Analog Devices AN767 Application Notes

AN-767

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • www.analog.com

Using Absolute Output iMEMS® Gyroscopes with Ratiometric ADCs

by Harvey Weinberg

iMEMS 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 corre­sponds 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 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.

The ADC’s transfer function when measuring the gyro’s

2.5 V output is

A/D

= (2.5/A/D

OUT

REF

)  2

n

where:

n is the number of bits of the A/D converter.

A/D

is the ADC’s reference voltage.

REF

Rearranging the equation:

A/D

= (2.5/A/D

REF

OUT

)  2

n

So the correction factor is

CF = (A/D

-

where REF

is the ideal ADC reference voltage.

IDEA L

REF

/ REF

IDEAL

)

For example:

CF = A/D

/ 5 for a 5 V system.

REF

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 sup­ply 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

AN05353–0–2/05(0)

© 2005 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.

–2–