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AN-1021
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Te l: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
MMSE-Based Multipoint Calibration Algorithm for Touch Screen Applications
by Ning Jia
Rev. 0
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One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved.
INTRODUCTION
Modern devices typically use LCDs with touch screen
technology as user interfaces. Due to their simple construction
and well-known operation, resistive-type touch screens are the
most popular in cost-conscious designs. However, the mechanical
misalignments and scaling factors of resistive-type touch screens
affect the X and Y coordinates produced by the touch screens.
Therefore, it is difficult to perfectly align the coordinates of the
touch screen to the display (LCD or otherwise) behind it. When
the final product that includes a touch screen comes out of the
box, a calibration algorithm must first be done.
The classical calibration algorithm for a touch screen is a three-
point calibration algorithm that uses three reference points. The
classical three-point calibration algorithm is both efficient and
effective, but its performance is low when the touch screen is
comparatively large. This application note proposes the minimum
mean square error (MMSE)-based multipoint calibration algorithm,
which uses more than three reference points, for resistive-type
touch screens. Both mathematical deduction and experimentation
prove that this algorithm is more accurate than the classical three-
point calibration algorithm.
