Circuit Note CN-0303 Devices Connected/Referenced High Precision, 70 g, 250 g, 500 g ADXL001 Circuits from the Lab reference designs are engineered and Single Axis Accelerometer tested for quick and easy system integration to help solve todays Precision, Low Noise, CMOS, Rail-to-Rail analog, mixed-signal, and RF design challenges. For more AD8606 Input/Output, Dual Package Op-Amp information and/or support, visit www.analog.com/CN0303. 2.35 V to 5.25 V, Low Power, Single-Channel, AD7476 1 MSPS, 12-Bit ADC MEMS-Based Vibration Analyzer with Frequency Response Compensation The circuit provides a low power solution suitable for bearing EVALUATION AND DESIGN SUPPORT analysis, engine monitoring and shock detection. Circuit Evaluation Boards CN-0303 Circuit Evaluation Board (EVAL-CN303-SDPZ) The Analog Devices, Inc. proprietary fifth-generation iMEMs Includes ADXL001 Evaluation Board (EVAL-ADXL001-70Z) process enables the ADXL001 accelerometer to provide dynamic and connecting ribbon cable range that extends from 70 g to 500 g in combination with System Demonstration Platform (EVAL-SDP-CB1Z) 22 kHz of bandwidth. Design and Integration Files The AD8606 is a precision, low noise, dual op amp used to Schematics, Layout Files, Bill of Materials 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 circuit in Figure 1 offers a high linearity, low noise, wide- the AD7476 low power, single-channel, 12-bit, successive bandwidth vibration sensing solution. It is ideal for applications approximation analog-to-digital converter (SAR ADC). 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 EVAL-CN0303-SDPZ 5V 5V 5V 5V 10F 0.1pF 100k 0.1F 5V 5V 5V V DD 200k V V DD DD2 AD7476 107k EVAL-SDP-CB1Z 100pF 100k ST X SCLK OUT 100k A1 ADXL001 100pF 200k A2 V SDATA IN 107k COM CS GND 100k 100k 100nF GND 100k 100k GND GND NOTE: A1 AND A2 ARE 1/2 AD8606 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 20132016 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 11054-001CN-0303 Circuit Note PIN 8 CIRCUIT DESCRIPTION Accelerometer Output Characteristics 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 The output voltage sensitivity is ratiometric with respect to the Positive X-Axis Direction supply voltage. For a 3.3 V supply, the nominal output sensitivity is Interfacing to the ADC 16 mV/g. For a 5 V supply, the sensitivity is 24.2 mV/g. To digitize the acceleration information, the accelerometer output The zero-g output level is also ratiometric and is nominally voltage range must fall inside the ADC input voltage range. The equal to V /2. DD AD7476 input voltage range is 0 V to V (5 V). The ADXL001 DD The ADXL001 requires only one 0.1 F decoupling capacitor as output voltage range is 0.2 V to V 0.2 V (4.8 V). Based on S long as there is no noise present at the 1 MHz internal clock this information, any acceleration sensed by the accelerometer frequency. If necessary, larger bulk capacitors (1 F to 10 F) or is digitized requiring no additional amplifiers or buffers. ferrite beads can be included. Because the VDD power supply of the AD7476 serves as the Accelerometer Physical Operation ADC reference, an external reference is not required. In The ADXL001uses silicon on insulator (SOI) MEMS addition, the entire circuit is ratiometric with the supply technology and takes advantage of mechanically coupled but because the same V supply also drives the ADXL001. DD electrically isolated differential sensing cells. Figure 2 is a Frequency Response simplified view of one of the differential sensor cell blocks. Each The frequency response of the accelerometer is the most sensor block includes several differential capacitor unit cells. important characteristic in the system and is shown in Figure 4. Each cell is composed of fixed plates attached to the device As the frequency of the signal goes beyond approximately 2 kHz layer, and movable plates attached to the sensor frame. to 3 kHz, there is an increase in the gain of the accelerometer. Displacement of the sensor frame changes the differential At the resonant frequency of the beam (22 kHz), there is capacitance. On-chip circuitry measures the capacitance change approximately 7 dB (2.24) of peaking in the output voltage of and converts it into an output voltage. the device. This peaking has significant ramifications for the output voltage of the accelerometer. ANCHOR 15 MOVABLE PLATE FRAME 12 CAPACITORS 9 UNIT FIXED SENSING PLATES 6 CELL 3 UNIT 0 FORCING MOVING CELL PLATE 3 6 9 ANCHOR 12 Figure 2. Simplified View of Sensor Under Acceleration 15 1 10 100 1k 10k 100k FREQUENCY (Hz) The sensor device is micro-machined in plane in the SOI device Figure 4. ADXL001 Frequency Response layer. Trench isolation is used to electrically isolate, but mechanically couple, the differential sensing elements. Single- Consider a 20 g acceleration at 10 kHz. The expected output crystal silicon springs suspend the structure over the handle voltage, assuming a zero-g output voltage of 2.5 V and a wafer and provide resistance against acceleration forces. sensitivity of 24.2 mV/g is: The ADXL001 is an x-axis acceleration and vibration-sensing 2.5 V + (0.0242 20) = 2.984 V device. It produces a positive-going output voltage for vibration However, this voltage is increased by approximately 2 dB of toward its Pin 8 marking as shown in Figure 3. peaking, causing the actual output voltage to be: 2 dB = 20 log (V /2.984 V) 10 OUT VOUT = 3.757 V Rev. A Page 2 of 6 ACCELERATION 11054-002 RESPONSE (dB) 11054-003 11054-004