Direct detect sensor for flat panel displays

Abstract

Each sensor of a linear array of sensors includes, in part, a sensing electrode and an associated feedback circuit. The sensing electrodes are adapted to be brought in proximity to a flat panel having formed thereon a multitude of pixel electrodes in order to capacitively measure the voltage of the pixel electrodes. Each feedback circuit is adapted to actively drive its associated electrode via a feedback signal so as to maintain the voltage of its associated electrode at a substantially fixed bias. Each feedback circuit may include an amplifier having a first input terminal coupled to the sensing electrode and a second input terminal coupled to receive a biasing voltage. The output signal of the amplification circuit is used to generate the feedback signal that actively drives the sensing electrode. The biasing voltage may be the ground potential.

1 . An apparatus operative to test an array of pixels formed on a panel, the apparatus comprising:
 a first linear array of N sensors each sensor comprising:   a sensing electrode adapted to be capacitively coupled to a pixel electrode disposed on the panel; and   an associated feedback network configured to maintain the voltage of the sensing electrode at a substantially constant voltage when the sensing electrode is positioned in proximity of the pixel electrode to be capacitively coupled thereto, said first linear array of sensors adapted to be scanned over the panel at a continuous rate.   
   
   
       2 . The apparatus of  claim 1  wherein said feedback network comprises:
 an operational amplifier comprising a first input terminal coupled to the sensing electrode and a second input terminal coupled to receive a first voltage supply, wherein an output signal of the operational amplifier is used to generate a feedback signal adapted to drive the first input terminal of the operational amplifier.   
   
   
       3 . The apparatus of  claim 2  wherein said first voltage supply is the ground potential. 
   
   
       4 . The apparatus of  claim 2  wherein said feedback network further comprises a capacitive elements disposed between the first input terminal of the amplifier and an output terminal of the amplifier. 
   
   
       5 . The apparatus of  claim 2  further comprising:
 a second linear array of N sensors spaced at a distance D1 away from said first linear array of N sensors, said second linear array of N sensors configured to be scanned over a row of the panel pixels at a known time period after the first linear array of sensors are scanned over the row of the panel pixels, wherein said second linear array of sensors are adapted to capacitively measure voltages present on the panel pixels.   
   
   
       6 . The apparatus of  claim 5  further comprising:
 a third linear array of N sensors spaced at a distance D2 away from said first linear array of N sensors, said third linear array of N sensors configured to be scanned over a row of the panel pixels at a known time period after the first linear array of sensors are scanned over the row of the panel pixels.   
   
   
       7 . The apparatus of  claim 6  wherein each of the sensors of the second and third linear arrays comprises:
 a sensing electrode adapted to be capacitively coupled to a pixel electrode disposed on the panel; and   an associated feedback network configured to maintain the voltage of the sensing electrode associated therewith at a substantially constant voltage when positioned in proximity of the pixel electrode to be capacitively coupled thereto.   
   
   
       8 . The apparatus of  claim 6  wherein each of the feedback networks of the second and third linear array of sensors comprises an operational amplifier comprising a first input terminal coupled to the associated sensing electrode and a second input terminal adapted to receive a first biasing voltage, wherein an output signal of the operational amplifier is used to generate a feedback signal adapted to drive the first input terminal of the operational amplifier. 
   
   
       9 . A method of testing a panel having formed thereon a plurality of pixels, the method comprising:
 capacitively coupling a sensing electrode to a pixel electrode at time T 1  to sense the pixel electrode voltage; and   maintaining said sensing electrode pixel at a substantially constant voltage via a feedback signal generated in accordance with the sensed pixel electrode voltage, wherein said sensing electrode is disposed in a linear of array of sensors.   
   
   
       10 . The method of  claim 9  further comprising:
 supplying the sensed pixel electrode voltage to a first input terminal of an amplifying circuit; and   generating the feedback signal from an output voltage generated by the amplifying circuit.   
   
   
       11 . The method of  claim 10  wherein said amplifying circuit comprises an operational amplifier. 
   
   
       12 . The method of  claim 10  further comprising:
 supplying a biasing voltage to a second input terminal of the amplifying circuit.   
   
   
       13 . The method of  claim 12  wherein said biasing voltage is the ground potential. 
   
   
       14 . The method of  claim 12  further comprising:
 capacitively coupling the output terminal of the amplifying circuit to the first input terminal of the amplifying circuit.   
   
   
       15 . The method of  claim 10  further comprising:
 capacitively coupling a second sensing electrode to the pixel electrode at time T 2  to sense the pixel electrode voltage; wherein T 2  and T 1  are spaced in time by a predefined value; and   maintaining said second sensing electrode pixel at a substantially constant voltage via a feedback signal generated in accordance with the pixel electrode voltage sensed by the second sensed electrode.   
   
   
       16 . The method of  claim 15  further comprising:
 capacitively coupling a third sensing electrode to the pixel electrode at time T 3  to sense the pixel electrode voltage; wherein T 3  and T 1  are spaced in time by a predefined value; and   maintaining said third sensing electrode pixel at a substantially constant voltage via a feedback signal generated in accordance with the pixel electrode voltage sensed by the third sensed electrode.   
   
   
       17 . The method of  claim 16  wherein each of the second and third sensors further comprises:
 a sensing electrode adapted to be capacitively coupled to the pixel electrode disposed on the panel; and   an associated feedback network configured to maintain the voltage of the sensing electrode associated therewith at a substantially constant voltage when positioned in proximity of the pixel electrode to be capacitively coupled thereto.   
   
   
       18 . The method of  claim 9  wherein said pixel electrode receives a DC voltage before being capacitively coupled to the sensing electrode.