AK4183_06 Datasheet, PDF (6/17 Pages) –

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
Output Code
(∆VREF-1.5LSB)~ ∆VREF
FFFH
(∆VREF-2.5LSB) ~ (∆VREF-1.5LSB)
FFEH
---------
---------
0.5LSB ~ 1.5LSB
001H
0 ~ 0.5LSB
000H
∆VREF: (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 (∆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 2.5µs (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
X-Plate
XP
Y-Plate
VCC
YP-Driver SW ON
X-Plate
XP
Y-Plate
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
XN-Driver SW ON
YN
YN Touch Screen
YN-Driver SW ON
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
XN
YN
YP
c) 4-wire Touch Screen Construction
Figure 2 Axis Measurements
X-Plate (Top side)
Y-Plate (Bottom side)
MS0500-E-00
6
2006/04