Position digitizer circuit for a moveable instrument

The invention is a digitizer circuit in a peripheral system for a computer comprising a moveable instrument and a planar grid in which an excitation signal in one of said instrument and grid generates an induced signal in the other one of said instrument and grid, and wherein the phase difference between said excitation and induced signals is indicative of the position of said instrument with respect to said grid, comprising means for summing said excitation and induced signals to produce a sum signal and means for producing an output signal whose frequency is a function of the amplitude of said sum signal.