Touchpad - Theory of Operation

Theory of Operation

There are two principal means by which touchpads work. In the matrix approach, a series of conductors are arranged in an array of parallel lines in two layers, separated by an insulator and crossing each other at right angles to form a grid. A high frequency signal is applied sequentially between pairs in this two-dimensional grid array. The current that passes between the nodes is proportional to the capacitance. When a virtual ground, such as a finger, is placed over one of the intersections between the conductive layer some of the electrical field is shunted to this ground point, resulting in a change in the apparent capacitance at that location. This method received U.S. Patent 5,305,017 awarded to George Gerpheide in April 1994.

The capacitive shunt method, described in an application note by Analog Devices, senses the change in capacitance between a transmitter and receiver that are on opposite sides of the sensor. The transmitter creates an electric field which oscillates at 200–300 kHz. If a ground point, such as the finger, is placed between the transmitter and receiver, some of the field lines are shunted away, decreasing the apparent capacitance.