Patent ID: 7312409

Claim:
A position detecting circuit for a touch pad, said touch pad including a sensing pad and a sensing pen, the position detecting circuit comprising: a waveform generator, for generating an input signal, wherein said input signal is capable of gradually moving a position of a zero voltage on said sensing pad in a predetermined direction; a filter, for receiving a sensing signal sensed by said sensing pen, and filtering and outputting said sensing signal; an amplifier, coupled to said filter, for receiving and amplifying said filtered sensing signal; an envelope detector, coupled to said amplifier, for detecting said filtered-amplified sensing signal and generating an envelope signal; a zero voltage detector, coupled to said envelope detector, for receiving said envelope signal and generating an output signal, wherein said output signal is capable of determining as to when a zero voltage occurs; and a controller, coupled to said waveform generator and said zero voltage detector, for controlling said waveform generator to generate said input signal and determining a position of said sensing pen on said sensing pad responsive to said output signal from said zero voltage detector, wherein said position of said zero voltage moves back and forth in said predetermined direction.