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Interference suppression apparatus for a capacitive keyboard

  • US 4,543,564 A
  • Filed: 03/26/1982
  • Issued: 09/24/1985
  • Est. Priority Date: 04/03/1981
  • Status: Expired due to Fees
First Claim
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1. An apparatus for the suppression of interference in a capacitive keyboard of the type including a series of sensitive keys on an outer surface accessible to a user, each key having a transmitting electrode excited by an alternating signal supplied by a transmitting line, and a receiving electrode selectively capacitively coupled to the transmitting electrode and connected to supply a receiving line, the receiving lines providing a corresponding set of unconditioned operating signals having amplitude variations dependent on the effect of the possible presence of the finger of a user on the sensitive key,said apparatus further comprising means for establishing the difference between the unconditioned operating signal from each receiving line and the signal from a reference line in the keyboard to supply a set of conditioned operating signals free from any unwanted electromagnetic component which has entered the keyboard, said means for establishing the difference including at least one subtractor into which signals from the receiving lines are passed.

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