Circuit Note CN-0533 Devices Connected/Referenced Low noise, high frequency, 50 g MEMS ADXL1002 Accelerometer Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help Industrial Current Out Driver, Single- solve todays analog, mixed-signal, and RF design AD5749 Supply, 55 V Maximum Supply, challenges. For more information and/or support, visit Programmable Ranges www.analog.com/CN0533. Precision Wide Supply, High Output LT6654 Drive, Low Noise Reference 10 kHz MEMS Accelerometer with 4 mA to 20 mA Output for Condition-Based Monitoring Applications While some applications place the accelerometer close to the EVALUATION AND DESIGN SUPPORT supporting circuitry (either on the same board or off board Circuit Evaluation Boards through a short cable), others require the accelerometer to be CN-0533 Circuit Evaluation Board (EVAL-CN0533-EBZ) some distance away, which limits connectivity options. MEMS Aluminum mounting block (EVAL-XLMOUNT1) accelerometer outputs are typically analog voltage and/or digital Design and Integration Files 2 (typically, serial peripheral interface (SPI) or I C), none of Schematics, Layout Files, Bill of Materials which are well suited to driving long cables. It is possible to CIRCUIT FUNCTION AND BENEFITS convert to a high speed digital interface such as USB, low Condition-based monitoring (CbM) is one form of predictive voltage digital signaling (LVDS), or Ethernet. However, the maintenance that uses various sensors to assess the operating additional power, size, and cost are not practical. status of equipment over time. The collected sensor data is used In contrast, analog current loop data transmission, such as the to establish baseline trends, which then help diagnose or even 4 mA to 20 mA industry standard, offers good noise immunity, predict failure. Utilizing CbM, maintenance is performed when robustness in electromagnetic interference (EMI) prone needed as opposed to the conventional periodic preventive environments, high bandwidth, and up to a 20 meter long wire maintenance model, saving both time and cost. data transmission, all while keeping the number of devices in Vibration monitoring is a common type of CbM measurement. the circuit board to a few components. Furthermore, the 4 mA Changes in vibration trends are often indicative of wear or other to 20 mA signaling standard is supported by nearly all legacy failure modes. To measure vibration data, high bandwidth industrial data acquisition (DAQ) systems and can be adapted (10 kHz and more), ultralow noise (100 g/Hz or lower) simply to the modern, Industry 4.0 smart sensor node. MEMS accelerometers are a cost effective and reliable choice. Rev. 0 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages Tel: 781.329.4700 www.analog.com due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last Fax: 781.461.3113 2020 Analog Devices, Inc. All rights reserved. page) CN-0533 Circuit Note V CC V CC C5 AVDD V IN VREF 1F V OUT DVCC LT6654 HW SELECT IOUT SCLK/OUTEN GND CLRSEL RESET AGND R8 AD5749 100k FAULT/TEMP NC/IFAULT V SDO DD R1 R2 TP3 STANDBY V VIN OUT TP2 680 1. 3k C1 C2 R11 ST 4.7nF 4.7nF 470k R12 ADXL1002 470k AGND AGND AGND AGND TP1 R13 OR 15k V PAD SS AGND AGND AGND Figure 1. EVAL-CN0533-EBZ Simplified Circuit Diagram The AD5749 directly translates the ADXL1002 voltage output CIRCUIT DESCRIPTION signal to a 4 mA to 20 mA current output, with minimal impact The circuit shown in Figure 1 is a simplified schematic of a on the printed circuit board (PCB) footprint and offers up to a MEMS accelerometer vibration sensing solution whose voltage 50 kHz bandwidth and good noise immunity. output is converted into a 4 mA to 20 mA analog signal. Many of 4 mA to 20 mA drivers in the market consist of current 4 mA to 20 mA Current Loop and Interface 2 output, digital-to-analog converters (DACs) and require SPI or I C The 4 mA to 20 mA current loop has been an industry analog external controllers. The AD5749 4 mA to 20 mA driver has the signaling standard since the 1950s. The main advantage of this additional advantage of a standalone operation mode (hardware signal standard is that virtually no attenuation over long cables mode). exists, which results in more robustness in EMI prone In hardware mode, set the HW SELECT pin high. The R0 to environments, such as industrial and factory settings. On the R3 and RSET pins are all tied low to set the AD5749 output contrary, if a voltage output is used, there is voltage drop over range to 4 mA to 20 mA, which means no external micro- long cables (greater than 10 meters) due to the cable resistance, controller is required to configure the AD5749 output range. resulting in loss of sensor data and incorrect sensor readings. To improve the stability of the output current over temperature, The reference design shown in Figure 1 consists of a single axis connect an external low drift resistor between the REXT1 and ADXL1002 MEMS accelerometer, whose analog voltage output REXT2 pins. is converted to the 4 mA to 20 mA signaling standard by using On the DAQ front-end circuitry (not included), only one an AD5749 voltage to current converter. The AD5749 input current to voltage (I-V) conversion amplifier is required. The (V ) swings from 0 V to 4.096 V, and the ADXL1002 analog IN transimpedance (I-V resistance) must be set according to the output voltage (V ) swings from 0 V to V , thus V must be OUT DD DD input range of the DAQ front-end circuitry. set to 4.096 V. Therefore, the LT6654AMPS6-4.096 was chosen to supply the 4.096 V with a 10 ppm/C temperature stability over the 55C to 125C temperature range. A 2-pole RC low- pass filter with a 3 dB bandwidth of 36 kHz is placed between V and V . This filter limits broadband noise and attenuates OUT IN the noise component at 200 kHz from the internal clock of the ADXL1002, which may alias in-band depending on the sample rate and filtering characteristics of the DAQ circuitry of the application. Rev. 0 Page 2 of 5 REXT1 REXT2 AD2/R2 AD1/R1 AD0/R3 NC CLEAR SDIN/R0 SYNC/RSET EPAD GND 21931-001