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ADXL150AQC Datasheet(PDF) 7 Page - Analog Devices |
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ADXL150AQC Datasheet(HTML) 7 Page - Analog Devices |
7 / 15 page ADXL150/ADXL250 –7– REV. 0 THEORY OF OPERATION The ADXL150 and ADXL250 are fabricated using a propri- etary surface micromachining process that has been in high volume production since 1993. The fabrication technique uses standard integrated circuit manufacturing methods enabling all the signal processing circuitry to be combined on the same chip with the sensor. The surface micromachined sensor element is made by deposit- ing polysilicon on a sacrificial oxide layer that is then etched away leaving the suspended sensor element. Figure 14 is a simplified view of the sensor structure. The actual sensor has 42 unit cells for sensing acceleration. The differential capacitor sensor is composed of fixed plates and moving plates attached to the beam that moves in response to acceleration. Movement of the beam changes the differential capacitance, which is measured by the on chip circuitry. The sensor has 12-unit capacitance cells for electrostatically forcing the beam during a self-test. Self-test is activated by the user with a logic high on the self-test input pin. During a logic high, an electrostatic force acts on the beam equivalent to approximately 20% of full-scale acceleration input, and thus a proportional voltage change appears on the output pin. When activated, the self-test feature exercises both the entire mechani- cal structure and the electrical circuitry. BEAM FIXED PLATE UNIT CELL ANCHOR PLATE CAPACITANCES Figure 14. Simplified View of Sensor Under Acceleration All the circuitry needed to drive the sensor and convert the capacitance change to voltage is incorporated on the chip requiring no external components except for standard power supply decou- pling. Both sensitivity and the zero-g value are ratiometric to the supply voltage, so that ratiometeric devices following the accelerometer (such as an ADC, etc.) will track the accelerom- eter if the supply voltage changes. The output voltage (VOUT) is a function of both the acceleration input (a) and the power supply voltage (VS) as follows: VOUT = VS/2 – (Sensitivity × VS 5V × a) Both the ADXL150 and ADXL250 have a 2-pole Bessel switched- capacitor filter. Bessel filters, sometimes called linear phase filters, have a step response with minimal overshoot and a maxi- mally flat group delay. The –3 dB frequency of the poles is preset at the factory to 1 kHz. These filters are also completely self-contained and buffered, requiring no external components. MEASURING ACCELERATIONS LESS THAN 50 g The ADXL150/ADXL250 require only a power supply bypass capacitor to measure ±50 g accelerations. For measuring ±50 g accelerations, the accelerometer may be directly connected to an ADC (see Figure 25). The device may also be easily modified to measure lower g signals by increasing its output scale factor. The scale factor of an accelerometer specifies the voltage change of the output per g of applied acceleration. This should not be confused with its resolution. The resolution of the device is the lowest g level the accelerometer is capable of measuring. Resolu- tion is principally determined by the device noise and the mea- surement bandwidth. The zero g bias level is simply the dc output level of the accelerom- eter when it is not in motion or being acted upon by the earth’s gravity. Pin Programmable Scale Factor Option In its normal state, the ADXL150/ADXL250’s buffer amplifier provides an output scale factor of 38 mV/g, which is set by an internal voltage divider. This gives a full-scale range of ±50 g and a nominal bandwidth of 1 kHz. A factor-of-two increase in sensitivity can be obtained by con- necting the VOUT pin to the offset null pin, assuming that it is not needed for offset adjustment. This connection has the effect of reducing the internal feedback by a factor of two, doubling the buffer’s gain. This increases the output scale factor to 76 mV/g and provides a ±25 g full-scale range. Simultaneously, connecting these two pins also increases the amount of internal post filtering, reducing the noise floor and changing the nominal 3 dB bandwidth of the ADXL150/ ADXL250 to 500 Hz. Note that the post filter’s “Q” will also be reduced by a factor of √2 from 0.58 (Bessel response) to a much gentler “Q” value of 0.41. The primary effect of this change in “Q” is only at frequencies within two octaves of the corner frequency; above this the two filter slopes are essentially the same. In applications where a flat response up to 500 Hz is needed, it is better to operate the device at 38 mV/g and use an external post filter. Note also that connecting VOUT to the offset pin adds a 30 k Ω load from V OUT to VS/2. When swinging ±2 V at VOUT, this added load will consume ±60 µA of the ADXL150/ ADXL250’s 100 µA (typical) output current drive. |
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