SX8650 Datasheet, PDF (23/56 Pages) Semtech Corporation – World’s Lowest Power & Smallest Footprint 4-wire Resistive Touchscreen Controller with 15kV ESD

SX8650
World’s Lowest Power & Smallest Footprint 4-wire
Resistive Touchscreen Controller with 15kV ESD
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
4.2. Coordinates Measurement
The top resistive sheet (Y) is biased with a voltage source. Resistors R3 and R4 determine a voltage divider proportional to
the Y position of the contact point. Since the converter has a high input impedance, no current flows through R1 so that the
voltage X+ at the converter input is given by the voltage divider created by R3 and R4.
The X coordinate is measured in a similar fashion with the bottom resistive sheet (X) biased to create a voltage divider by
R1 and R2, while the voltage on the top sheet is measured through R3. Figure 20 shows the coordinates measurement
setup. The resistance R_T is the resistance obtained when a pressure is applied on the screen. R_T is created by the contact
area of the X and Y resistive sheet and varies with the applied pressure.
Ypos
X+
Y+
R1
R3
R_T
+
Vr-ef
R2
R4
X+
Y+
Xpos
R1
R3
+
Vr-ef
R_T
R2
R4
X-
Y-
X-
Y-
Figure 20. Ordinate (Y) and abscissa (X) coordinates measurement setup
The X and Y position are found by:
Xpos
=
4095
⋅
-------^R----2--------
R1 + R2
Ypos
=
4095
⋅
-------^R----4--------
R3 + R4
ACS Revision V2.15/October 2009
©2009 Semtech Corp.
Page 22
www.semtech.com