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ADXL202AE Arkusz danych(PDF) 12 Page - Analog Devices |
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ADXL202AE Arkusz danych(HTML) 12 Page - Analog Devices |
12 / 12 page REV. A –12– ADXL202E OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 8-Terminal Ceramic Leadless Chip Carrier (E-8) BOTTOM VIEW 1 3 5 7 0.025 (0.64) 0.075 (1.91) 0.099 (2.50) 0.099 (2.50) 0.015 (0.38) R0.008 (0.20) 0.015 (0.38) 0.050 (1.27) 0.197 (5.00) SQ 0.177 (4.50) SQ TOP VIEW R0.028 (0.70) 0.008 (0.20) 0.070 (1.78) CONTROLLING DIMENSIONS ARE IN MILLIMETERS 0.050 (1.27) 0.050 (1.27) USING THE ADXL202E IN VERY LOW POWER APPLICATIONS An application note outlining low power strategies for the ADXL202E is available. Some key points are presented here. It is possible to reduce the ADXL202E’s average current from 0.6 mA to less than 20 µA by using the following techniques: 1. Power Cycle the accelerometer. 2. Run the accelerometer at a Lower Voltage, (Down to 3 V). Power Cycling with an External A/D Depending on the value of the XFILT capacitor, the ADXL202E is capable of turning on and giving a good reading in 1.6 ms. Most microcontroller based A/Ds can acquire a reading in another 25 µs. Thus it is possible to turn on the ADXL202E and take a reading in <2 ms. If we assume that a 20 Hz sample rate is sufficient, the total current required to take 20 samples is 2 ms × 20 samples/s × 0.6 mA = 24 µA average current. Running the part at 3 V will reduce the supply current from 0.6 mA to 0.4 mA, bringing the average current down to 16 µA. The A/D should read the analog output of the ADXL202E at the XFILT and YFILT pins. A buffer amplifier is recommended, and may be required in any case to amplify the analog output to give enough resolution with an 8-bit to 10-bit converter. Power Cycling When Using the Digital Output An alternative is to run the microcontroller at a higher clock rate and put it into shutdown between readings, allowing the use of the digital output. In this approach the ADXL202E should be set at its fastest sample rate (T2 = 0.5 ms), with a 500 Hz filter at XFILT and YFILT. The concept is to acquire a reading as quickly as pos- sible and then shut down the ADXL202E and the microcontroller until the next sample is needed. In either of the above approaches, the ADXL202E can be turned on and off directly using a digital port pin on the microcontroller to power the accelerometer without additional components. CALIBRATING THE ADXL202E/ADXL210 The initial value of the offset and scale factor for the ADXL202E will require calibration for applications such as tilt measurement. The ADXL202E architecture has been designed so that these calibrations take place in the software of the microcontroller used to decode the duty cycle signal. Calibration factors can be stored in EEPROM or determined at turn-on and saved in dynamic memory. For low g applications, the force of gravity is the most stable, accurate and convenient acceleration reference available. A reading of the 0 g point can be determined by orientating the device par- allel to the earth’s surface and then reading the output. A more accurate calibration method is to make measurements at +1 g and –1 g. The sensitivity can be determined by the two measurements. To calibrate, the accelerometer’s measurement axis is pointed directly at the earth. The 1 g reading is saved and the sensor is turned 180 ° to measure –1 g. Using the two readings, the sensi- tivity is: Let A = Accelerometer output with axis oriented to +1 g Let B = Accelerometer output with axis oriented to –1 g then: Sensitivity = [A – B]/2 g For example, if the +1 g reading (A) is 55% duty cycle and the –1 g reading (B) is 32% duty cycle, then: Sensitivity = [55% – 32%]/2 g = 11.5%/g These equations apply whether the output is analog or duty cycle. Application notes outlining algorithms for calculating accelera- tion from duty cycle and automated calibration routines are available from the factory. |
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