[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
Output Code
FFFH
(ΔVREF-1.5LSB)~
ΔVREF
FFEH
(ΔVREF-2.5LSB) ~ (ΔVREF-
1.5LSB)
---------
---------
0.5LSB ~ 1.5LSB
001H
0 ~ 0.5LSB
000H
ΔVREF:
(VREF+) – (VREF-)
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 (ΔVREF) is the differential
voltage between the non-inverting terminal and the inverting terminal of the measured axis (e.g. X-axis measurement:
ΔVREF
= V
XP
– V
XN
). The voltage difference on the A/D converter (ΔAIN) is the voltage between non-inverting
terminals of the non-measured axis and the inverting terminal of the measured axis. (E.g.
ΔAIN=
(AIN+) - (AIN-) =
V
YP
-V
XN
) The voltage difference (ΔAIN) 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 2.5μs (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.
VCC
XP-Driver SW ON
XP
VCC
X-Plate
Y-Plate
YP-Driver SW ON
XP
X-Plate
Y-Plate
VREF+
AIN+
YP
VREF+
AIN+
YP
ADC
VREF-
AIN-
ADC
VREF-
AIN-
XN
XN
XN-Driver SW ON
YN
YN-Driver SW ON
YN
Touch Screen
a)
X-Position Measurement Differential Mode
b)
Y-Position Measurement Differential Mode
The X-plate and Y-plate are connected on the dotted line when the panel is touched.
XP
X-Plate (Top side)
XN
YN
c)
Y-Plate (Bottom side)
YP
4-wire Touch Screen Construction
Figure 2. Axis Measurements
MS0500-E-01
6
2008/12
AKM [ ASAHI KASEI MICROSYSTEMS ]
