[ASAHI KASEI]
A/D Converter for Touch Screen
The AK4533 has 12 bit resolution Successive Approximation Resistor (SAR) A/D converter for touch screen, and the
measurement of buttery voltage.
As A/D converter uses capacitive redistribution architecture, internal capacitors also operate as sample/hold circuit.
TSY_TOP pin or TSX_TOP pin, which is used for detecting the pressed position, is selected as analog input to ADC.
VA is referred to as full scale of ADC. When BTI_1 pin, or BTI_2 pin is selected for the measurement of buttery
voltage as analog input to A/D converter, TPVREFIN is referred to as full scale of ADC.
Tracking time, which is defined as the time to store the charge into internal capacitors, depends on the output
impedance (Rin) of touch screen. If Rin is 500 Ω, at least 1.5 µs is required for tracking time. The AK4533 uses 3
TADCK cycles for tracking: If TADCK is 2MHz, tracking time is 0.5µs * 3 cycles = 1.5µs. If Rin is larger value at the
measurement of buttery voltage, not only enough tracking time is required, but also the clock frequency of TADCK
should be lower than 2MHz.
Data format of Touch Screen A/D converter is binary as Table 6.
Input Voltage
(VREF-1.5LSB) - VREF
(VREF-2.5LSB) - (VREF-1.5LSB)
---------
Output Code
FFF
FFE
---------
001
0.5LSB - 1.5LSB
0 - 0.5LSB
000
VREF : VA or TPVREFIN
Table 6. Data format
Figure 17 shows the touch screen controller block, and the connection between touch screen and the AK4533. Two
times of A/D conversion is needed to detect the pressed position: one is for X-axis, and another is for Y-axis. A pair of
terminals of touch screen is biased while another terminal is hi-Z state. ADC measures the voltage that is induced to
TOP side of hi-Z terminals. For example, if TSX_TOP and TSX BTM are biased to VA, and AGND respectively, the
ADC can output the dot on X-axis by measuring TSY_TOP voltage.
TSX_TOP
SCLK
Control
Logic
CS
TSY_TOP
Touch
Screen
SDTI
TSX_BTM
TSY_BTM
SDTO
Battery1
VA
TPVREFIN
Battery2
BTI_1
BTI_2
TADE
VREF
TADCK
12bit
ADC
(SAR type)
TADRDY
TADOUT
AIN
INT
PW
Figure 17. Connection between AK4533 and Touch Screen
<Revision 0.9a>
28
July 00
AKM [ ASAHI KASEI MICROSYSTEMS ]
