Touch panel with automatic nulling

A touch panel device for determining the touch location at which a touch sensing surface of the device is touched comprising:

• signal generator means for producing an alternating-current voltage output;
• transformer means having a primary winding means connected to the signal generator output so as to be driven by the signal generator means, the transformer means also having a secondary winding means with first and second secondary winding outputs and a center tap, the secondary winding means providing a panel scanning signal as the primary winding means is driven by the signal generator output;
• switching means for selectively coupling the secondary winding means to the touch sensing surface so as to selectively apply the panel scanning signal to the touch sensing surface and generate touch current signals at the center tap upon touching of the panel, the switching means comprising means for selectively coupling one of the secondary winding outputs to a first side of the touch sensing surface and the other of the secondary winding outputs to a second side of the touch sensing surface, the second side being located across the touch sensing surface in a first direction from the first side, so as to establish a voltage gradient across the touch sensing surface in the first direction from the first to the second sides and so as to provide a first touch current signal at the center tap when the touch sensing surface is touched, the switching means also comprising means for selectively coupling one of the secondary winding outputs to a third side of the touch sensing surface and the other of the secondary winding outputs to a fourth side of the touch sensing surface, the fourth side being located across the touch sensing surface in a second direction from the first side, so as to establish a voltage gradient across the touch sensing surface in the second direction from the third to the fourth sides and so as to provide a second touch current signal at the center tap when the touch sensing surface. is touched, the switching means comprising means for selectively and simultaneously coupling one of the secondary winding outputs to at least two of the first, second, third and fourth sides of the touch sensing surface to provide an impedance touch current signal at the center tap when the touch sensing surface is touched;
• panel output signal processing means having a first input coupled to the center tap of the transformer means for receiving the first, second and impedance touch current signals, the panel output signal processing means comprising means for determining the touch location along the first and second directions from the touch current signals;
• and
• the panel output signal processing means including nulling circuit means for automatically nulling the first, second and impedance touch current signals when the touch sensing surface is untouched.