ANALOG DEVICES AN-376 APPLICATION NOTE ONE TECHNOLOGY WAY e P.O. BOX 9106 e NORWOOD, MASSACHUSETTS 02062-9106 e 617/329-4700 Using the ADXL50EM Accelerometer Evaluation Module by Charles Kitchin, Bob Briano, and Mike Shuster IMPORTANT FEATURES 1. Complete Mechanically Assembled Acceleration Measuring System . Rugged, Nonresonant, Mechanical Mounting . Measures Accelerations Up to +50 g . Operates on Single +5 V dc Supply . Precalibrated, Buffered Output . Nominal Sensitivity: 19 mV/g . Nominal 0 g Level: +1.8 V . Built-in Self-Test Feature . Wide Frequency Response: DC to 1 kHz OOnN DA & WN FUNCTIONAL DESCRIPTION The ADXL50 evaluation module simplifies the evalua- tion, experimentation and testing of the ADXL50 accel- erometer IC in a wide variety of accelerometer applications with a minimum of design effort. Figure 1 shows the evaluation modules schematic (see the ADXL50 data sheet for more complete informa- tion regarding the ADXL50 IC). The module's output sensitivity, zero g offset level, and bandwidth may be easily modified by the user. The module has been care- fully designed so that mechanical resonances are minimized. CONSTRUCTION DETAILS On the top side of the modules PC board is an ADXL50JH accelerometer and the boards ground plane. On the reverse side of the PC board are its surface- mount components, which are industry standard 0805 size capacitors and resistors. Connection points for +5 V, ground, self-test, 3.4 V refer- ence and Voyr are shown in Figure 2. These are con- nected to the cable. i; FT 7 DEMODULATOR CAPACITOR ) REFERENCE OUTPUT WHITE | 0.022, ADXL50JH (WHITE) SELF TEST INPUT | oscitator sENsoR PREAMP (YELLOW) | DECOUPLING CAPACITOR Vour | (GREEN) | 0.022,F | @ | COM | | C3 | RIA $re Cp enn | (BLACK) | Vv | Lo +5 (RED) Figure 1. ADXL50EM Evaluation Module SchematicBOTTOM VIEW @ = GROUND TOP VIEW REF +5 N ee Figure 2. ADXL50EM Module Printed Circuit Board Layout ADXL50EM MODULE SPECIFICATIONS The ADXL50EM module is configured to provide a nomi- nal output sensitivity of 19 mV/g with a 0 g level of +1.8 V. Except for variations due to the +1% tolerance of the two 0.5% resistors used in the module, it has the same specifi- cations as the ADXL50JH grade devices. For all other specifications, refer to the ADXL50 data sheet. Note that both sensitivity and 0 g offset can be user trimmedsee modifying the ADXL50EM module. Output Sensitivity & Polarity: In the ADXL50EM module the preamplifier output of the ADXL50 (Vp,) is internally connected to the ADXL50s buffer amplifiers input (-Vin) terminal. Therefore, the output voltage polarity will be op- posite that occurring at the Vpg terminal of the ADXL50. Modifying the ADXL50EM Module: The output sensitivity, 0 g offset level, and circuit bandwidth can each be modi- fied by removing the printed circuit board from the mod- ule and replacing or adding surface mount components. Nonsurface mount resistors or capacitors should not be used because they can resonate. Figure 2 shows the lay- out of the modules printed circuit board. Access to the board is provided by removing the four screws on the bot- tom of the module. Standard Modifications Using the On-Chip Buffer Amplifier: 1. To add a one pole low pass filter, connect a chip capaci- tor of the desired value across resistor R3 (see the one pole post filtering section of the ADXL50 data sheet for details on component selection). . To modify for ac coupling (i.e., high pass filtering), re- move resistor R1 and solder capacitor CF1 and resistor R1a to the footprints adjacent to R1. . To add bandpass filtering, perform Steps 1 and 2 above. 4. To perform a 0 g offset trim, temporarily connect a 500 kQ trim potentiometer across the footprints labeled R2 and adjust the trim potentiometer for the desired 0g level (above +1.8 V). Remember to orient the cube so that its axis of sensitivity is horizontal. Remove the trim potentiometer, measure its value with an ohmmeter, and then use the closest standard resistor value for R2. . To perform a sensitivity (scale factor) trim, remove resistor R1, temporarily insert a 100 kQ trim potentio- meter in its place and adjust for +1 g using the earths gravity (see the ADXL50 data sheet for more details). Remove the trim potentiometer and insert the closest standard value resistor onto the footprints. When reinstalling the board, carefully hand tighten the boards mounting screws. !f the board is to be removed and used outside the module, it should be attached to a rigid surface using four mounting screws with standoffs to provide clearance below the board. MECHANICAL CHARACTERISTICS In many accelerometer applications, proper mounting techniques are crucial in order to eliminate unwanted mechanical resonances (see the application note AN-372 for more details). Mechanical resonances produce vibra- tions which, to the accelerometer, are indistinguishable from an actual acceleration. The ADXL50EM module was specifically designed to minimize the majority of these resonances. Resonances were minimized by making the board size as small as possible, by using only surface mount compo- nents, and by using a very flexible interconnection cable. The circuit board is attached to the modules shell using 4/40 screws in all four corners of the board providing the proper mechanical seating for vibration and shock testing.Figure 3 shows the output amplitude vs. frequency of the ADXL50 accelerometer under ideal conditions. Al- though steps were taken to reduce resonances, the evaluation module demonstrates how the ADXL50 will actually behave when mounted on a typical printed cir- cuit board. The graphs of Figures 4, 5, and 6 show the relative output amplitude vs. frequency of the ADXL50EM module when mounted in a mechanical 190 a 1 19 4 E Sh 2 0.022uF rh NN a 19 \_| 0.015yF 4 z on | 4| 7 \ \ 0.005.F \\ 0.19 0 rc en, SSS | . -90 Ss -160 100 1k 10k FREQUENCY - Hz Figure 3. ADXL50EM Frequency Response for Various Demodulator Capacitors shaker and. accelerated along the X, Y or Z axis. Note that the scale of Figure 4 is 3 dB per division and 10 dB per division for the other two figures. Figure 4 shows a resonance at approximately 7 kHz due to the ADXL50 ICs package flexing on its leads as the PC board moves below it. Since the X axis amplitude includes the signal, its resonant peak appears small compared with the Y and Z axis amplitudes which contain virtually no signal and are normalized to the system noise. The resonance can vary in both amplitude and frequency depending on how securely the device is attached to the PC board. Note that there are several easy ways to reduce these resonances. The most obvious is to simply low pass fil- ter the ADXL50s output above its 1 kHz typical band- width, by adding a capacitor across resistor R3, the ADXL50 buffers feedback resistor. Several mechanical cures may also be employed, such as potting the entire cavity of the module with a suitable material. Paraffin wax will allow removal of the circuit board while harder substances such as epoxy make a permanent bond. Pot- ting the cavity will dramatically reduce the resonance amplitude. Alternatively, the header package of the ADXL50 can be epoxied to the board at the seating plane (the center of its pin circle) which will cause the resonant peak to shift higher in frequency to approximately 12 kHz. PHASE SHIFT Degrees NORMALIZED OUTPUT AMPLITUDE 100 1k 10k FREQUENCY - Hz Figure 4. Normalized Amplitude vs. Frequency of the ADXL50EM Module When Accelerated Along the Sensitive X Axis +50 +30 +20 +10 NORMALIZED OUTPUT AMPLITUDE l 3 6 8 8 00 1k 10k FREQUENCY - Hz Figure 5. Normalized Amplitude vs. Frequency of the ADXL50EM Module When Accelerated Along Its Transverse Y Axis +10 NORMALIZED OUTPUT AMPLITUDE 68 B84. 8 tk 10k FREQUENCY - Hz Figure 6. Normalized Amplitude vs. Frequency of the ADXL50EM Module When Accelerated Along Its Transverse Z" AxisCONNECTIONS & ORIENTATION Figure 7 shows the connector pinout and wire color code for the ADXL50EM module. To avoid ground loop cur- rents, the connectors ground or common lead should be a separate wire from that connecting the cables shield +5V ST GND to ground. Figure 8 provides mechanical dimensions of (RED) (YELLOW) (BLACK) the ADXL50EM module. VreF Vout SHIELD (WHITE) (GREEN) Figure 7. ADXL50EM Module Connector Pinout (As Seen Looking Down on Connector Pins) 10 - 32 THREAD HOLES 1.125 + 0.005 0 (28.6 + 0.13) 6 OUTPUT 1.125 + 0.005 VOLTAGE (28.6 + 0.13) POLARITY 0 AXIS OF MODULE WEIGHT: 2.3 02. SENSITIViTy 9 (65.2 grams) Ly 1.137 + 0.010 (28.88 + 0.254) DIMEMSIONS SHOWN ARE IN INCHES AND (mm). NOTE: THE METAL CASE OF THE EVALUATION MODULE IS ISOLATED. Figure 8. ADXL50EM Module Mechanical Dimensions E1910-5-4/94 PRINTED IN U.S.A.