EVAL-CN0303-SDPZ

Circuit Note CN-0303 Devices Connected/Referenced Circuits from the Lab® reference designs 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/CN0303. ADXL001 High Precision, ±70 g, ±250 g, ±500 g Single Axis Accelerometer AD8606 Precision, Low Noise, CMOS, Rail-to-Rail Input/Output, Dual Package Op-Amp AD7476 2.35 V to 5.25 V, Low Power, Single-Channel, 1 MSPS, 12-Bit ADC MEMS-Based Vibration Analyzer with Frequency Response Compensation The circuit provides a low power solution suitable for bearing analysis, engine monitoring and shock detection. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0303 Circuit Evaluation Board (EVAL-CN303-SDPZ) Includes ADXL001 Evaluation Board (EVAL-ADXL001-70Z) and connecting ribbon cable System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The Analog Devices, Inc. proprietary fifth-generation iMEMs® process enables the ADXL001 accelerometer to provide dynamic range that extends from ±70 g to ±500 g in combination with 22 kHz of bandwidth. The AD8606 is a precision, low noise, dual op amp used to create an analog bi-quad filter that flattens the output frequency response of the accelerometer. CIRCUIT FUNCTION AND BENEFITS The ADXL001 output voltage is converted into a digital word by the AD7476 low power, single-channel, 12-bit, successive approximation analog-to-digital converter (SAR ADC). The circuit in Figure 1 offers a high linearity, low noise, widebandwidth vibration sensing solution. It is ideal for applications requiring a large dynamic range (±70 g, ±250 g, or ±500 g) and a flat frequency response from dc to 22 kHz. EVAL-ADXL001-70Z 5V 5V EVAL-CN0303-SDPZ 5V 5V 10µF 100kΩ 0.1pF 0.1µF 5V 5V VDD 200kΩ VDD VDD2 100kΩ XOUT ADXL001 100pF 200kΩ AD7476 107kΩ 100pF A1 107kΩ COM EVAL-SDP-CB1Z SCLK 100kΩ A2 VIN SDATA CS GND 100nF 100kΩ 100kΩ GND 100kΩ GND GND 100kΩ NOTE: A1 AND A2 ARE 1/2 AD8606 11054-001 ST 5V Figure 1. Single Axis Vibration Analysis System (Simplified Schematic: Decoupling and All Connections Not Shown) Rev. A Circuits from the Lab reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2013–2016 Analog Devices, Inc. All rights reserved. CN-0303 Circuit Note PIN 8 CIRCUIT DESCRIPTION Accelerometer Output Characteristics The output voltage sensitivity is ratiometric with respect to the supply voltage. For a 3.3 V supply, the nominal output sensitivity is 16 mV/g. For a 5 V supply, the sensitivity is 24.2 mV/g. The zero-g output level is also ratiometric and is nominally equal to VDD/2. The ADXL001 requires only one 0.1 μF decoupling capacitor as long as there is no noise present at the 1 MHz internal clock frequency. If necessary, larger bulk capacitors (1 μF to 10 μF) or ferrite beads can be included. Accelerometer Physical Operation The ADXL001uses silicon on insulator (SOI) MEMS technology and takes advantage of mechanically coupled but electrically isolated differential sensing cells. Figure 2 is a simplified view of one of the differential sensor cell blocks. Each sensor block includes several differential capacitor unit cells. Each cell is composed of fixed plates attached to the device layer, and movable plates attached to the sensor frame. Displacement of the sensor frame changes the differential capacitance. On-chip circuitry measures the capacitance change and converts it into an output voltage. ANCHOR 11054-003 The ADXL001 is specified and tested for 3.3 V and 5 V supplies. Although operational with a supply voltage anywhere between 3 V and 6 V, optimum overall performance is achieved at 5 V. Figure 3. ADXL001 XOUT Voltage Increases with Acceleration in the Positive X-Axis Direction Interfacing to the ADC To digitize the acceleration information, the accelerometer output voltage range must fall inside the ADC input voltage range. The AD7476 input voltage range is 0 V to VDD (5 V). The ADXL001 output voltage range is 0.2 V to VS − 0.2 V (4.8 V). Based on this information, any acceleration sensed by the accelerometer is digitized requiring no additional amplifiers or buffers. Because the VDD power supply of the AD7476 serves as the ADC reference, an external reference is not required. In addition, the entire circuit is ratiometric with the supply because the same VDD supply also drives the ADXL001. Frequency Response The frequency response of the accelerometer is the most important characteristic in the system and is shown in Figure 4. As the frequency of the signal goes beyond approximately 2 kHz to 3 kHz, there is an increase in the gain of the accelerometer. At the resonant frequency of the beam (22 kHz), there is approximately 7 dB (×2.24) of peaking in the output voltage of the device. This peaking has significant ramifications for the output voltage of the accelerometer. 15 MOVABLE FRAME 12 9 UNIT SENSING CELL FIXED PLATES 6 RESPONSE (dB) ACCELERATION PLATE CAPACITORS UNIT FORCING CELL MOVING PLATE 3 0 –3 –6 11054-004 –9 11054-002 ANCHOR –12 Figure 2. Simplified View of Sensor Under Acceleration –15 The sensor device is micro-machined in plane in the SOI device layer. Trench isolation is used to electrically isolate, but mechanically couple, the differential sensing elements. Singlecrystal silicon springs suspend the structure over the handle wafer and provide resistance against acceleration forces. The ADXL001 is an x-axis acceleration and vibration-sensing device. It produces a positive-going output voltage for vibration toward its Pin 8 marking as shown in Figure 3. 1 10 100 1k FREQUENCY (Hz) 10k 100k Figure 4. ADXL001 Frequency Response Consider a 20 g acceleration at 10 kHz. The expected output voltage, assuming a zero-g output voltage of 2.5 V and a sensitivity of 24.2 mV/g is: 2.5 V + (0.0242 × 20) = 2.984 V However, this voltage is increased by approximately 2 dB of peaking, causing the actual output voltage to be: 2 dB = 20 log10 (VOUT/2.984 V) VOUT = 3.757 V Rev. A | Page 2 of 6 Circuit Note CN-0303 15 The difference between the expected output voltage and the actual output voltage is a significant source of error: 12 9 Error = 3.757 V − 2.984 V = 0.773 V 6 RESPONSE (dB) It is important to correct for this error to ensure accuracy, and an analog bi-quad filter was designed specifically for this purpose. The details of its implementation are discussed in the Filter Design section. Accelerometer Range Reduction ADXL001 RESPONSE 3 0 FILTER RESPONSE –3 SYSTEM RESPONSE –6 It is important to note that, as the frequency response of the accelerometer begins to peak, the useable acceleration range of the device decreases. Consider a 70 g acceleration at 20 kHz. The expected output voltage is: 2.5 V + (24.2 mV/g × 70 g) = 4.194 V 11054-005 –9 –12 –15 1 10 100 1k FREQUENCY (Hz) 10k 100k Figure 5. ADXL001 Frequency Response, Filter Frequency Response and System Frequency Response Incorporating the ~7dB peaking effect: 7 dB = 20 log10 (VOUT /4.194V) VOUT = 9.389 V Because the ADXL001 has a 5 V supply rail, the output limits at approximately 0.2 V and 4.8 V. The largest measureable g-force therefore depends on the frequency of vibration. Additional headroom of ±0.5 V must be allowed due to the variation in the zero-g offset voltage. The zero-g offset variation limits the maximum usable output voltage range to ±1.8 V, corresponding to approximately ±70 g for vibration frequencies less than approximately 2 kHz. As the vibration frequency increases from about 2 kHz to 22 kHz, the maximum allowable g-force before output saturation is gradually reduced by 7 dB (×2.24) to ±31 g. As long as the maximum g-force is less than ±31 g, the filter provides a flat frequency response to 22 kHz without saturation and loss of information. Filter Design To compensate for the gain peak found in the accelerometer frequency response, an analog, bi-quad, notch filter was implemented. The quality factor (Q = 2.5) and resonant frequency of the beam (22 kHz) are both found in the specifications table of the ADXL001 data sheet. By creating a notch filter with a peak of approximately −7 dB at 22 kHz, the frequency response of the accelerometer can be flattened, removing much of the difficulty in measuring vibration at higher frequencies. Figure 5 shows the frequency response for the filter, the accelerometer, and the entire signal chain. The data was taken using a sine wave input to the EVALCN0303-SDPZ board to simulate the accelerometer output. The notch filter was designed by modifying an example circuit found in “Passive and Active Network Analysis and Synthesis” by Aram Budak, October 1991 (ISBN-13: 978-0881336252. The transfer function for this compensator is the inverse of the previously derived transfer function. The Multisim™ Circuit Design Suite was used to simulate and verify the notch filter transfer function. The filter parameters were specified as Q = 2.5, center frequency = 22 kHz, notch depth = 7 dB. Test Results Two basic tests verify the performance of the system. First, a signal generator was used to drive a constant amplitude sine wave of varying frequency into the filter. Measuring the input and output voltages and plotting 20 log10(VOUT /VIN) provided the frequency response of the analog filter shown in Figure 5. Second, the frequency response of the entire signal chain was verified to ensure the performance of the design. To more accurately verify the system frequency response, a signal generator was used to simulate the ADXL001 output. For testing purposes, a 5 g acceleration was simulated and driven into the filter over a 50 kHz frequency range. If the ADXL001 is subject to a sinusoidal ±5 g acceleration in the sensitive axis, it outputs a corresponding ac voltage: ±5 g × 0.0242V/g = ±0.121 V This voltage is centered on the zero-g output, 2.5 V. The signal generator drives this voltage into the filter. An oscilloscope was used to measure the peak output voltage of the filter. This voltage was converted back into g’s (divide g’s by the sensitivity) and compared to the original input acceleration. Plotting 20log10 (VOUT/VIN) provides the frequency response of the system. Rev. A | Page 3 of 6 CN-0303 Circuit Note It is important to adjust the signal generator output voltage based on the peaking found in the frequency response of the accelerometer. For a 10 kHz frequency, the output voltage of the signal generator must be increased by approximately 1.8 dB to accurately represent the output voltage of the accelerometer experiencing a 5 g acceleration. The results of Figure 5 show the removal of the large peak found in the frequency response of the accelerometer. The −3 dB bandwidth is approximately 23 kHz. A small amount of ripple is seen in the passband immediately prior to the roll off caused by a slight misalignment in the peak of the accelerometer frequency response and the notch in the filter response. The power supply to the ADXL001 is decoupled with a 0.1 μF capacitor to properly suppress noise and reduce ripple. The capacitor must be placed as close to the device possible. Power supply lines must have as large a trace width as possible to provide low impedance paths and reduce glitch effects on the supply line. Clocks and other fast switching digital signals must be shielded from other parts of the board by digital ground. A complete design support package for this circuit note can be found at www.analog.com/CN0303-DesignSupport. 11054-007 A Wavetek Model 81 Pulse/Function Generator was used to generate a 2 kHz sine wave and connected directly to the filter input. Figure 6 shows a screen shot of the CN0303 Evaluation Software displaying conversion data from the AD7476 ADC and plotting the data in a graph. The sampling rate was 1 MSPS. 11054-006 Figure 7. Photo of EVAL-CN0303-SDPZ Figure 6. Screenshot of CN0303 Evaluation Software Digitizing a 2 kHz Sine Wave at a 1 MSPS Sampling Rate PCB Layout Considerations In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The printed circuit board (PCB) must isolate the digital and analog sections as much as possible. The PCB for this system was constructed in a four-layer stack up with large area ground plane layers and power plane polygons. See the MT-031 Tutorial for more discussion on layout and grounding, and see the MT-101 Tutorial for information on decoupling techniques. COMMON VARIATIONS To produce a more complete vibration sensing solution, a two axis (ADXL2xx family) or three axis (ADXL3xx family) accelerometer can be used in place of the ADXL001. By measuring acceleration in the second and third spatial dimensions, the user can write custom software allowing a more accurate and complete vibration sensing system. The EVAL-ADXL001-70Z board is connected to the EVALCN0303-SDPZ circuit board by a flexible ribbon cable. This allows the user to isolate the EVAL-CN0303-SDPZ from any vibrations that could cause potentially damaging mechanical stress to the circuit board, as well as allowing the user to place the ADXL001 directly at the vibration source. Rev. A | Page 4 of 6 Circuit Note CN-0303 CIRCUIT EVALUATION AND TEST  This circuit uses the EVAL-SDP-CB1Z system demonstration platform (SDP) board and the EVAL-CN0303-SDPZ circuit board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance.     This circuit also uses the EVAL-ADXL001-70Z evaluation board (included with the EVAL-CN0303-SDPZ board). The output connector (P1) of this PCB must be wired to the input connector (J6) of the EVAL-CN0303-SDPZ PCB using the ribbon cable supplied. Getting Started Load the evaluation software by placing the CN0303 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. The EVAL-CN0303-SDPZ board contains the circuit to be evaluated, as described in this circuit note. The EVAL-SDP-CB1Z evaluation board is used with the CN0303 Evaluation Software to capture the data from the EVAL-CN0303-SDPZ circuit board. The EVAL-ADXL001-70Z board contains the ADXL001 IC. Functional Block Diagram A functional block diagram of the test setup is shown in Figure 8. The signal generator is used to simulate the ADXL001 output. The EVAL-ADXL001-70Z board can be connected to EVAL-CN0303-SDPZ board for actual vibration measurements by using the ribbon cable to connect the 5-pin header (P1) of the EVAL-ADXL001-70Z to the 5-pin header (J6) of the EVALCN0303-SDPZ. Equipment Needed The following equipment is needed:  PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit) EVAL-CN0303-SDPZ circuit board EVAL-CN0303-SDPZ EVAL-CFTL-6V-PWRZ 6V WALL WART J5 10µF 100kΩ OSCILLOSCOPE PROBE 1 (TP VOUT) 5V 100kΩ 5V SIGNAL OUT 100pF OSCILLOSCOPE PROBE 2 (TP3) A1 200kΩ VDD AD7476 107kΩ 100pF PC 0.1pF USB 5V 200kΩ SIGNAL GENERATOR 5V A2 107kΩ VIN SDATA CS J6 100nF 100kΩ 100kΩ J1 120-PIN EVAL-SDP-CB1Z GND 100kΩ GROUND USB SCLK 100kΩ 100kΩ 11054-008  EVAL-ADXL001-70Z evaluation board and ribbon cable (included with EVAL-CN0303-SDPZ board) EVAL-SDP-CB1Z SDP board CN0303 Evaluation Software Power supply: +6.0 V or +6.0 V wall wart Function generator (Wavetek 81 or equivalent) for generating simulated ADXL001 output if desired GND Figure 8. Test Setup Functional Block Diagram Rev. A | Page 5 of 6 CN-0303 Circuit Note Setup LEARN MORE Connect the 120-pin connector (J1) on the EVAL-CN0303-SDPZ circuit board to the connector on the EVAL-SDP-CB1Z SDP board. Use nylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. CN-0303 Design Support Package: http://www.analog.com/CN0303-DesignSupport Connect the 5-pin header (P1) of the EVAL-ADXL001-70Z evaluation board to the 5-pin header (J6) of the EVAL-CN0303SDPZ circuit board using the ribbon cable. Connect a +6.0 V power supply to connector J5 of the EVALCN0303-SDPZ board. Connect the USB cable supplied with the SDP board to the USB port on the PC. Do not connect the USB cable to the mini USB connector on the SDP board at this time. Test MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. AN-688 Application Note, Phase and Frequency Response of iMEMS Accelerometers and Gyros, Analog Devices Linear Circuit Design Handbook, Hank Zumbahlen, Analog Devices Aram Budak, Passive and Active Network Analysis and Synthesis, October 1991, ISBN-13: 978-0881336252. Launch the evaluation software, and connect the USB cable from the PC to the USB mini-connector on the SDP board. Data Sheets and Evaluation Boards After USB communications are established, the SDP board can now be used to send, receive, and capture serial data from the EVAL-CN0303-SDPZ board. System Demonstration Platform (EVAL-SDP-CB1Z) CN-0303 Circuit Evaluation Board (EVAL-CN0303-SDPZ) Information regarding the SDP board can be found in the SDP User Guide. Information and details regarding test setup and calibration, and how to use the evaluation software for data capture can be found in the software Readme file found at www.analog.com/CN0303-UserGuide. ADXL001 Data Sheet ADXL001 Evaluation Board (EVAL-ADXL001-70Z) AD8606 Data Sheet AD7476 Data Sheet AD7476 Evaluation Board REVISION HISTORY 9/2016—Rev. 0 to Rev. A Changed EVAL-SDP-CS1Z to EVAL-SDP-CB1Z ..... Throughout 7/2013—Revision 0: Initial Version (Continued from first page) Circuits from the Lab reference designs 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 reference designs 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 reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2013–2016 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN11054-0-9/16(A) Rev. A | Page 6 of 6