Pen location detection

Apparatus comprising:

• memory configured to store pre-defined response function, wherein the response function relates an output signal from a digitizer sensor junction to distance of a passive pen tip from the junction;
• and
• a circuit configured to:
• detect outputs from digitizer sensor junctions;
• identify, based on the outputs, an area on the digitizer sensor likely to include output caused by touch of a passive pen tip;
• apply a maximum likelihood cost function at points within the area to identify likely coordinates of the passive pen tip on the digitizer sensor;
• and
• select most likely coordinates for the location of the passive pen tip,
• wherein the most likely coordinates are defined based on the output signal in the area and the response function.

A method for mapping a relationship between a signal and a location of a pen tip touching a sensing surface of a grid based digitizer sensor, the method comprising:

• (a) providing a grid sensor;
• (b) providing a pen;
• (c) placing a tip of the pen at a plurality of locations relative to a first junction of the grid sensor;
• (d) measuring a signal produced by the tip of the pen at the grid sensor junction when the tip of the pen is at each one of the plurality of locations;
• and
• (e) mapping a relationship between the signal and the plurality of locations.