LMH2110 LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range Literature Number: SNWS022B LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range General Description Features The LMH2110 is a 45 dB Logarithmic RMS power detector particularly 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 encountered in W-CDMA and LTE cell phones. The RMS measurement topology inherently ensures a modulation insensitive 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 intercept only, reducing calibration effort significantly. The device 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 directional 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 prevent 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 microSMD package. Typical Application 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 Output Voltage and Log Conformance Error vs. RF Input Power at 1900 MHz 30064804 30064838 (c) 2010 National Semiconductor Corporation 300648 www.national.com LMH2110 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range October 13, 2010 LMH2110 Storage Temperature Range Junction Temperature (Note 3) Maximum Lead Temperature (Soldering,10 sec) 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 RF Input Input power DC Voltage Enable Input Voltage -65C to 150C 150C 260C 5.5V Operating Ratings 12 dBm 1V GND-0.4V <VEN and VEN< Min (VDD-0.4, 3.6V) ESD Tolerance (Note 2) Human Body Model Machine Model Charge Device Model (Note 1) Supply Voltage Temperature Range RF Frequency Range RF Input Power Range 2.7V to 5V -40C to +85C 50 MHz to 8 GHz -40 dBm to 5 dBm Package Thermal Resistance JA (Note 3) 2000V 200V 1000V 166.7C/W 2.7V and 4.5V DC and AC Electrical Characteristics Unless otherwise specified: all limits are guaranteed to; TA = 25C, 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 3.7 2.9 4.8 5.5 5.9 mA 3.7 4.7 5 4.6 5.7 6.1 3.5 4.7 5 4.6 5.7 6.1 Supply Interface IBAT Supply Current Active mode: EN = High, no signal present at RFIN. Shutdown: EN = Low, no VBAT= 2.7V signal present at RFIN. VBAT= 4.5V EN = Low, RFIN = 0 dBm, VBAT= 2.7V 1900 MHz VBAT= 4.5V PSRR Power Supply Rejection Ratio (Note 8) RFIN = -10 dBm, 1900 MHz, 2.7V<VBAT<5V 45 56 A A dB Logic Enable Interface VLOW EN Logic Low Input Level (Shutdown mode) VHIGH EN Logic High Input Level IEN Current into EN Pin 0.6 V 50 nA V 1.1 Input / Output Interface 44 50 56 0 1.5 8 mV 0.2 2 3 RIN Input Resistance VOUT Minimum Output Voltage (Pedestal) No input Signal ROUT Output Impedance EN = High, RFIN = -10 dBm, 1900 MHz, ILOAD = 1 mA, DC measurement IOUT Output Short Circuit Current Sinking, RFIN = -10 dBm, OUT connected to 2.5V 37 32 42 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 en Output Referred Noise (Note 8) RFIN = -10 dBm, 1900 MHz, output spectrum at 10 kHz www.national.com 2 mA 50 3 nA V/Hz vN Parameter Condition Min (Note 5) Integrated Output Referred Noise Integrated over frequency band (Note 8) 1 kHz - 6.5 kHz, RFIN = -10 dBm, 1900 MHz Typ (Note 6) Max (Note 5) Units VRMS 210 Timing Characteristics tON Turn-on Time from shutdown RFIN = -10 dBm, 1900 MHz, EN LowHigh transition to OUT at 90% 15 tR Rise Time (Note 8) Signal at RFIN from -20 dBm to 0 dBm, 10% to 90%, 1900 MHz 2.2 s tF Fall Time (Note 8) Signal at RFIN from 0 dBm to -20 dBm, 90% to 10%, 1900 MHz 31 s 19 s 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 PMAX Maximum Power level, top end of dynamic range 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 -39 dBm 7 1 dB Log Conformance Error (ELC) 46 45 3 dB Log Conformance Error (ELC) 51 50 0.5 dB Input referred Variation over Temperature (EVOT), from PMIN dB 42 RFIN = 900 MHz, fit range -20 dBm to -10 dBm (Note 7) PMIN Minimum Power level, bottom end of dynamic range PMAX Maximum Power level, top end of dynamic range VMIN Minimum Output Voltage At PMIN VMAX Maximum Output Voltage At PMAX KSLOPE Logarithmic Slope PINT Logarithmic Intercept DR Dynamic Range for specified accuracy EMOD Input referred Variation due to Modulation Log Conformance Error within 1 dB -38 dBm 0 3 mV 1.58 43.9 46.0 mV/dB -37.4 -37.0 -36.7 dBm 1 dB Log Conformance Error (ELC) 38 37 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 W-CDMA Release 6/7/8, -38 dBm<RFIN<-5 dBm 0.08 LTE, -38 dBm<RFIN<-5 dBm 0.19 3 V 41.8 dB dB www.national.com LMH2110 Symbol 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 PMAX Maximum Power level, top end of dynamic range VMIN Minimum Output Voltage At PMIN VMAX Maximum Output Voltage At PMAX KSLOPE Logarithmic Slope PINT Logarithmic Intercept DR Dynamic Range for specified accuracy EMOD Input referred Variation due to Modulation Log Conformance Error within 1 dB -36 dBm 0 3 mV 1.5 V 41.8 43.9 46.1 mV/dB -35.5 -35.1 -34.7 dBm 1 dB Log Conformance Error (ELC) 36 36 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 W-CDMA Release 6/7/8, -38 dBm<RFIN<-5 dBm 0.09 LTE, -38 dBm<RFIN<-5 dBm 0.18 dB dB 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 PMAX Maximum Power level, top end of dynamic range 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 -31 dBm 6 1 dB Log Conformance Error (ELC) 37 36 3 dB Log Conformance Error (ELC) 44 42 0.5 dB Input referred Variation over Temperature (EVOT), from PMIN dB 39 RFIN = 5800 MHz, fit range -20 dBm to 3 dBm (Note 7) PMIN Minimum Power level, bottom end of dynamic range PMAX Maximum Power level, top end of dynamic range VMIN Minimum Output Voltage At PMIN VMAX Maximum Output Voltage At PMAX KSLOPE Logarithmic Slope 42.5 44.8 47.1 mV/dB PINT Logarithmic Intercept -22.0 -21.0 -19.9 dBm www.national.com Log Conformance Error within 1 dB -22 dBm 10 3 mV 1.34 4 V DR Parameter Dynamic Range for specified accuracy Condition Min (Note 5) Typ (Note 6) 1 dB Log Conformance Error (ELC) 32 31 3 dB Log Conformance Error (ELC) 39 37 0.5 dB Input referred Variation over Temperature (EVOT), from PMIN Max (Note 5) Units dB 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). FieldInduced 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) , JA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) - TA)/JA. 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. 5 www.national.com LMH2110 Symbol 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 A1 VDD Positive Supply Voltage. C1 GND Power Ground. Power Ground. May be left floating in case grounding is not feasible. Power Supply Description B2 GND Logic Input C2 EN Analog Input B1 RFIN RF input signal to the detector, internally terminated with 50. Output A2 OUT Ground referenced detector output voltage. 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. Ordering Information Package 6-Bump microSMD www.national.com Part Number LMH2110TM LMH2110TMX Package Marking P Transport Media 250 Units Tape and Reel 3k Units Tape and Reel 6 NSC Drawing Status TMD06BBA Released LMH2110 Block Diagram 30064806 LMH2110 7 www.national.com LMH2110 Typical Performance Characteristics Unless otherwise specified: TA = 25C, VBAT = 2.7V, RFin = 1900 MHz CW (Continuous Wave, unmodulated). Specified errors are input referred. Supply Current vs. Supply Voltage (Active) Supply Current vs. Supply Voltage (Shutdown) 30064812 30064811 Supply Current vs. Enable Voltage (EN) Supply Current vs. RF Input Power 30064814 30064813 Sourcing Output Current vs. RF Input Power Sinking Output Current vs. RF Input Power 30064861 www.national.com 30064862 8 Power Supply Rejection Ratio vs. Frequency 30064863 30064865 Output Voltage Noise vs. Frequency Output Voltage vs. RF Input Power 30064864 30064817 Output Voltage vs. Frequency Log Slope vs. Frequency 30064818 30064815 9 www.national.com LMH2110 RF Input Impedance vs. Frequency, Resistance (R) and Reactance (X) LMH2110 Log Intercept vs. Frequency Output Voltage and Log Conformance Error vs. RF Input Power at 50 MHz 30064816 30064819 Log Conformance Error (50 units) vs. RF Input Power at 50 MHz Temperature Variation vs. RF Input Power at 50 MHz 30064820 30064821 Temperature Variation (50 units) vs. RF Input Power at 50 MHz Output Voltage and Log Conformance Error vs. RF Input Power at 900 MHz 30064822 30064827 www.national.com 10 LMH2110 Log Conformance Error (50 units) vs. RF Input Power at 900 MHz Temperature Variation vs. RF Input Power at 900 MHz 30064828 30064829 Temperature Variation (50 units) vs. RF Input Power at 900 MHz 1 dB Power Step Error vs. RF Input Power at 900 MHz 30064830 30064831 10 dB Power Step Error vs. RF Input Power at 900 MHz W-CDMA Variation vs. RF Input Power at 900 MHz 30064832 30064833 11 www.national.com LMH2110 LTE Variation vs. RF Input Power at 900 MHz Output Voltage and Log Conformance Error vs. RF Input Power at 1900 MHz 30064834 30064838 Log Conformance Error (50 units) vs. RF Input Power at 1900 MHz Temperature Variation vs. RF Input Power at 1900 MHz 30064839 30064840 Temperature Variation (50 units) vs. RF Input Power at 1900 MHz 1 dB Power Step Error vs. RF Input Power at 1900 MHz 30064841 www.national.com 30064842 12 LMH2110 10 dB Power Step Error vs. RF Input Power at 1900 MHz W-CDMA Variation vs. RF Input Power at 1900 MHz 30064843 30064844 LTE Input referred Variation vs. RF Input Power at 1900 MHz Output Voltage and Log Conformance Error vs. RF Input Power at 3500 MHz 30064845 30064849 Log Conformance Error (50 units) vs. RF Input Power at 3500 MHz Temperature Variation vs. RF Input Power at 3500 MHz 30064850 30064851 13 www.national.com LMH2110 Temperature Variation (50 units) vs. RF Input Power at 3500 MHz Output Voltage and Log Conformance Error vs. RF Input Power at 5800 MHz 30064852 30064857 Log Conformance Error (50 units) vs. RF Input Power at 5800 MHz Temperature Variation vs. RF Input Power at 5800 MHz 30064858 30064859 Temperature Variation (50 units) vs. RF Input Power at 5800 MHz Output Voltage and Log Conformance Error vs. RF Input Power at 8000 MHz 30064860 30064866 www.national.com 14 LMH2110 Temperature Variation vs. RF Input Power at 8000 MHz 30064867 15 www.national.com LMH2110 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 transmit 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 measured 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 control 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 further reduce the ripple. Application Information The LMH2110 is a 45 dB Logarithmic RMS power detector particularly 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 applications. The LMH2110 has an RF frequency range from 50 MHz to 8 GHz. It provides an output voltage that relates 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 30064870 FIGURE 1. Transmit Power Control System 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: ACCURATE POWER MEASUREMENT Detectors have evolved over the years along with the communication standards. Newer communication standards like LTE and W-CDMA raise the need for more advanced accurate 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 measurement 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 average power content. The average power can be described by the following formula: (2) Implementing the exact formula for RMS can be difficult though. 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 calculate 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 (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 www.national.com 16 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 unmanageable. 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 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 piecewise linear approximation. Consequently, the LOG amp detector does not implement an exact power measurement, which implies a dependency on the signal shape. In systems using various modulation schemes calibration and lookup tables might be required. Peak Detectors A peak detector is one of the simplest types of detectors. According 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). RMS Detectors An RMS detector has a response that is insensitive to the signal shape and modulation form. This is because its operation is based on exact determination of the average power, i.e. it implements: (3) 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. 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 chosen, 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). Controlling the supply voltage of the PA can reduce power consumption significantly. The optimal mode of operation is to set 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 4). Subsequently, the details of the electrical interfacing are separately discussed for each pin. 17 www.national.com LMH2110 * Saw-tooth wave: VRMS = VPEAK/ 3 For more complex waveforms it is not always easy to determine 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 calibration a look-up table corrects for the error. Multiple look-up tables can be created for different modulation schemes. 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 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 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) (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-performance 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. in which i LF 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: RF Input RF systems typically use a characteristic impedance of 50. The LMH2110 is no exception to this. The RF input pin of the LMH2110 has an input impedance of 50. 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 amplifier. This can be done by selecting a directional coupler with the correct coupling factor. Since the LMH2110 has a constant input impedance, a resistive divider can also be used in stead of a directional coupler (Figure 5). (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: www.national.com 18 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 because 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: 30064881 FIGURE 5. Application with Resistive Divider Resistor R1 implements an attenuator together with the detector input impedance to match the output range of the PA to the input range of the LMH2110. The attenuation (AdB) realized by R1 and the effective input impedance of the LMH2110 equals: (9) f-3 dB = 1 / (2RSCS) Solving this expression for R1 yields: (11) (10) Suppose the desired attenuation is 30 dB with a given LMH2110 input impedance of 50, the resistor R1 needs to be 1531. A practical value is 1.5 k. Although this is a cheaper solution than the application with directional coupler, it also comes with a disadvantage. After calculating the resistor 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 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 resistor, 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. 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 ensures a fast settling time when the device returns from shutdown 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 usually 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 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 LMH2110 with a high impedance shutdown mode save power in pulse mode systems. This is because the capacitor CS doesn't need to be fully re-charged each cycle. 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 efficiency. 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 devices start to conduct when the enable voltage range is exceeded and excessive current will be drawn. A correct 19 www.national.com LMH2110 operation is not guaranteed then. The absolute maximum ratings are also exceeded when the enable (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 k in series with the enable input to limit the current. LMH2110 To determine the log conformance error two steps are required: 1. Determine the best fitted line at 25C. 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 specified 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: 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 variety of parameters. This section discusses the key parameters. 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 dynamic 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 specified error metric is within a predefined window. An explanation of these errors is given in the following paragraphs. (13) Where VOUT is the measured output voltage at a power level at PIN at a temperature. KSLOPE 25C (mV/dB) and PINT 25C (dBm) are the parameters of the best fitted line of the 25C 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 temperature variation are included in this error metric. This is because the measured data for all temperatures is compared to the fitted line at 25C. The measurement result of a typical LMH2110 in Figure 8 shows a dynamic range of 36 dB for ELC= 1dB. Log Conformance error The LMH2110 implements a logarithmic function. In order to describe how close the transfer is to an ideal logarithmic function 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). 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 25C is subtracted from the output voltage at another temperature. Subsequently, it is translated into an input referred error by dividing it by KSLOPE at 25C. The equation for variation over temperature is given by: EVOT = (VOUT_TEMP - VOUT 25C) / KSLOPE 25C 30064873 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. FIGURE 7. Ideal Logarithmic Response www.national.com (14) 20 LMH2110 30064842 30064840 FIGURE 9. EVOT vs. RF Input Power at 1900 MHz FIGURE 10. 1 dB Step Error 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 temperature by determining the maximum difference between two output voltages for a 1 dB step within the power range: 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 temperature when a 10 dB power step is made. The 10 dB step at 25C is taken as a reference. To determine the 10 dB step error first the output voltage levels (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 levels at temperature T (PT and PT+X). The difference between those two power levels subtracted by 10 results in the 10 dB step error. 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 sensitivity 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 average 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. 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 30064844 FIGURE 12. 10 dB Step Error vs. RF Input Power at 1900 MHz 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 application. The LMH2110 is designed to be used in RF applications, having a characteristic impedance of 50. To achieve this impedance, the input of the LMH2110 needs to be connected via a 50 transmission line. Transmission lines can be created on PCBs using microstrip or (grounded) coplanar waveguide (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. 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 because the measurement is not an actual RMS measurement and is therefore waveform dependent. To calculate the variation due to modulation (EMOD), the measurement 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 temperature. 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 applied 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. 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 8 GHz Logarithmic RMS Power Detector with 45 dB dynamic range Notes 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(R) 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 www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise(R) Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagicTM www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise(R) Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION ("NATIONAL") PRODUCTS. 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