
[AK4184] 
MS0603-E-00 
 2007/04 
10
■
 Touch Screen Position Detection 
XY-coordinate detection (XY-position) from the touch panel is determined by the control command (A1, A0) bits of 
the touch screen control command. Coordinate detection is in 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 input (
Δ
AIN) of the A/D converter is the voltage between the non-inverting 
terminal (V
XP
) of the non-target axis and the inverting terminal (V
XN
) of the target axis. (E.g. 
Δ
AIN = (AIN+)-(AIN-) 
= V
YP
 – V
XN
)  The input voltage charges the internal capacitor during the sampling time period. No current flows into 
the internal capacitors after the capacitor has been fully charged. The required settling time to charge the internal 
capacitor array depends on the source impedance (Rin). The source impedance is the resistance of the touch panel 
plate. If the source impedance is 600
Ω
, the settling time needs to be at least 1.5
μ
s. The maximum throughput of the 
A/D converter is 125kHz. If the source impedance of the analog input is larger than 600
Ω
, longer settling time is 
required. The position from 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 
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 
AVDD 
AVDD 
Touch Screen
Y-Plate
Figure 3. Axis Measurement