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10_Gyroscopes with Ratiometric ADCs.pdf
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AN-767 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com i MEMS gyroscopes are often used with low cost ratiometric ADCs integral to many microcontrollers. This application note outlines how to interface a gyro’s absolute (invariant with supply voltage variations) output with a ratiometric ADC. GENERAL CONCEPTS Ratiometric ADCs return a numeric value that corre- sponds to the ratio of the input voltage to the supply voltage. For example, an 8-bit ratiometric ADC that has a supply voltage of 5.00 V and 1.99 V at its input would return a value (in bits) of Output = 2 8 (1.99 V/5.00 V) = 102 If the supply voltage changes by –5% (to 4.75 V) the con- verter would return a value approximately 5% greater or Output = 2 8 (1.99 V/4.75 V) = 107 Since supply voltage can rarely be counted on to be very accurate or constant, some provision must be made for conversion errors when interfaced to transducers with absolute outputs. USING THE GYRO’S REFERENCE OUTPUT All Analog Devices gyros have a 2.5 V reference output that can be used to reduce conversion errors. The general idea is that by performing an A/D conversion on a known voltage (the gyro’s 2.5 V reference), one can calculate what the ADC’s reference voltage is and an appropriate correction factor. Since the supply voltage can vary, one should perform the 2.5 V reference and gyro rate output conversions as close to simultaneously as possible. Using Absolute Output iMEMS ® Gyroscopes with Ratiometric ADCs by Harvey Weinberg The ADC’s transfer function when measuring the gyro’s 2.5 V output is A/D OUT = (2.5/A/D REF ) 2 n where: n is the number of bits of the A/D converter. A/D REF is the ADC’s reference voltage. Rearranging the equation: A/D REF = (2.5/A/D OUT ) 2 n So the correction factor is CF = (A/D REF / REF IDEAL ) where REF IDEAL is the ideal ADC reference voltage. For example: CF = A/D REF / 5 for a 5 V system. Finally the corrected A/D value is A/D CORRECTED = A/D OUT CF CONCLUSION Using a ratiometric ADC with an absolute output device like iMEMS gyros can result in significant errors if the sup- ply voltage can vary.The 2.5 V output on Analog Devices iMEMS gyros allows the user to compensate for these errors by simple calculations as presented above. REV. 0
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10_Gyroscopes with Ratiometric ADCs.pdf
Transcript of 10_Gyroscopes with Ratiometric ADCs.pdf
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AN-767APPLICATION NOTE
One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 Tel: 781/329-4700 Fax: 781/326-8703 www.analog.com
iMEMS gyroscopes are often used with low cost ratiometric ADCs integral to many microcontrollers. This application note outlines how to interface a gyros absolute (invariant with supply voltage variations) output with a ratiometric ADC.
GENERAL CONCEPTSRatiometric ADCs return a numeric value that corre- sponds to the ratio of the input voltage to the supply voltage. For example, an 8-bit ratiometric ADC that has a supply voltage of 5.00 V and 1.99 V at its input would return a value (in bits) of
Output = 28 (1.99 V/5.00 V) = 102
If the supply voltage changes by 5% (to 4.75 V) the con-verter would return a value approximately 5% greater or
Output = 28 (1.99 V/4.75 V) = 107
Since supply voltage can rarely be counted on to be very accurate or constant, some provision must be made for conversion errors when interfaced to transducers with absolute outputs.
USING THE GYROS REFERENCE OUTPUTAll Analog Devices gyros have a 2.5 V reference output that can be used to reduce conversion errors. The general idea is that by performing an A/D conversion on a known voltage (the gyros 2.5 V reference), one can calculate what the ADCs reference voltage is and an appropriate correction factor. Since the supply voltage can vary, one should perform the 2.5 V reference and gyro rate output conversions as close to simultaneously as possible.
Using Absolute Output iMEMS Gyroscopes with Ratiometric ADCsby Harvey Weinberg
The ADCs transfer function when measuring the gyros 2.5 V output is
A/DOUT = (2.5/A/DREF) 2n
where:
n is the number of bits of the A/D converter.
A/DREF is the ADCs reference voltage.
Rearranging the equation:
A/DREF = (2.5/A/DOUT) 2n
So the correction factor is
CF = (A/DREF / REFIDEAL)
where REFIDEAL is the ideal ADC reference voltage.
For example:
CF = A/DREF / 5 for a 5 V system.
Finally the corrected A/D value is
A/DCORRECTED = A/DOUT CF
CONCLUSIONUsing a ratiometric ADC with an absolute output device like iMEMS gyros can result in signicant errors if the sup-ply voltage can vary. The 2.5 V output on Analog Devices iMEMS gyros allows the user to compensate for these errors by simple calculations as presented above.
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GENERAL CONCEPTSUSING THE GYROS REFERENCE OUTPUTCONCLUSION
