
ASAHI KASEI
[AK4183]
A/D Converter for Touch Screen
The AK4183 incorporates a 12-bit successive approximation resistor (SAR) A/D converter for position measurement. The 
architecture is based on a capacitive redistribution algorithm, and an internal capacitor array functions as the sample/hold (S/H) 
circuit.  
The SAR A/D converter output is a straight binary format as shown below: 
Input Voltage 
(
VREF-1.5LSB)~ 
VREF 
(
VREF-2.5LSB) ~ (
VREF-1.5LSB) 
--------- 
0.5LSB ~ 1.5LSB 
0 ~ 0.5LSB 
VREF: (VREF+) – (VREF-) 
MS0500-E-00 
 2006/04 
6
Output Code 
FFFH 
FFEH 
--------- 
001H 
000H 
Table 1 Output Code 
 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 (
VREF) is the differential voltage between the non-
inverting terminal and the inverting terminal of the measured axis (e.g. X-axis measurement: 
VREF = V
XP
 – V
XN
).   The voltage 
difference on the A/D converter (
AIN) is the voltage between non-inverting terminals of the non-measured axis and the inverting 
terminal of the measured axis. (E.g. 
AIN= (AIN+) - (AIN-) = V
YP
-V
XN
) The voltage difference (
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
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
VCC 
VCC
Touch Screen
Y-Plate
Figure 2 Axis Measurements