
ASAHI KASEI
[AK4182]
MS0411-E-00 
 2005/08 
8
YN 
XN 
YP 
ON 
VREF
VREF- 
ADC 
AIN+ 
AIN- 
XP 
ON 
a) X-Position Measurement Differential Mode 
b) Y-Position Measurement Differential Mode 
YN 
XN 
YP 
ON 
VREF
VREF-
ADC 
AIN+ 
AIN- 
XP 
ON
Figure 2 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 point of power consumption, single-
ended mode has more advantages. However the differential mode is generally used to get more accurate measurement in position 
detecting.  
 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)]