EVAL-CN0178-SDPZ

Circuit Note CN-0178 Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0178. Devices Connected/Referenced ADL5902 50 MHz to 9 GHz, 65 dB TruPwr™ Detector AD7466 Micropower, 12-Bit, 200 kSPS SAR ADC Software-Calibrated, 50 MHz to 9 GHz, RF Power Measurement System EVALUATION AND DESIGN SUPPORT The measurement result is provided as serial data at the output of a 12-bit ADC (AD7466). A simple 4-point system calibration at ambient temperature is performed in the digital domain. Circuit Evaluation Boards CN-0178 Circuit Evaluation Board (EVAL-CN0178-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The interface between the RF detector and the ADC is straightforward, consisting of two signal scaling resistors and no active components. In addition, the ADL5902 internal 2.3 V reference voltage provides the supply and reference voltage for the micropower ADC. The AD7466 has no pipeline delay and is operated as a read-only SAR ADC. CIRCUIT FUNCTION AND BENEFITS This circuit uses the ADL5902 TruPwr™ detector to measure the rms signal strength of RF signals with varying crest factors (peak-to-average ratio) over a dynamic range of approximately 65 dB and operates at frequencies from 50 MHz up to 9 GHz. +5V +5V C3 0.1µF C7 0.1µF C4 100pF C5 100pF VPOS VPOS 3 10 TEMPERATURE SENSOR ADL5902 RFIN R3 60.4Ω C10 100pF The overall circuit achieves temperature stability of approximately ±0.5 dB. C9A 0.1µF 8 TEMP C10A 10µF VDD 1 INHI INLO 14 X2 C12 100pF NC IDET VIN R10 3 1.21kΩ 15 LINEAR-IN-dB VGA (NEGATIVE SLOPE) 7 X2 VSET G=5 6 R11 2kΩ VOUT BIAS AND POWERDOWN CONTROL 5 11 1 R9 1430Ω 12 9 VTGT R10 3.74kΩ CS DGND C9 0.1µF AD7466 2 4 COMM DATA CS GND COMM R11 2kΩ 09331-001 R12 301Ω VREF CLK SDATA 5 6 CLPF 26pF SCLK 4 CONTROL LOGIC VREF 2.3V NC 13 TADJ 12-BIT SUCCESSIVE APPROXIMATION ADC ITGT 2 NC 16 T/H Figure 1. Software-Calibrated RF Power Measurement System Rev.A Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2010–2011 Analog Devices, Inc. All rights reserved. CN-0178 Circuit Note Data is shown for the two devices operating over a −40°C to +85°C temperature range. The transfer function of the detector can be approximated by the equation VOUT = SLOPE_DETECTOR × (PIN − INTERCEPT) CIRCUIT DESCRIPTION CODE = SLOPE × (PIN − INTERCEPT) where SLOPE is the combined slope of the detector, the scaling resistors, and the ADC, and has the unit of counts/dB; PIN and INTERCEPT still have the unit of dBm. Figure 3 shows a typical detector power sweep in terms of input power and observed ADC output codes for a 700 MHz input signal. 4096 6 +85°C CODE –40°C CODE +25°C CODE +25°C ERROR 4-POINT CAL @ 0dBm, –20dBm, –45dBm, AND, –58dBm +85°C ERROR 4-POINT CAL –40°C ERROR 4-POINT CAL 3755 3413 3072 5 4 3 2731 2 2389 1 2048 0 1707 –1 1365 –2 1024 –3 683 –4 341 –5 A typical plot of detector output voltage vs. input power is shown in Figure 2 (without output scaling). 0 –70 –6 –60 –50 –40 –30 PIN (dBm) –20 –10 0 10 ERROR (dB) The ADC full-scale voltage is equal to 2.3 V. The maximum detector output voltage (when operating in its linear input range) is approximately 3.5 V (see ADL5902 data sheet figures 6, 7, 8, 12, 13, and 14) and must, therefore, be scaled down by a factor of 0.657 before driving the AD7466. This scaling is implemented using a simple resistor divider R10 and R11 (1.21 kΩ and 2.0 kΩ). These values provide an actual scaling factor of 0.623, which ensures that the ADL5902 RF detector does not overdrive the ADC by building in some room for resistor tolerance. At the output of the ADC, VOUT is replaced by the ADC’s output code, and the equation can be rewritten as 09331-003 The power supply voltage and reference voltage for the AD7466 12-bit ADC are provided by the ADL5902 internal 2.3 V reference. Because the AD7466 consumes so little current (16 µA when sampling at 10 kSPS), the ADL5902’s reference voltage output can supply the ADC, as well as the temperature compensating and rms accuracy-scaling network consisting of R9, R10, R11, and R12. where SLOPE_DETECTOR is in mV/dB; INTERCEPT is the x-axis intercept with a unit of dBm; PIN is the input power in dBm. ADC CODE The RF signal being measured is applied to the input of the ADL5902, a linear-in-dB rms-responding rms detector. The external 60.4 Ω resistor, R3, combined with the relatively high input impedance of the ADL5902 ensures a broadband 50 Ω match to the RF input. The ADL5902 is configured in its so-called “measurement mode,” with the VSET and VOUT pins connected together. In this mode the output voltage is proportional to the logarithm of the rms value of the input. In other words, the reading is presented directly in decibels and is scaled to 1.06 V per decade, or 53 mV/dB. Figure 3. ADC Output Code and Error vs. RF Input Power @ 700 MHz 4.0 3.5 Overall SLOPE and INTERCEPT will vary from system to system. This variation is caused by part to part variations in the transfer function of the RF detector, the scaling resistors, and the ADC. As a result, a system level calibration is required to determine the complete system SLOPE and INTERCEPT. In this application, a 4-point calibration is used to correct for some nonlinearity in the RF detector’s transfer function, particularly at the low end. This 4-point calibration scheme yields three SLOPE and three INTERCEPT calibration coefficients, which should be stored in nonvolatile RAM (NVM) after calibration. 3.0 VOUT (V) 2.5 2.0 1.5 1.0 0 –70 –60 –50 INTERCEPT –40 –30 PIN (dBm) –20 –10 0 10 09331-002 0.5 Figure 2. ADL5902 RMS Detector, Output Voltage vs. Input Power @ 900 MHz Rev. A| Page 2 of 5 Circuit Note CN-0178 Figure 4 and Figure 5 show the performance of the circuit at 1 GHz and 2.2 GHz, respectively. 4096 6 +85°C CODE –40°C CODE +25°C CODE +25°C ERROR 4-POINT CAL @ 0dBm, –20dBm, –45dBm, AND, –58dBm +85°C ERROR 4-POINT CAL –40°C ERROR 4-POINT CAL The SLOPE and INTERCEPT calibration coefficients are calculated using the equations ADC CODE 3072 INTERCEPT1= CODE_1/(SLOPE_ADC × PIN_1) This calculation is then repeated using CODE_2/CODE_3 and CODE_3/CODE_4 to calculate SLOPE2/INTERCEPT2 and SLOPE3/INTERCEPT3, respectively. The six calibration coefficients should then be stored in NVM along with CODE_1, CODE_2, CODE_3, and CODE_4. 2 2389 1 2048 0 1707 –1 1365 –2 1024 –3 683 –4 341 –5 0 –70 –6 –60 –20 –10 0 10 6 +85°C CODE –40°C CODE +25°C CODE +25°C ERROR 4-POINT CAL @ 0dBm, –20dBm, –45dBm, AND, –58dBm +85°C ERROR 4-POINT CAL –40°C ERROR 4-POINT CAL ADC CODE 3072 Figure 3 also includes plots of error vs. temperature. In this case the measured ADC codes at +85°C and −40°C are compared to the straight line equations at ambient. This is consistent with a real world system where system calibration is generally only practical at ambient temperature. –30 4096 PIN = (CODE/SLOPE) + INTERCEPT = (CODE/SLOPE) + INTERCEPT – PIN_TRUE –40 Figure 4. ADC Output Code and Error vs. RF Input Power @ 1 GHz 3413 Error (dB) = Calculated RF Power − True Input Power –50 PIN (dBm) 3755 Figure 3 also shows the transfer function variation of the circuit vs. the above straight line equations. This error function is caused by bending at the edges of the transfer function, small ripple in the linear operating range, and drift over temperature. The error is expressed in dB using the equation 3 2731 When the circuit is in operation in the field, these calibration coefficients are used to calculate an unknown input power level, PIN, using the equation In order to retrieve the appropriate SLOPE and INTERCEPT calibration coefficients during circuit operation, the observed CODE from the ADC must be compared to CODE_1, CODE_2, CODE_3, and CODE_4. For example if the CODE from the ADC is between CODE_1 and CODE_2, then the SLOPE1 and INTERCEPT1 should be used. This step can also be used to provide an underrange or overrange warning. For example, if the CODE from the ADC is greater than CODE_1 or less than CODE_4, it indicates that the measured power is outside of the calibration range 4 5 4 3 2731 2 2389 1 2048 0 1707 –1 1365 –2 1024 –3 683 –4 341 –5 0 –70 –6 –60 –50 –40 –30 –20 –10 0 10 PIN (dBm) ERROR (dB) SLOPE1 = ( CODE _1 – CODE_2)/(PIN_1 − PIN_2) 5 ERROR (dB) 3413 09331-004 3755 09331-005 The calibration is performed by applying four known signal levels to the ADL5902 and measuring the corresponding output codes from the ADC. The calibration points chosen should be within the linear operating range of the device. In this example, calibration points at 0 dBm, −20 dBm, −45 dBm, and −58 dBm were used. Figure 5. ADC Output Code and Error vs. RF Input Power @ 2.2 GHz The performance of this or any high speed circuit is highly dependent on proper PCB layout. This includes, but is not limited to, power supply bypassing, controlled impedance lines (where required), component placement, signal routing, and power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial, and article, A Practical Guide to High-Speed Printed-CircuitBoard Layout, for more detailed information regarding PCB layout.) A complete design support package for this circuit note can be found at www.analog.com/CN0178-DesignSupport. Rev. A| Page 3 of 5 CN-0178 Circuit Note COMMON VARIATIONS Functional Block Diagram For applications that require less RF detection range, the AD8363 rms detector can be used. The AD8363 has a detection range of 50 dB and operates at frequencies up to 6 GHz. For non-rms detection applications, the AD8317/AD8318/AD8319 or ADL5513 can be used. These devices offer varying detection ranges and have varying input frequency ranges up to 10 GHz (see CN-0150 for more details). See Figure 1 of this circuit note for the circuit block diagram, and the file “EVAL-CN0178-SDPZ-SCH-Rev0.pdf ” for the circuit schematics. This file is contained in the CN0178 Design Support Package. The AD7466 is a single channel, 12-bit ADC with SPI interface. If the end application requires a multichannel ADC, the dual 12-bit AD7887 can be used. In multichannel applications that require multiple ADC and DAC channels, the AD7294 can be used. In addition to providing four 12-bit DAC outputs, this subsystem chip includes four uncommitted ADC channels, two high-side current sense inputs, and three temperature sensors. Current and temperature measurements are digitally converted and available to read over the I2C-compatible interface. CIRCUIT EVALUATION AND TEST This circuit uses the EVAL-CN0178-SDPZ circuit board and the EVAL-SDP-CB1Z System Demonstration Platform (SDP) evaluation board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance. The EVAL-CN0178-SDPZ board contains the circuit to be evaluated, as described in this note, and the SDP evaluation board is used with the CN0178 evaluation software to capture the data from the EVAL-CN0178-SDPZ circuit board. Equipment Needed • PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit) • EVAL-CN0178-SDPZ Circuit Evaluation Board Setup Connect the 120-pin connector on the EVAL-CN0178-SDPZ circuit board to the connector marked “CON A” on the EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware should be used to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. Using an appropriate RF cable, connect the RF signal source to the EVAL-CN0178-SDPZ board via the SMA RF input connector. With power to the supply off, connect a +6 V power supply to the pins marked “+6 V” and “GND” on the board. If available, a +6 V "wall wart" can be connected to the barrel connector on the board and used in place of the +6 V power supply. Connect the USB cable supplied with the SDP board to the USB port on the PC. Note: Do not connect the USB cable to the mini USB connector on the SDP board at this time. Test Apply power to the +6 V supply (or “wall wart”) connected to EVAL-CN0178-SDPZ circuit board. Launch the Evaluation software, and connect the USB cable from the PC to the USB mini-connector on the SDP board. Once USB communications are established, the SDP board can now be used to send, receive, and capture serial data from the EVAL-CN0178-SDPZ board. The data in this circuit note were generated using a Rohde & Schwarz SMT-03 RF signal source, and an Agilent E3631A power supply. The signal source was set to the frequencies indicated in the graphs, and the input power was stepped and data recorded in 1 dB increments. Temperature testing was performed using a Test Equity Model 107 environmental chamber. The CN0178-SDPZ evaluation board was placed in the chamber via a slot in the test chamber door, with the SDP evaluation board extending outside. • EVAL-SDP-CB1Z SDP Evaluation Board • CN0178 Evaluation Software • Power supply: +6 V, or +6 V “wall wart” Information and details regarding how to use the evaluation software for data capture can be found in the CN0178 Evaluation Software ReadMe file. • Environmental chamber • RF signal source • Coaxial RF cable with SMA connectors Information regarding the SDP board can be found in the SDP User Guide. Getting Started Load the evaluation software by placing the CN0178 Evaluation Software disc in the CD drive of the PC. Using "My Computer," locate the drive that contains the evaluation software disc and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software. Rev. A| Page 4 of 5 Circuit Note CN-0178 LEARN MORE Data Sheets and Evaluation Boards CN0178 Design Support Package: http://www.analog.com/CN0178-DesignSupport CN-0178 Circuit Evaluation Board (EVAL-CN0178-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) SDP User Guide ADL5902 Data Sheet Ardizzoni, John. A Practical Guide to High-Speed PrintedCircuit-Board Layout, Analog Dialogue 39-09, September 2005. ADL5902 Evaluation Board AD7466 Data Sheet CN-0150 Circuit Note, Software-Calibrated, 1 MHz to 8 GHz, 70 dB RF Power Measurement System Using the AD8318 Logarithmic Detector , Analog Devices. AD7466 Evaluation Board MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”, Analog Devices. 3/11—Rev. 0 to Rev. A REVISION HISTORY MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs), Analog Devices. MT-077 Tutorial, Log Amp Basics, Analog Devices. Added Evaluation and Design Support Section ............................ 1 Added Circuit Evaluation and Test Section ................................... 4 10/10—Rev. 0: Initial Version MT-078 Tutorial, High Speed Log Amps, Analog Devices. MT-081 Tutorial, RMS-to-DC Converters, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Whitlow, Dana. Design and Operation of Automatic Gain Control Loops for Receivers in Modern Communications Systems. Chapter 8. Analog Devices Wireless Seminar. 2006. I2C refers to a communications protocol originally developed by Philips Semiconductors (now NXP Semiconductors). (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2010–2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09331-0-3/11(A) Rev. A| Page 5 of 5