
[AK4673] 
MS0670-E-00 
2007/09 
- 70 - 
■
 The Position Detection of Touch Screen 
A position detecting (X, Y position) on the touch panel is selected by the control command via the A2, A1, A0 bits in the 
control register.  The mode of the position detecting is differential mode, the full scale (
Δ
VREF) is the differential voltage 
between the non-inverting terminal and the inverting terminal of the measured axis (e.g. X-axis measurement: 
Δ
VREF = 
V
XP
 – V
XN
).   The voltage difference on the A/D converter (
Δ
AIN) is the voltage between non-inverting terminals of the 
non-measured axis and the inverting terminal of the measured axis. (E.g. 
Δ
AIN= (AIN+) - (AIN-) = V
YP
-V
XN
) The 
voltage difference (
Δ
AIN) is charged to the internal capacitor array during the sampling period. No current flows into the 
internal capacitor after the capacitor has been charged completely.  
The required settling time to charge the internal capacitor array depends on the source impedance (Rin).  If the source 
impedance is 600 ohm, the settling time needs at least 2.5
μ
s (1 clock cycle period of SCL 400 KHz) 
The position on the touch screen is detected by taking the voltage of one axis when the voltage is supplied between the 
two terminals of another axis.  At least two A/D conversions are needed to get the two-dimensional (X/Y axis) position.    
The X-plate and Y-plate are connected on the dotted line when the panel is touched. 
XP 
XN 
X-Plate (Top side) 
Y-Plate (Bottom side) 
c) 4-wire Touch Screen Construction
Figure 63 Axis Measurements 
X-Plate 
Y-Plate 
X-Plate 
YP 
YN 
YN 
XN 
YP 
XN-Driver SW ON 
VREF+ 
VREF- 
ADC 
AIN+ 
AIN- 
XP 
XP-Driver SW ON 
a) X-Position Measurement Differential Mode 
b) Y-Position Measurement Differential Mode 
YN 
XN 
YP 
YN-Driver SW ON
VREF+ 
VREF-
ADC 
AIN+ 
AIN- 
XP 
YP-Driver SW ON
TSVDD 
TSVDD
Touch Screen 
Y-Plate 
The differential mode position detection is typically more accurate than the single-ended mode.  As the full scale of 
single-ended mode is fixed to the VCC, input voltage may exceed the full-scale reference voltage.  This problem does not 
occur in differential mode.  In addition to this, the differential mode is less influenced by power supply voltage variation 
due to the ratio-metric measurement.