[AK4183]
The differential mode position detection is typically more accurate than the single-ended mode. As the full scale of single-ended
mode is fixed to the VCC, input voltage may exceed the full-scale reference voltage. This problem does not occur in differential
mode. In addition to this, the differential mode is less influenced by power supply voltage variation due to the ratio-metric
measurement.
■
The Pen Pressure Measurement
The touch screen pen pressure can be derived from the measurement of the contact resistor between two plates. The
contact resistance depends on the size of the depressed area and the pressure. The area of the spot is proportional to the
contact resistance. This resistance (Rtouch) can be calculated using two different methods.
The first method is that when the total resistance of the X-plate sheet is already known. The resistance, Rtouch, is
calculated from the results of three conversions, X-position, Z1-Position, and Z2-Position, and then using the following
formula:
Rtouch = (Rxplate) * (Xposition/4096) * [(Z2/Z1) – 1]
The second method is that when both the resistances of the X-plate and Y-plate are known. The resistance, Rtouch, is
calculated from the results of three conversions, X-position, Y-Position, and Z1-Position, and then using the following
formula:
Rtouch = (Rxplate*Xposition/4096)*[(4096/Z1) – 1] – Ryplate*[1 – (Yposition/4096)]
VCC
YP-Driver SW ON
YP
VCC
YP-Driver SW ON
YP
XP
VREF+
AIN+
Rtouch
VREF+
AIN+
XP
Rtouch
ADC
VREF-
AIN-
ADC
VREF-
AIN-
XN
XN
XN-Driver SWON
YN
XN-Driver SW ON
YN
a)
Z1-Position Measurement
b)
Z2-Position Measurement
Figure 3. Pen Pressure Measurements
MS0500-E-01
7
2008/12
AKM [ ASAHI KASEI MICROSYSTEMS ]
