ASAHI KASEI
[AK4181A]
A/D Converter for Touch Screen
The AK4181A incorporates a 12-bit successive approximation resistor (SAR) A/D converter for position measurement,
temperature, and battery voltage. The architecture is based on 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
(∆VREF-1.5LSB)~
∆VREF
FFEH
(∆VREF-2.5LSB) ~ (∆VREF-1.5LSB)
---------
---------
0.5LSB ~ 1.5LSB
001H
0 ~ 0.5LSB
000H
∆VREF:
(VREF+) – (VREF-)
Table 1 Output Code
The full scale (∆VREF) of the A/D converter depends on the input mode. The AK4181A is controlled by the 8 bit serial command
on DIN.
Analog Inputs
Analog input is selected via the A2, A1, A0 and SER/ DFR bits in the control register. If the analog inputs are selected to the X
or Y-axis, SER/ DFR = “0”, which means 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:(XP) – (XN)). Analog non-inverting
input to A/D converter is the non-inverting terminal of the non-measured axis while the inverting input is the inverting terminal of
the measured axis. If the SER/ DFR bit is set to “1” which means single-ended mode, the full scale of A/D converter (∆VREF)
is the internal reference voltage or external reference voltage. Note that SER/ DFR bit should be set to ”0” if IN2 is selected as
analog input; nevertheless, IN2 is actually measured by single-ended mode.
Tracking time is the period from the falling edge of 5
th
DCLK to that of 8
th
DCLK after the detection of START bit during
CSN=”L”.
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 1.428µs (3tDCLKat 2.1MHz). The maximum throughput of A/D converter is 125 kHz.
If the source impedance of analog input or battery input is larger than 600 ohm, longer tracking time is required.
The Position Detection of Touch Screen
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.
MS0313-E-00
7
2004/06
AKM [ ASAHI KASEI MICROSYSTEMS ]
