65.6.15.3 4-wire Position Measurement Method
As shown in the above figure, to detect the position of a contact, a supply is first applied from top to bottom. Due to the linear resistance of the film, there is a voltage gradient from top to bottom. When a contact is performed on the screen, the voltage propagates at the point the two surfaces come into contact. If the input impedance on the right and left electrodes is high enough, the film intrinsic resistor does not affect this voltage.
For the horizontal direction, the same method is used, but by applying supply from left to right. The range depends on the supply voltage and on the loss in the switches that connect to the top and bottom electrodes.
In an ideal world (linear, with no loss through switches), the horizontal position is equal to:
VYM / VDD or VYP / VDD.
The implementation with on-chip power switches is shown in the figure below. The voltage measurement at the output of the switch compensates for the switches loss.
It is possible to correct for switch loss by performing the operation:
[VYP - VXM] / [VXP - VXM].
This requires additional measurements, as shown in the figure below.
