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ADXL105 Arkusz danych(PDF) 8 Page - Analog Devices |
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ADXL105 Arkusz danych(HTML) 8 Page - Analog Devices |
8 / 8 page REV. A ADXL105 –8– Dynamic Operation In applications where only dynamic accelerations (vibration) are of interest, it is often best to ac-couple the accelerometer output as shown in Figures 15c and 15d. The advantage of ac coupling is that 0g offset variability (part to part) and drifts are eliminated. Low Power Operation The most straightforward method of lowering the ADXL105’s power consumption is to minimize its supply voltage. By lower- ing VDD from 5 V to 2.7 V the power consumption goes from 9.5 mW to 3.5 mW. There may be reasons why lowering the supply voltage is impractical in many applications, in which case the best way to minimize power consumption is by power cycling. The ADXL105 is capable of turning on and giving an accurate reading within 700 µs (see Figure 18). Most microcontrollers can perform an A-to-D conversion in under 25 µs. So it is prac- tical to turn on the ADXL105 and take a reading in under 750 µs. Given a 100 Hz sample rate the average current required at 2.7 V would be: 100 samples/s × 750 µs × 1.3 mA = 97.5 µA Figure 18. Typical Turn-On Response at VDD = 5 V Note that if a filter is used in the UCA, sufficient time must be allowed for the settling of the filter as well. Broadband Operation The ADXL105 has a number of characteristics that permits operation over a wide frequency range. Its frequency and phase response is essentially flat from dc to 10 kHz (see Figures 12 and 13). Its sensitivity is also constant over temperature (see Figure 3). In contrast, most accelerometers do not have linear response at low frequencies (in many cases, no response at very low frequencies or dc), and often have a large sensitivity tem- perature coefficient that must be compensated for. In addi- tion, the ADXL105’s noise floor is essentially flat from dc to 5 kHz where it gently rolls off (see Figure 7). The beam reso- nance at 16 kHz can be seen in Figure 7 where there is a small noise peak (+5 dB) at the beam’s resonant frequency. There are no other significant noise peaks at any frequency. The resonant frequency of the beam in the ADXL105 deter- mines its high frequency limit. However the resonant frequency of the Cerpak package is typically around 7 kHz. As a result, it is not unusual to see 6 dB peaks occurring at the package reso- nant frequency (as shown in Figures 12 and 13). Indeed, the PCB will often have one or more resonant peaks well below 7 kHz. Therefore, if the application calls for accurate operation at or above 6 kHz the ADXL105 should be glued to the PCB in order to eliminate the amplitude response peak due to the pack- age, and careful consideration should be given to the PCB mechanical design. CALIBRATING THE ADXL105 The initial value of the offset and scale factor for the ADXL105 will require dc calibration for applications such as tilt measurement. For low g applications, the force of gravity is the most stable, accurate and convenient acceleration reference available. An approximate reading of the 0 g point can be determined by orienting the device parallel to the Earth’s surface and then reading the output. For high accuracy, a calibrated fixture must be used to ensure exact 90 degree orientation to the 1 g gravity signal. An accurate sensitivity calibration method is to make a measure- ment at +1 g and –1 g. The sensitivity can be determined by the two measurements. This method has the advantage of being less sensitive to the alignment of the accelerometer because the on axis signal is proportional to the Cosine of the angle. For ex- ample, a 5 ° error in the orientation results in only a 0.4% error in the measurement. To calibrate, the accelerometer 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 and sensi- tivity is calculated: Sensitivity = [1 g Reading – (–1 g Reading)]/2 V/g VDD TOUT ST COM AOUT VMID VIN VNIN UCAOUT UNCOMMITTED AMPLIFIER ADXL105 X SENSOR TEMP SENSOR VDD 0.22 F 50 COM +V A-TO-D CONVERTER DOUT VREF AIN COM Figure 17. Reducing Noise on VDD OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 14-Lead Cerpak (QC-14) 1 7 8 14 0.310 (7.874) 0.275 (6.985) 0.415 (10.541) MAX PIN 1 0.300 (7.62) 0.419 (10.643) 0.394 (10.008) 0.345 (8.763) 0.290 (7.366) 0.050 (1.270) 0.016 (0.406) 8 0 0.0125 (0.318) 0.009 (0.229) SEATING PLANE 0.170 (4.318) 0.135 (3.429) 0.190 (4.826) 0.140 (3.556) 0.020 (0.508) 0.013 (0.330) 0.050 (1.27) BSC 0.020 (0.508) 0.004 (0.102) |
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