.com
LMH2120
Output Voltage vs. RF Input Power at 2600 MHz
30055742
Lin Conformance vs. RF Input Power at 2600 MHz
30055743
Lin Conformance (50 units) vs.
RF Input Power at 2600 MHz
30055744
Temperature Variation vs. RF Input Power at 2600 MHz
30055745
Temperature Variation (50 units) vs.
RF Input Power at 2600 MHz
30055746
Output Voltage vs. RF Input Power at 3500 MHz
30055747
www.national.com 14
LMH2120
Lin Conformance vs. RF Input Power at 3500 MHz
30055748
Lin Conformance (50 units) vs.
RF Input Power at 3500 MHz
30055749
Temperature Variation vs. RF Input Power at 3500 MHz
30055750
Temperature Variation (50 units) vs.
RF Input Power at 3500 MHz
30055751
Output Voltage vs. RF Input Power at 5800 MHz
30055752
Lin Conformance vs. RF Input Power at 5800 MHz
30055753
15 www.national.com
LMH2120
Temperature Variation vs. RF Input Power at 5800 MHz
30055754
www.national.com 16
LMH2120
Application Information
The LMH2120 is a 40 dB Linear RMS power detector partic-
ularly suited for accurate power measurements of modulated
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-CDMA ap-
plications. The LMH2120 has an RF frequency range from
50 MHz to 6 GHz. It provides an output voltage that relates
linearly to the RF input power in volt. Its output voltage is
highly insensitive to temperature and supply variations.
TYPICAL APPLICATION
The LMH2120 can be used in a wide variety of applications
like LTE, W-CDMA, CDMA and GSM. This section discusses
the LMH2120 in a typical transmit power control loop for such
applications.
Transmit-power-control-loop circuits make the transmit-pow-
er level insensitive to power amplifier (PA) inaccuracy. This is
desirable because 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 inac-
curacy of the PA is eliminated from the overall accuracy of the
transmit power level. The accuracy of the transmit power level
now depends on the RF detector accuracy instead. The
LMH2120 is especially suited for transmit-power control ap-
plications, 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 LMH2120 through a directional coupler. The
measured output voltage of the LMH2120 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
LMH2120 is typically low enough, an optional low-pass filter
can be placed in between the LMH2120 and the ADC to fur-
ther reduce the ripple.
30055770
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 volt-
age.
According to aforementioned formula for power, an exact
power measurement can be done by measuring the RMS
voltage (VRMS) of a signal. The RMS voltage is described by:
(2)
Implementing the exact formula for RMS can be difficult how-
ever. A simplification can be made in determining the average
power when information about the waveform is available. 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 measuring
peak voltage rather than measuring the RMS voltage. To cal-
culate the RMS value (and therewith the average power), the
measured peak voltage is translated into an RMS voltage
based on the waveform characteristics. A few examples:
•Sine wave: VRMS = VPEAK / √2
•Square wave: VRMS = VPEAK
•Saw-tooth wave: VRMS = VPEAK / √3
17 www.national.com
LMH2120
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 therefore become impracti-
cal. 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 type of detector, storing
the highest value arising in a certain time window. However,
a peak detector is typically used with a relatively long holding
time when compared to the carrier frequency and a relatively
short holding time with respect to the envelope frequency. In
this way a peak detector is used as AM demodulator or en-
velope tracker (Figure 2).
30055780
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; subsequently, the RC filter determines the av-
eraging (holding) time. The selection of the holding time con-
figures 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.
In contrast, a configuration with a relatively large time con-
stant measures the maximum (peak) voltage of a signal.
30055788
FIGURE 3. Diode Detector
Since peak detectors measure a peak voltage, their response
is inherently dependent 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 particularly 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 ap-
plication due to different applied modulation schemes.
LMH2120 RF POWER DETECTOR
For optimal performance, the LMH2120 needs to be config-
ured correctly in the application. The detector will be dis-
cussed by means of its block diagram (Figure 4). Details of
the electrical interfacing are separately discussed for each pin
below.
www.national.com 18
LMH2120
30055771
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 LMH2120 by means of a multiplier
and a low-pass filter in a negative-feedback loop. A simplified
block diagram of the LMH2120 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)
i1 = 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 LMH2120 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 linear transfer for this RF detector, the feedback net-
work implements a linear function as well resulting in an
overall transfer function for the LMH2120 of:
(8)
in which k is the conversion gain. Note that as a result of the
feedback loop a square root is also implemented, yielding the
RMS function.
Given this architecture for the RF detector, the high perfor-
mance of the LMH2120 can be understood. In theory the
accuracy of the linear transfer is set by:
•The linear 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. Thus, a very accurate LIN-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 LMH2120 is no exception to this. The RF input pin of the
LMH2120 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 LMH2120 needs
to be aligned with the output power range of the power am-
19 www.national.com
LMH2120
plifier. This can be done by selecting a directional coupler with
the appropriate coupling factor.
Since the LMH2120 has a constant input impedance, a re-
sistive divider can also be used instead of a directional cou-
pler (Figure 5).
30055781
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
with the input range of the LMH2120. The attenuation (AdB)
realized by R1 and the effective input impedance (RIN) of the
LMH2120 equals:
(9)
Solving this expression for R1 yields:
(10)
Suppose the desired attenuation is 30 dB with a given
LMH2120 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 has a disadvantage. After calculating the resistor value it is
possible that the realized attenuation is less than expected.
This is because of the parasitic capacitance of resistor R1
which results in a lower actual realized attenuation. 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 resistor to resis-
tor, exact determination of the realized attenuation can be
difficult. A way to reduce the parasitic capacitance of resistor
R1 is to realize it as a series connection of several separate
resistors.
Enable
To save power, the LMH2120 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 LMH2120 is switched to high-impedance. This
high impedance prevents the discharge of the optional low-
pass filter which is good for power efficiency. Using the shut-
down 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 excess current flowing into the en-
able pin. Also enable voltage levels lower than 400 mV below
GND should be prevented. In both cases the ESD devices
start to conduct when the enable voltage range is exceeded
and excess current will be drawn. A correct operation is not
guaranteed then. The absolute maximum ratings are also ex-
ceeded when EN is switched to HIGH (from shutdown to
active mode) while the supply voltage 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 current.
Output
The output of the LMH2120 provides a DC voltage that is a
measure for the applied RF power to the input pin. The output
voltage has a linear-in-V 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 LMH2120’s output is small; therefore,
additional filtering is usually not needed. This is because its
internal averaging mechanism reduces the ripple significant-
ly. For some modulation types having very high peak-to-
average ratios or low-frequency components in the amplitude
modulation, 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 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)
30055772
FIGURE 6. Low-Pass Output Filter for Residual Ripple
Reduction
The output impedance of the LMH2120 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, discharging of the capacitor should be minimized in
shutdown mode. The connected ADC input should thus have
a high input impedance to prevent a possible 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 LMH2120 with a high-impedance shutdown
www.national.com 20
LMH2120
mode saves power in pulse mode systems. This is because
the capacitor CS doesn’t need to be fully recharged each cy-
cle.
Supply
The LMH2120 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
LMH2120 ensures that the performance is constant over its
power supply range.
SPECIFYING DETECTOR PERFORMANCE
The performance of the LMH2120 can be expressed by a va-
riety of parameters. This section discusses the key parame-
ters.
Dynamic Range
The LMH2120 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:
•Linear conformance error, ELC
•Variation over temperature error, EVOT
•1 dB step error, E1 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.
Linear Conformance error
The LMH2120 implements a linear detection function. In order
to describe how close the transfer is to an ideal linear function
the linear conformance error is used. To calculate the linear
conformance error the detector transfer function is modeled
as a linear-in-V relationship between the input power and the
output voltage.
The ideal linear-in-V transfer is modeled by 2 parameters:
•Slope, KSLOPE
•Intercept, PINT
and is described by:
VOUT = KSLOPE (PIN – PINT) (12)
where VOUT is the output voltage in dBV, KSLOPE is the slope
of the function in dB/dB, PIN the input power level in dBm and
PINT is the power level in dBm at which the function intersects
VOUT = 0 dBV = 1V (See Figure 7).
30055773
FIGURE 7. Ideal Linear Response
To determine the linear 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 linear conformance is spec-
ified as an input referred error. The output referred error is
therefore divided by the KSLOPE to obtain the input referred
error. The linear conformance error is calculated by the fol-
lowing equation:
(13)
where VOUT (T) is the measured output voltage at a power level
at PIN at a specific temperature. KSLOPE 25°C (dB/dB) and
PINT 25°C (dBm) are the parameters of the best fitted line of the
25°C transfer.
Figure 8 shows that both the error with respect to the ideal
LIN response as well as the error due to temperature 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 LMH2120 in Figure
8 shows a dynamic range of 35 dB for ELC= ±1dB.
21 www.national.com
LMH2120
30055734
FIGURE 8. ELC vs. RF input Power at 1900 MHz
Variation over Temperature Error
In contrast to the linear 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 (14)
The variation over temperature is shown in Figure 9, where a
dynamic range of 40 dB is obtained for EVOT = ±0.5 dB.
30055736
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. This condition is often used to define a
useful dynamic range of the detector.
The 1 dB step error is calculated in 2 steps:
1. Determine the maximum sensitivity.
2. Calculate the 1 dB step error.
First the maximum sensitivity (SMAX) is calculated per tem-
perature. It is defined as the maximum difference between
two output voltages for a 1 dB step within the power range:
SMAX = VOUT P+1 - VOUT P (15)
The 1dB error is than calculated by:
E1 dB = (SACTUAL - SMAX) / SMAX (16)
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 36 dB over temperature is obtained
for E1dB = ±0.3 dB.
30055738
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 for a 10 dB power step. 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 out-
put voltages are used to determine the corresponding power
levels at temperature T (PT and PT+X). The difference be-
tween those two power levels minus 10 results in the 10 dB
step error.
www.national.com 22
LMH2120
30055787
FIGURE 11. Graphical Representation of 10 dB Step Error
Calculations
Figure 12 shows the typical 10 dB step error at 1900 MHz,
where a dynamic range of 35 dB is obtained for E10dB = ±1dB.
30055739
FIGURE 12. 10 dB Step Error vs.
RF Input Power at 1900 MHz
Variation due to Modulation
RMS power detectors, such as the LMH2120 are inherently
insensitive to different modulation schemes. This in contrast
to traditional detectors like peak detectors and LOG AMP de-
tectors, where modulation forms with high peak-to-average
ratios (PAR) can cause significant output variation. This is
because the measurement of these detectors is not an actual
RMS measurement and is therefore waveform dependent.
To be able to compare the various detector types on modu-
lation sensitivity, the variation due to modulation parameter is
used. To calculate the variation due to modulation (EMOD), the
measurement result for an unmodulated RF carrier is sub-
tracted from the measurement result for a modulated RF
carrier. The calculations are similar to those for variation over
temperature. The variation due to modulation can be calcu-
lated by:
EMOD = (VOUT_MOD - VOUT_CW) / KSLOPE (17)
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 0.16 dB is obtained for a dynamic range from
-34 dBm to -2 dBm.
30055740
FIGURE 13. Variation due to Modulation for W-CDMA at
1900 MHz
TEMPERATURE BEHAVIOR
The specified temperature range of the LMH2120 is from
-40°C to 85°C. The RF detector is, to a certain extent how-
ever, still functional outside these temperature limits. Fig-
ures 14, 15, 16 show the detector behavior for temperatures
from -50°C up to 125°C in steps of 25°C. The LMH2120 is still
very accurate within a dynamic range from -35 dBm to
5 dBm. On the upper and lower end the curves deviate in a
gradual way, the lowest temperature on the bottom side and
the highest temperature on top side.
30055793
FIGURE 14. VOUT vs. RF Input Power at 1900 MHz
23 www.national.com
LMH2120
30055790
FIGURE 15. Linear Conformance Error vs. RF Input
Power at 1900 MHz
30055791
FIGURE 16. Temperature Variation vs. RF Input Power at
1900 MHz
LAYOUT RECOMMENDATIONS
As with any other RF device, careful attention must be paid
to the board layout. If the board layout isn’t properly designed,
performance might be less than can be expected for the ap-
plication.
The LMH2120 is designed to be used in RF applications,
having a characteristic impedance of 50Ω. To achieve this
impedance, the input of the LMH2120 needs to be connected
via a 50Ω transmission line. Transmission lines can be cre-
ated on PCBs using microstrip or (grounded) coplanar
waveguide (GCPW) configurations.
In order to minimize injection of RF interference into the
LMH2120 through the supply lines, the PCB traces for VDD
and GND should be minimized for RF signals. This can be
done by placing a decoupling capacitor between the VDD and
GND. It should be placed as close as possible, to the VDD and
GND pins of the LMH2120.
www.national.com 24
LMH2120
Physical Dimensions inches (millimeters) unless otherwise noted
6-Bump microSMD
NS Package Number UMD06AAA
X1 = 0.825 mm ± 0.030 mm
X2 = 1.225 mm ± 0.030 mm
X3 = 0.425 mm ± 0.045 mm
25 www.national.com
LMH2120
Notes
LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range
For more National Semiconductor product information and proven design tools, visit the following Web sites at:
www.national.com
Products Design Support
Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench
Audio www.national.com/audio App Notes www.national.com/appnotes
Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns
Data Converters www.national.com/adc Samples www.national.com/samples
Interface www.national.com/interface Eval Boards www.national.com/evalboards
LVDS www.national.com/lvds Packaging www.national.com/packaging
Power Management www.national.com/power Green Compliance www.national.com/quality/green
Switching Regulators www.national.com/switchers Distributors www.national.com/contacts
LDOs www.national.com/ldo Quality and Reliability www.national.com/quality
LED Lighting www.national.com/led Feedback/Support www.national.com/feedback
Voltage References www.national.com/vref Design Made Easy www.national.com/easy
PowerWise® Solutions www.national.com/powerwise Applications & Markets www.national.com/solutions
Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero
Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic
PLL/VCO www.national.com/wireless PowerWise® Design
University
www.national.com/training
THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION
(“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY
OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO
SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS,
IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS
DOCUMENT.
TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT
NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL
PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR
APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND
APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE
NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS.
EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO
LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE
AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR
PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY
RIGHT.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR
SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and
whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected
to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform
can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness.
National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other
brand or product names may be trademarks or registered trademarks of their respective holders.
Copyright© 2010 National Semiconductor Corporation
For the most current product information visit us at www.national.com
National Semiconductor
Americas Technical
Support Center
Email: support@nsc.com
Tel: 1-800-272-9959
National Semiconductor Europe
Technical Support Center
Email: europe.support@nsc.com
National Semiconductor Asia
Pacific Technical Support Center
Email: ap.support@nsc.com
National Semiconductor Japan
Technical Support Center
Email: jpn.feedback@nsc.com
www.national.com
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic."Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products Applications
Audio www.ti.com/audio Communications and Telecom www.ti.com/communications
Amplifiers amplifier.ti.com Computers and Peripherals www.ti.com/computers
Data Converters dataconverter.ti.com Consumer Electronics www.ti.com/consumer-apps
DLP®Products www.dlp.com Energy and Lighting www.ti.com/energy
DSP dsp.ti.com Industrial www.ti.com/industrial
Clocks and Timers www.ti.com/clocks Medical www.ti.com/medical
Interface interface.ti.com Security www.ti.com/security
Logic logic.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense
Power Mgmt power.ti.com Transportation and Automotive www.ti.com/automotive
Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video
RFID www.ti-rfid.com
OMAP Mobile Processors www.ti.com/omap
Wireless Connectivity www.ti.com/wirelessconnectivity
TI E2E Community Home Page e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright ©2011, Texas Instruments Incorporated
