AK4180 Datasheet, PDF (8/17 Pages) Asahi Kasei Microsystems – Touch Screen Controller

[ASAHI KASEI]
[AK4180]
ON
XP
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
ON
YN
ON
XP
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
YN
ON
a) X-Position Measurement
Differential Mode
b) Y-Position Measurement
Differential Mode
Figure 1 Axis Measurement
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 internal (or external) reference voltage, 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.
However, note that the touch screen driver switch is still ON and the current flows even for the A/D conversion time. On the other
hand, the touch screen driver switch is ON only for the tracking time, 3tDCLK. From the power consumption, single-ended mode
has more advantage.
„ 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 uses the total resistance of the X-plate sheet. The resistance, Rtouch, is calculated from the results of three
conversions, X-position, Z1-Position, and Z2-Position, using the following formula:
Rtouch = (Rxplate) * (Xposition/4096) * [ (Z2/Z1) – 1]
The second method uses the resistances of both the X-plate and Y-plate. The resistance, Rtouch, is calculated from the results of
three conversions, X-position, Y-Position, and Z1-Position, using the following formula:
Rtouch = (Rxplate*Xposition/4096)*[(4096/Z1) – 1] – Ryplate*[1 – (Yposition/4096)]
<MS0195-E-01>
8
2003/02