LMH2120 LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range Literature Number: SNWS021B LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range General Description Features The LMH2120 is a 40 dB Linear 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 6 GHz. It provides an accurate, temperature and supply insensitive, output voltage that relates linearly to the RF input power in volt. The LMH2120's excellent conformance to a linear 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 directional 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 prevent 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. Typical Application 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 Output Voltage vs. RF Input Power at 1900 MHz 30055733 30055701 (c) 2010 National Semiconductor Corporation 300557 www.national.com LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range October 13, 2010 LMH2120 Storage Temperature Range -65C to 150C Junction Temperature (Note 3) 150C For soldering specifications: See product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf 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 RF Input Input power DC Voltage Enable (EN) Input Voltage 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 2000V 200V 1000V 2.7V to 5V -40C to +85C 50 MHz to 6 GHz -35 dBm to 5 dBm Package Thermal Resistance JA (Note 3) 166.7C/W 2.7 V and 4.5V DC and AC Electrical Characteristics Unless otherwise specified, all limits are guaranteed to TA = 25C, 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) 2.9 3.5 4.0 3.8 4.7 5.0 4.7 5.7 6.1 VBAT = 2.7V 3.8 4.7 5.0 VBAT = 4.5V 4.7 5.7 6.1 Units Supply Interface IDD Supply Current Active mode: EN = High, no signal present at RFIN. Shutdown: EN = LOW, VBAT = 2.7V no signal present at RFIN VBAT = 4.5V EN = LOW, RFIN = 0 dBm, 1900 MHz PSRR Power Supply Rejection Ratio RFIN = -10 dBm, 1900 MHz, 2.7V < VBAT < 5V 50 60 mA 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 1.1 50 nA 50 56 18 29 33 mV 1 2 3 Input / Output Interface RIN Input Resistance VOUT Minimum Output Voltage (Pedestal) No Input Signal ROUT Output Resistance EN = HIGH, RFIN = -10 dBm, 1900 MHz, ILOAD = 1 mA, DC measurement IOUT Output Sinking Current RFIN = -10 dBm, 1900 MHz, OUT connected to 2.5V 30 25 42 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 en Output Referred Noise (Note 8) RFIN = -10 dBm, 1900 MHz, output spectrum at 10 kHz www.national.com 44 2 mA 80 5 nA V/Hz vn Parameter Condition Min (Note 5) Output Referred Noise Integrated Integrated over frequency band 1 kHz (Note 8) 6.5 kHz, RFIN = -10 dBm, 1900 MHz Typ (Note 6) Max (Note 5) Units VRMS 390 Timing Characteristics tON Turn-on Time from shutdown RFIN = -10 dBm, 1900 MHz, EN LOWto-HIGH transition to OUT at 90% 13 tR Rise Time Signal at RFIN from -20 dBm to 0 dBm, 10% to 90%, 1900 MHz 7 s tF Fall Time Signal at RFIN from 0 dBm to -20 dBm, 90% to 10%, 1900 MHz 18 s 18 s 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 PMAX Maximum Power Level, top end of Dynamic Range VMIN Minimum Output Voltage At PMIN 31 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 DR Dynamic Range for specified Accuracy 1 dB Lin Conformance Error (ELC) 37 36 41 40 3 dB Lin Conformance Error (ELC) 44 43 48 47 41 45 -37 dBm 4 0.5 dB Variation over Temperature (EVOT) mV -5.3 dBm dB RFIN = 900 MHz (Note 7) PMIN Minimum Power Level, bottom end of Dynamic Range Lin Conformance Error within 1 dB PMAX Maximum Power Level, top end of Dynamic Range VMIN Minimum Output Voltage At PMIN 33 mV VMAX Maximum Output Voltage At PMAX 2.5 V KSLOPE Linear Slope PINT Linear Intercept VOUT = 0 dBV DR Dynamic Range for specified Accuracy dBm 5 1 -4.2 -4.0 1 dB Lin Conformance Error (ELC) 36 33 40 37 3 dB Lin Conformance Error (ELC) 45 44 48 47 41 44 0.5 dB Variation over Temperature (EVOT) EMOD Input referred Variation due to Modulation -35 0.3 dB Error for a 1dB Power Step (E1dB) 41 40 1 dB Error for a 10dB Power Step (E10dB) 45 W-CDMA Release 6/7/8, -35 dBm<RFIN<-3 dBm 0.15 LTE, -35 dBm<RFIN<-3 dBm 0.29 3 dB/dB -3.8 dBm dB dB www.national.com LMH2120 Symbol 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 PMAX Maximum Power Level, top end of Dynamic Range VMIN Minimum Output Voltage At PMIN 30 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 DR Dynamic Range for specified Accuracy 1 dB Lin Conformance Error (ELC) 35 31 38 35 3 dB Lin Conformance Error (ELC) 44 41 48 45 35 40 dBm 4 0.5 dB Variation over Temperature (EVOT) EMOD Input referred Variation due to Modulation -34 0.3 dB Error for a 1dB Power Step (E1dB) 39 36 1 dB Error for a 10dB Power Step (E10dB) 35 W-CDMA Release 6/7/8, -34 dBm<RFIN<-2 dBm 0.16 LTE, -34 dBm<RFIN<-2 dBm 0.24 mV -1.4 dBm dB dB RFIN = 2600 MHz (Note 7) PMIN Minimum Power Level, bottom end of Dynamic Range Lin Conformance Error within 1 dB PMAX Maximum Power Level, top end of Dynamic Range VMIN Minimum Output Voltage At PMIN 31 mV VMAX Maximum Output Voltage At PMAX 1.5 V KSLOPE Linear Slope PINT Linear Intercept VOUT = 0 dBV 0.8 1.7 DR Dynamic Range for specified Accuracy 1 dB Lin Conformance Error (ELC) 32 29 36 33 3 dB Lin Conformance Error (ELC) 43 40 45 42 34 39 -30 dBm 6 1 0.5 dB Variation over Temperature (EVOT) dB/dB 2.6 dBm dB RFIN = 3500 MHz (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 32 VMAX Maximum Output Voltage At PMAX 1.1 V KSLOPE Linear Slope 1 dB/dB PINT Linear Intercept www.national.com Lin Conformance Error within 1 dB -26 dBm 7 VOUT = 0 dBV 4.4 4 5.5 mV 6.7 dBm DR Parameter Dynamic Range for specified Accuracy Condition Min (Note 5) Typ (Note 6) 1 dB Lin Conformance Error (ELC) 30 27 33 30 3 dB Lin Conformance Error (ELC) 39 36 42 40 27 35 0.5 dB Variation over Temperature (EVOT) Max (Note 5) Units dB 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: 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 Symbol 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 Power Supply microSMD Name A1 VDD Positive Supply Voltage. GND Ground. Both C1 and B2 need to be connected to GND. C1 B2 EN Description Logic Input C2 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 . Output A2 OUT Ground referenced detector output voltage. Ordering Information Package 6-Bump micro SMD www.national.com Part Number LMH2120UM LMH2120UMX Package Marking R Transport Media 250 Units Tape and Reel 3k Units Tape and Reel 6 NSC Drawing Status UMD06AAA Released LMH2120 Block Diagram 30055703 LMH2120 7 www.national.com LMH2120 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) 30055711 30055710 Supply Current vs. Enable Voltage (EN) Supply Current vs. RF Input Power 30055712 30055713 Output Sourcing Current vs. RF Input Power Output Sinking Current vs. RF Input Power 30055761 www.national.com 30055762 8 Power Supply Rejection Ratio vs. Frequency 30055763 30055765 Output Voltage Noise vs. Frequency Lin Slope vs. Frequency 30055715 30055764 Lin Intercept vs. Frequency Output Voltage vs. RF Input Power 30055716 30055717 9 www.national.com LMH2120 RF Input Impedance vs. Frequency, Resistance (R) and Reactance (X) LMH2120 Output Voltage vs. Frequency Output Voltage vs. RF Input Power at 50 MHz 30055718 30055719 Lin Conformance vs. RF Input Power at 50 MHz Lin Conformance (50 units) vs. RF Input Power at 50 MHz 30055720 30055721 Temperature Variation vs. RF Input Power at 50 MHz Temperature Variation (50 units) vs. RF Input Power at 50 MHz 30055722 30055723 www.national.com 10 Lin Conformance vs. RF Input Power at 900 MHz 30055724 30055725 Lin Conformance (50 units) vs. RF Input Power at 900 MHz Temperature Variation vs. RF Input Power at 900 MHz 30055727 30055726 Temperature Variation (50 units) vs. RF Input Power at 900 MHz 1 dB Power Step Error vs. RF Input Power at 900 MHz 30055729 30055728 11 www.national.com LMH2120 Output Voltage vs. RF Input Power at 900 MHz LMH2120 10 dB Power Step Error vs. RF Input Power at 900 MHz W-CDMA variation vs. RF Input Power at 900 MHz 30055731 30055730 LTE variation vs. RF Input Power at 900 MHz Output Voltage vs. RF Input Power at 1900 MHz 30055732 30055733 Lin Conformance vs. RF Input Power at 1900 MHz Lin Conformance (50 units) vs. RF Input Power at 1900 MHz 30055734 30055735 www.national.com 12 LMH2120 Temperature Variation vs. RF Input Power at 1900 MHz Temperature Variation (50 units) vs. RF Input Power at 1900 MHz 30055736 30055737 1 dB Power Step Error vs. RF Input Power at 1900 MHz 10 dB Power Step Error vs. RF Input Power at 1900 MHz 30055738 30055739 W-CDMA variation vs. RF Input Power at 1900 MHz LTE variation vs. RF Input Power at 1900 MHz 30055740 30055741 13 www.national.com LMH2120 Output Voltage vs. RF Input Power at 2600 MHz Lin Conformance vs. RF Input Power at 2600 MHz 30055742 30055743 Lin Conformance (50 units) vs. RF Input Power at 2600 MHz Temperature Variation vs. RF Input Power at 2600 MHz 30055745 30055744 Temperature Variation (50 units) vs. RF Input Power at 2600 MHz Output Voltage vs. RF Input Power at 3500 MHz 30055747 30055746 www.national.com 14 LMH2120 Lin Conformance vs. RF Input Power at 3500 MHz Lin Conformance (50 units) vs. RF Input Power at 3500 MHz 30055748 30055749 Temperature Variation vs. RF Input Power at 3500 MHz Temperature Variation (50 units) vs. RF Input Power at 3500 MHz 30055750 30055751 Output Voltage vs. RF Input Power at 5800 MHz Lin Conformance vs. RF Input Power at 5800 MHz 30055752 30055753 15 www.national.com LMH2120 Temperature Variation vs. RF Input Power at 5800 MHz 30055754 www.national.com 16 The LMH2120 is a 40 dB Linear 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 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-power level insensitive to power amplifier (PA) inaccuracy. This is 30055770 FIGURE 1. Transmit-Power Control System the power is dissipated, and VRMS is the equivalent RMS voltage. 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: 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 however. 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 * Saw-tooth wave: VRMS = VPEAK / 3 (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 17 www.national.com LMH2120 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 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 LMH2120 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 transmitpower control system. The output power of the PA is measured 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 control 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 further reduce the ripple. Application Information LMH2120 For more complex waveforms it is not always easy to determine the exact relationship between RMS value and peak value. A peak measurement can therefore 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. 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 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 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 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 particularly 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 application due to different applied modulation schemes. 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 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. In contrast, a configuration with a relatively large time constant measures the maximum (peak) voltage of a signal. www.national.com LMH2120 RF POWER DETECTOR For optimal performance, the LMH2120 needs to be configured correctly in the application. The detector will be discussed by means of its block diagram (Figure 4). Details of the electrical interfacing are separately discussed for each pin below. 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 described 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) 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 performance 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. 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: (7) RF Input RF systems typically use a characteristic impedance of 50; the LMH2120 is no exception to this. The RF input pin of the LMH2120 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 LMH2120 needs to be aligned with the output power range of the power am- 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 net19 www.national.com LMH2120 higher than 3V to prevent excess current flowing into the enable 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 exceeded 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 k in series with the enable input to limit the current. plifier. This can be done by selecting a directional coupler with the appropriate coupling factor. Since the LMH2120 has a constant input impedance, a resistive divider can also be used instead of a directional coupler (Figure 5). 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 significantly. For some modulation types having very high peak-toaverage 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: 30055781 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 with the input range of the LMH2120. The attenuation (AdB) realized by R1 and the effective input impedance (RIN) of the LMH2120 equals: f-3 dB = 1 / (2RSCS) (9) (11) Solving this expression for R1 yields: (10) Suppose the desired attenuation is 30 dB with a given LMH2120 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 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 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. 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 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, 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 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 lowpass filter which is good for 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 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 cycle. 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 variety of parameters. This section discusses the key parameters. 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 dynamic 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 specified error metric is within a predefined window. An explanation of these errors is given in the following paragraphs. 30055773 FIGURE 7. Ideal Linear Response To determine the linear 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 linear 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 linear conformance error is calculated by the following equation: 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) (13) where VOUT (T) is the measured output voltage at a power level at PIN at a specific temperature. KSLOPE 25C (dB/dB) and PINT 25C (dBm) are the parameters of the best fitted line of the 25C 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. (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). 21 www.national.com LMH2120 First the maximum sensitivity (SMAX) is calculated per temperature. 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. 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 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 (14) 30055738 The variation over temperature is shown in Figure 9, where a dynamic range of 40 dB is obtained for EVOT = 0.5 dB. 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 temperature for a 10 dB power step. 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 25C 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 minus 10 results in the 10 dB step error. 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. www.national.com 22 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. 30055740 FIGURE 13. Variation due to Modulation for W-CDMA at 1900 MHz TEMPERATURE BEHAVIOR The specified temperature range of the LMH2120 is from -40C to 85C. The RF detector is, to a certain extent however, still functional outside these temperature limits. Figures 14, 15, 16 show the detector behavior for temperatures from -50C up to 125C in steps of 25C. 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. 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 detectors, 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 modulation 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 subtracted 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 calculated by: EMOD = (VOUT_MOD - VOUT_CW) / KSLOPE 30055793 FIGURE 14. VOUT vs. RF Input Power at 1900 MHz (17) where VOUT_MOD is the measured output voltage for an applied power level of a modulated signal, VOUT_CW is the output 23 www.national.com LMH2120 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. LMH2120 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 application. The LMH2120 is designed to be used in RF applications, having a characteristic impedance of 50. To achieve this impedance, the input of the LMH2120 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 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. 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 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 6 GHz Linear RMS Power Detector with 40 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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