Abstract: A conventional three-electrode wedge-and-strip pattern in a digitizer tablet surface is used with a conventional cursor or stylus. A phantom cursor electrode is located at predetermined coordinates of the active area of the digitizer tablet beneath the X, Y and balance (B) electrodes and separated therefrom by a layer of dielectric material, preferably an air gap. Prior to use of the digitizer apparatus, the phantom cursor electrode is electrostatically coupled to the X, Y and B electrodes in order to obtain data signals representing the phantom cursor's measured coordinates without compensation for resolution distortion. A microprocessor produces a correction factor using the data signals and the predefined coordinates of the phantom cursor electrode, which correction factor is applied in normal use to compensate for resolution distortion resulting from the unavoidable gaps between the X, Y and B electrodes.

Abstract: A charge ratio tablet having three patterned electrodes for determining the position coordinates of a stylus relative to a first area on the tablet is provided with a menu in a second area. The menu includes first and second pluralities of menu electrodes, the three menu electrodes of the first pluralilty having a first geometric pattern and the three menu electrodes of the second pluralilty having a second geometric pattern. The patterned electrodes and menu electrodes are driven in accordance with a predetermined sequence. The processing circuitry determines the position coordinates of the stylus if the signal induced in the stylus in response to driving of the three patterned electrodes simultaneously has at least a predetermined magnitude. The processing circuitry carries out operations other than position coordinate determination if instead the stylus is capacitively coupled to a driven menu electrode. The processing circuitry discriminates which menu electrode is capacitively coupled to the stylus.