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Offset calibration of the MMA8451, 2, 3Q

Posted: 13 Sep 2010 ?? ?Print Version ?Bookmark and Share

Keywords:MMA8451? offset calibration? accelerometer?

Various sources of error in an accelerometer need to be understood and, in some cases, compensated before use. Understanding these sources of errors can be as important as understanding how the accelerometer operates within an application. One of the leading sources of error in an accelerometer is attributed to the 0g-offset or bias error, which is defined as "the difference between the measured value of the sensor from the true zero value".

Accelerometers are "trimmed" for offset and sensitivity in the factory by adjusting the offset trim codes and gain by using factory programmable registers within the part. Accelerometers are trimmed within a narrow test target in the manufacturing process to calibrate out these sources of error within the devices. Ultimately, the accuracy of the test station and its ability to calibrate determine how well the devices are initially calibrated during the manufacturing process.

End users then solder the accelerometers onto a PCB upon which various mechanical strains are produced:
? On the package from mounting the part to the PCB;
? From the PCB mount holes or screws;
? From other components placed close to the sensor.

Such conditions can cause further offset or shifts. Other sources of shifts can also occur due to temperature changes and aging. All of these sources of errors may cause the accelerometer to appear to be tilted or rotated relative to the zero reference point. Re-calibration after the sensor has been PCB-mounted may be needed for other reasons as well. For example, an application may need to change the zero (0) reference point if the zero (0) position in a tilt application is always at a 15 angle, thus considering this the new "zero" position. These re-calibrations add to the end product manufacturing cost (by adding additional steps to assembly). Accordingly, the benefit of re-calibration will have to be weighed against these added costs.

View the PDF document for more information.

Click here to view related datasheets.

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