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ADXL350BCEZ-RL Arkusz danych(PDF) 30 Page - Analog Devices |
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ADXL350BCEZ-RL Arkusz danych(HTML) 30 Page - Analog Devices |
30 / 36 page ADXL350 Data Sheet Rev. 0 | Page 30 of 36 using the built-in offset registers. This results in the data acquired from the DATA registers already compensating for any offset. In a no-turn or single-point calibration scheme, the part is oriented such that one axis, typically the z-axis, is in the 1 g field of gravity and the remaining axes, typically the x-axis and y-axis, are in a 0 g field. The output is then measured by taking the average of a series of samples. The number of samples averaged is a choice of the system designer, but a recommended starting point is 0.1 sec worth of data for data rates of 100 Hz or greater. This corresponds to 10 samples at the 100 Hz data rate. For data rates less than 100 Hz, it is recommended that at least 10 samples be averaged together. These values are stored as X0g, Y0g, and Z+1g for the 0 g measurements on the x-axis and y-axis and the 1 g measure- ment on the z-axis, respectively. The values measured for X0g and Y0g correspond to the x- and y-axis offset, and compensation is done by subtracting those values from the output of the accelerometer to obtain the actual acceleration. XACTUAL = XMEAS − X0g YACTUAL = YMEAS − Y0g Because the z-axis measurement was done in a +1 g field, a no-turn or single-point calibration scheme assumes an ideal sensitivity, SZ for the z-axis. This is subtracted from Z+1g to attain the z-axis offset, which is then subtracted from future measured values to obtain the actual value: Z0g = Z+1g − SZ ZACTUAL = ZMEAS − Z0g The ADXL350 can automatically compensate the output for offset by using the offset registers (Register 0x1E, Register 0x1F, and Register 0x20). These registers contain an 8-bit, twos complement value that is automatically added to all measured acceleration values, and the result is then placed into the DATA registers. Because the value placed in an offset register is additive, a negative value is placed into the register to eliminate a positive offset and vice versa for a negative offset. The register has a scale factor of 7.8 mg/LSB and is independent of the selected g-range. As an example, assume that the ADXL350 is placed into full- resolution mode with a sensitivity of typically 512 LSB/g. The part is oriented such that the z-axis is in the field of gravity and x-, y-, and z-axis outputs are measured as +10 LSB, −13 LSB, and +9 LSB, respectively. Using the previous equations, X0g is +10 LSB, Y0g is −13 LSB, and Z0g is +9 LSB. Each LSB of output in full-resolution is 1.95 mg or one-quarter of an LSB of the offset register. Because the offset register is additive, the 0 g values are negated and rounded to the nearest LSB of the offset register: XOFFSET = −Round(10/4) = −3 LSB YOFFSET = −Round(−13/4) = 3 LSB ZOFFSET = −Round(9/4) = −2 LSB These values are programmed into the OFSX, OFSY, and OFXZ registers, respectively, as 0xFD, 0x03, and 0xFE. As with all registers in the ADXL350, the offset registers do not retain the value written into them when power is removed from the part. Power cycling the ADXL350 returns the offset registers to their default value of 0x00. Because the no-turn or single-point calibration method assumes an ideal sensitivity in the z-axis, any error in the sensitivity results in offset error. To help minimize this error, an additional measure- ment point can be used with the z-axis in a 0 g field and the 0 g measurement can be used in the ZACTUAL equation. USING SELF-TEST The self-test change is defined as the difference between the acceleration output of an axis with self-test enabled and the acceleration output of the same axis with self-test disabled (see Endnote 4 of Table 1). This definition assumes that the sensor does not move between these two measurements, because if the sensor moves, a non-self-test related shift corrupts the test. Proper configuration of the ADXL350 is also necessary for an accurate self-test measurement. The part should be set with a data rate that is greater than or equal to 100 Hz. This is done by ensuring that a value greater than or equal to 0x0A is written into the rate bits (Bit D3 through Bit D0) in the BW_RATE register (Address 0x2C). It is also recommended that the part be set to ±8 g mode to ensure that there is sufficient dynamic range for the entire self-test shift. This is done by setting Bit D3 of the DATA_FORMAT register (Address 0x31) and writing a value of 0x03 to the range bits (Bit D1 and Bit D0) of the DATA_FORMAT register (Address 0x31). This results in a high dynamic range for measurement and a 2 mg/LSB scale factor. After the part is configured for accurate self-test measurement, several samples of x-, y-, and z-axis acceleration data should be retrieved from the sensor and averaged together. The number of samples averaged is a choice of the system designer, but a recom- mended starting point is 0.1 sec worth of data, which corresponds to 10 samples at 100 Hz data rate. The averaged values should be stored and labeled appropriately as the self-test disabled data, that is, XST_OFF, YST_OFF, and ZST_OFF. Next, self-test should be enabled by setting Bit D7 of the DATA_FORMAT register (Address 0x31). The output needs some time (about four samples) to settle after enabling self-test. After allowing the output to settle, several samples of the x-, y-, and z-axis acceleration data should be taken again and averaged. It is recommended that the same number of samples be taken for this average as was previously taken. These averaged values should again be stored and labeled appropriately as the value with self- test enabled, that is, XST_ON, YST_ON, and ZST_ON. Self-test can then be disabled by clearing Bit D7 of the DATA_FORMAT register (Address 0x31). |
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