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ADXL210JE датащи(PDF) 11 Page - Analog Devices |
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ADXL210JE датащи(HTML) 11 Page - Analog Devices |
11 / 12 page REV. 0 ADXL210E –11– Power Cycling with an External A/D Depending on the value of the XFILT capacitor, the ADXL210E 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 ADXL210E 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 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 ADXL210E 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 ADXL210E 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 possible and then shut down the ADXL210E and the microcon- troller until the next sample is needed. In either of the above approaches, the ADXL210E can be turned on and off directly using a digital port pin on the microcontroller to power the accelerometer without additional components. CALIBRATING THE ADXL210E The initial value of the offset and scale factor for the ADXL210E will require calibration for applications such as tilt measurement. The ADXL210E architecture has been designed so that these calibra- tions 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 parallel 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 47% duty cycle, then: Sensitivity = [55% – 47%]/2 g = 4%/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. Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period, and Resolution of Duty Cycle Modulator Counter- ADXL210E Clock Counts RSET Sample Rate per T2 Counts Resolution T2 (ms) (k ) Rate (MHz) Cycle per g (mg) 1.0 124 1000 2.0 2000 80 12.50 1.0 124 1000 1.0 1000 40 25.00 1.0 124 1000 0.5 500 20 50.00 5.0 625 200 2.0 10000 400 2.50 5.0 625 200 1.0 5000 200 5.00 5.0 625 200 0.5 2500 100 10.00 10.0 1250 100 2.0 20000 800 1.25 10.0 1250 100 1.0 10000 400 2.50 10.0 1250 100 0.5 5000 200 5.00 USING THE ANALOG OUTPUT The ADXL210E was specifically designed for use with its digital outputs, but has provisions to provide analog outputs as well. Duty Cycle Filtering An analog output can be reconstructed by filtering the duty cycle output. This technique requires only passive components. The duty cycle period (T2) should be set to <1 ms. An RC filter with a 3 dB point at least a factor of >10 less than the duty cycle fre- quency is connected to the duty cycle output. The filter resistor should be no less than 100 k Ω to prevent loading of the output stage. The analog output signal will be ratiometric to the supply voltage. The advantage of this method is an output scale factor of approximately double the analog output. Its disadvantage is that the frequency response will be lower than when using the XFILT, YFILT output. XFILT, YFILT Output The second method is to use the analog output present at the XFILT and YFILT pin. Unfortunately, these pins have a 32 k Ω output impedance and are not designed to drive a load directly. An op amp follower may be required to buffer this pin. The advantage of this method is that the full 5 kHz bandwidth of the accelerometer is available to the user. A capacitor still must be added at this point for filtering. The duty cycle converter should be kept running by using RSET <10 M Ω. Note that the acceler- ometer offset and sensitivity are ratiometric to the supply voltage. The offset and sensitivity are nominally: 0 g Offset = VDD/2 ADXL210E Sensitivity = (20 mV VS)/g USING THE ADXL210E IN VERY LOW POWER APPLICATIONS An application note outlining low power strategies for the ADXL210E is available. Some key points are presented here. It is possible to reduce the ADXL210E’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). |
Аналогичный номер детали - ADXL210JE |
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Аналогичное описание - ADXL210JE |
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