DIGITIZED WRITING BOARD AND METHOD FOR COMPUTATION OF POSITION OF STYLUS ON IT

PURPOSE: To reduce common mode noise by obtaining the difference of a pair of signal tubes generated by an I-to-V amplifier by subtraction in the amplification of the difference or the like. CONSTITUTION: Four different amplifiers D1-D4 generate the four pairs of the difference corresponding to the four different pairs in the calculation of the (x) and (y) coordinates of a stylus 6. Actually, the outputs of the respective differential amplifiers D1-D4 are equal to (R2/R1) (V+-V-). In this case, V+ indicates the noninverted input of an operational amplifier and Vindicates inverted input. At the time of R1=R2, the output is equal to a true difference V2-V1. When R1 is not equal to R2, the difference is roughly calculated by R2/R1. The pairs of the difference are sent to processing circuits. The processing circuits can be in the form of a programmable digital computer or a dedicated calculation circuit for utilizing the pairs for the calculation of the (x) and (y) coordinates used in (x) and (y) calculation.