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±15kV ESD-Protected Touch-Screen
Controllers Include DAC and Keypad Controller
nal, reference, or both may not settle into their final
FORCE LINE
Y+
+AV DD
steady-state values before the ADC samples the inputs,
and the reference voltage may continue to change dur-
ing the conversion cycle. The MAX1233/MAX1234 can
be programmed to wait for a fixed amount of time after
a screen touch has been detected before beginning a
scan. Use the touch-screen settling control bits in the
ADC control register (Table 11) to set the settling delay
to between zero and 100ms.
The settling problem is amplified in some applications
where external filter capacitors may be required across
the touch screen to filter noise that may be generated
X+
SENSE LINE
+IN
-IN
+REF
CONVERTER
-REF
by the LCD panel or backlight circuitry, etc. The values
of these capacitors cause an additional settling time
requirement when the panel is touched. Any failure to
SENSE LINE
settle before conversion start may show up as a gain
error. Average the conversion result by writing to the
ADC control register, as shown in Table 10, to minimize
noise.
FORCE LINE
Y-
Touch-Pressure Measurement
The MAX1233/MAX1234 provide two methods of mea-
surement of the pressure applied to the touch screen.
(
)
? ? ? Z 1 ?
? ?
R TOUCH = R XPLATE × ? POSITION ? × ? ? 2 ? ? 1 ?
GND
Figure 12. Ratiometric Y-Coordinate Measurement
ence power-up status when the A/D0–A/D3 bits are
zero. These values can be set initially on power-up.
(Subsequently A/D0–A/D3 bits are not zero, and any
other value of these bits is exclusive to ADC resolution
programming.)
Differential Ratiometric Touch-Position
Measurement
The MAX1233/MAX1234 provide differential conver-
sions. Figure 12 shows the switching matrix configura-
tion for Y coordinate measurement. The +REF and -REF
inputs are connected directly to Y+ and Y-. The conver-
sion result is a percentage of the external resistances,
and is unaffected by variation in the total touch-screen
resistance or the on-resistance of the internal switching
Although 8-bit resolution is typically sufficient, the follow-
ing calculations use 12-bit resolution demonstrating the
maximum precision of the MAX1233/MAX1234. Figure 13
shows the pressure measurement block diagram.
The first method performs pressure measurements
using a known X-plate resistance. After completing
three conversions, X-position, Z1-position, and Z2 posi-
tion, use the following equation to calculate R TOUCH :
? X ? ? ? Z ? ?
? 4096 ?
The second method requires knowing both the X-plate
and Y-plate resistance. Three touch-screen conver-
sions are required in this method as well for measure-
ment of the X-position, Y-position, and Z- position of the
touch screen. Use the following equation to calculate
R TOUCH :
? ? ? Z 1 ?
?? ?
? ? Y ? ?
? ? R YPLATE × ? POSITION ? ?
? ? ?
? ?
? ? ?
matrix.  The  touch  screen  remains  powered  during  the
acquisition and conversion process.
Touch-Screen Settling
There are two mechanisms that affect the voltage level
at the point where the touch panel is pressed. One is
electrical ringing due to parasitic capacitance between
the top and bottom layers of the touch screen and the
other is the mechanical bouncing caused by vibration
of the top layer of the touch screen. Thus, the input sig-
?
? ? R ? ? X ? ? ? 4096 ?
R TOUCH = ? ? XPLATE ? × ? POSITION ? × ? ? ?
Z 1 ? ? 4096 ?
? 4096
? 1 ? ?
??
? ?
30
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