LMH2110

LMH2110 Evaluation Board LMH2110 Evaluation Board National Semiconductor Application Note 2014 Gerrit de Wagt and Jaimy de Koning May 4, 2010 General Description The evaluation board is designed to help the evaluation of the LMH2110, which 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 in- tercept 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. Figure 1 shows the LMH2110 Evaluation Board. 30109716 FIGURE 1. LMH2110 Evaluation Board Basic Operation The circuit operates with a single supply form 2.7V to 5V and has an RF power detection range from −40 dBm to 5 dBm. The board consist of a single LMH2110 along with external components soldered on a printed circuit board. External sup- ply voltages and input signals can be applied to the on-board connectors. The supply voltage is applied with connectors P21 (VDD) and P22 (GND). The RF input signal is applied by SMA connector P1. This RF signal is applied through an RF generator and is connected with a 50Ω SMA cable. The detector output can be measured via BNC connector P3. AN-2014 © 2010 National Semiconductor Corporation 301097 www.national.com AN-2014 Configuration The LMH2110 evaluation board can be configured via jumper settings. The device is active when EN = High. This can be accomplished by setting the jumper J4 to VDD or by using external control on P4 by setting the jumper J4 to EN. 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. To guarantee a correct operation a voltage divider formed by R2 and R3 is present on the evaluation board. 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 solution to protect the device is the resistor R1 of 1 kΩ in series with the enable input to limit the current. An overview of the various jumper positions on the board is given in Figure 2. The settings of these jumpers and their functions are listed in Table 1. 30109710 FIGURE 2. Jumper Positions TABLE 1. Jumper and Header Overview Bold face jumper settings refer to the factory default configuration. Jumper J4 Function Enable Jumper Position 1–2 3–4 5-6 Description Active, Connects Enable Pin to VDD External Control, Connects Enable Pin to Enable P4 Shutdown, Connects Enable Pin to GND www.national.com 2 AN-2014 Measurement Setup The performance of the LMH2110 can be measured with the setup shown in Figure 3. 30109712 FIGURE 3. Measurement Setup An external power supply provides a voltage of 2.7V to 5V to the evaluation board. An accurate and stable RF Signal Generator is used to produce the test signal. It is recommended to use low loss cables to ensure reliable measurement data. The detected output voltage can be measured with a Digital Voltage Meter (DVM). Figure 4 depicts the output voltage versus frequency for various power levels on RFIN. The frequency range is from 10 MHz to 10 GHz. Figure 5 depicts the output voltage versus RF input power for various frequencies. 30109709 FIGURE 5. Output Voltage vs. RF Input Power 30109708 FIGURE 4. Output Voltage vs. Freqency 3 www.national.com AN-2014 Schematic 30109705 FIGURE 6. Evaluation Board Schematic Bill of Material The Bill of Material (BOM) of the evaluation board is given in Table 2. TP1 U1 Test Point µSMD GND LMH2110TM TABLE 2. Bill of Material Designator C1, C2 C3, C4, C6 C5 C7 J4 P1 P21, P22 P3 P4 R1 R2, R3 R4 Description 0603 Capacitor 0603 Capacitor 0603 Capacitor 0603 Capacitor Header Connector Connector Connector Connector 0603 Resisitor 0603 Resistor 0603 Resisitor Comment 10 nF 10 pF 1 pF 10 µF 2x3 SMA Banana BNC BNC 1 kΩ 100 kΩ 0Ω Board Layout 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 than can be expected for the application. The LMH2110 is designed to be used in RF applications, having a characteristic impedance of 50Ω. To achieve this impedance, the input of the LMH2110 needs to be connected via a 50Ω transmission line. Transmission lines can be 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. Figure 7 shows the component locations of the LMH2110 evaluation board and Figure 8 shows the board layout of the LMH2110 evaluation board. www.national.com 4 AN-2014 LMH2110 Evaluation Board 30109711 FIGURE 7. Component Locations of Evaluation Board 30109715 FIGURE 8. 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