Cancellation of common-mode signals in digitizing tablet

• US 5,945,639 A
• Filed: 02/04/1994
• Issued: 08/31/1999
• Est. Priority Date: 02/04/1994
• Status: Expired due to Term

• Current Assignee: Taiwan Semiconductor Manufacturing Company Limited
• Original Assignee: Hyundai Electronics America Incorporated (SK Hynix Inc.)
• Inventors: Partow, Tony S., Petty, William K.
• Primary Examiner(s): Vo, Don N.
• Application Number:
  US08/192,078
• Time in Patent Office: 2,034 Days
• Field of Search: 178/18, 178/19, 178/18.03, 178/19.05, 178/19.01
• US Class Current: 178/18.03
• CPC Class Codes: G06F 3/0444 using a single conductive element covering the whole sensing surface, e.g. by sensing the electrical current flowing at the corners

Abstract

The invention concerns apparatus for processing signals induced by a stylus applied to a digitizing tablet. The tablet produces four signals, A, B, C, and D, indicative of four currents, induced at four different points on the tablet, by the stylus. The four signals indicate the position of the stylus. The invention derives stylus position based solely on two difference pairs (A-D) and (B-C). Further, the computation of the difference-pairs suppresses common-mode noise, which otherwise interferes with computation of stylus position.

Claims

1. In a digitizing tablet which produces four signals indicative of the distances of a stylus from four respective comers of the tablet, the improvement comprising:

a) computing x-y position of the stylus using a computation based on differences between four pairs of distance signals; and

b) deriving said differences using difference amplifiers which suppress common-mode signals in the distance signals.

2. A process for computing position of a stylus on a digitizing pad, comprising the following steps:

a) producing signals A, B, C, and D by four detectors, located at respective corners of the tablet, in response to a signal produced by the stylus;

b) using one or more difference amplifiers having a high CMRR, subtracting

C from A, to produce (A-C),

D from B, to produce (B-D),

D from A, to produce (A-D),

B from C, to produce (C-B), and

c) producing the following sums;

SUM 1;

(A-C)+(B-D) and

SUM 2 (A-D)+(C-B); and

d) Dividing both SUM 1 and SUM 2 by the quantity (A+B+C+D).

3. A process for computing position of a stylus on a digitizing pad, comprising the following steps:

a) producing signals A, B, C, and D by four detectors, in response to a signal produced by the stylus;

b) using one or more amplifiers to generate a plurality of difference signals from said signals A, B, C and D; and

c) adding together at least two of said difference signals to generate a first sum and adding together at least two of said difference signals to generate a second sum.

4. The method of claim 3 further comprising the step of dividing at least one of said first sum and said second sum by the quantity (A+B+C+D).