Using Absolute Output iMEMS® Gyroscopes with Ratiometric ADCs Har
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AN-767 APPLICATION NOTE
Using Absolute Output iMEMS® Gyroscopes with Ratiometric ADCs
Harvey Weinberg
iMEMS gyroscopes often used with cost ratiometric ADCs integral many microcontrollers. This application note outlines interface gyro's absolute (invariant with supply voltage variations) output with ratiometric ADC. GENERAL CONCEPTS Ratiometric ADCs return numeric value that corresponds ratio input voltage supply voltage. example, 8-bit ratiometric that supply voltage 5.00 1.99 input would return value bits) Output (1.99 V/5.00 supply voltage changes 4.75 converter would return value approximately greater Output (1.99 V/4.75 Since supply voltage rarely counted very accurate constant, some provision must made conversion errors when interfaced transducers with absolute outputs. USING GYRO'S REFERENCE OUTPUT Analog Devices gyros have reference output that used reduce conversion errors. general idea that performing conversion known voltage (the gyro's reference), calculate what ADC's reference voltage appropriate correction factor. Since supply voltage vary, should perform reference gyro rate output conversions close simultaneously possible.
ADC's transfer function when measuring gyro's output A/DOUT (2.5/A/DREF) where: number bits converter. A/DREF ADC's reference voltage. Rearranging equation: A/DREF (2.5/A/DOUT) correction factor (A/DREF REFIDEAL) where REFIDEAL ideal reference voltage. example: A/DREF system. Finally corrected value A/DCORRECTED A/DOUT CONCLUSION Using ratiometric with absolute output device like iMEMS gyros result significant errors supply voltage vary. output Analog Devices iMEMS gyros allows user compensate these errors simple calculations presented above.
REV.
2005 Analog Devices, Inc. rights reserved. Trademarks registered trademarks property their respective owners.
AN05353-0-2/05(0)
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