Abstract: This invention describes a method and apparatus for filtering noise from a measurement of the X and Y coordinates of a resistive digitizer. The method applies to both four and five wire resistive digitizers biased with a DC voltage. The same filtering and measurement apparatus can be used on both types of digitizers with inclusion of an extra signal pin to accommodate the sense lead of the five wire digitizer. This approach involves connecting the signal to be read to a filter, reading the filtered voltage, and disconnecting the signal from the filter before disconnecting bias voltage from the planes of the digitizer. A separate filter is used for the X and Y coordinate signals and each filter voltage can be read at any time before the next measurement. A reset voltage is available to an established reference on the filter capacitor when a “pen up” status is detected.

Abstract: The linearity of a digital to analog converter is greatly enhanced by applying the same current to each of the nodes of a R-2R ladder network. This is accomplished by breaking the conversion cycle into n time slots and sequencing the connection of the current sources such that each will be connected to a different node in each of the time slots. In this way all the current sources will be connected to each node once in the conversion cycle and the current flowing into the each node will be the average value of all of the currents. Since each node receives this same average current, the linearity of the digital to analog converter is enhanced and is not a function of the mismatch of the applied current sources.