Abstract: A tablet system has an improved operation for saving power. During a digitizing period, the tablet supplies power to resistive sheets that produce analog pen position signals and to the sample and hold circuit and the analog to digital converter that operate on these signals. During a wait state, power is removed from these components and a separate circuit detects contact with the tablet surface by a pen. A pen activity circuit that uses less power is also disclosed. The pen activity detecting circuit has a resistor in circuit with the two resistive sheets. This resistor and associated circuits are independent of the normal path for signals that denote pen position and thereby allow these components to be turned off. When the pen is positioned on the tablet surface, the two resistive sheets are brought into contact in the normal way for detecting the pen position. During a period of pen inactivity, this contact produces a current in the resistor.

Abstract: A digital tablet of the type that multiplexes pen position signals has filtering and digitizing circuits arranged in separate channels for each of the signals that is to be multiplexed. The tablet pen is connected to receive an oscillator signal and the signal is picked up at each of the four corners of the tablet and applied to the corresponding channel. The separate channels permit the filters to be made sharper. The disclosure also includes improved circuits for processing the pen position signals.