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Dual Ramp Analog to Digital Converter with Divide Function for Graphic Tablet

IP.com Disclosure Number: IPCOM000051164D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Dym, H: AUTHOR

Abstract

The analog to digital converter that is used in a capacitive graphics tablet is arranged to perform an analog division that normalizes the position signal from the pen (or stylus). This function is Coordinate Value = M Vx /VrS, where Vx and Vr are the pen position signal and a pen reference signal, respectively, M is a scaling factor, and S is an offset.

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Dual Ramp Analog to Digital Converter with Divide Function for Graphic Tablet

The analog to digital converter that is used in a capacitive graphics tablet is arranged to perform an analog division that normalizes the position signal from the pen (or stylus). This function is Coordinate Value = M Vx /VrS, where Vx and Vr are the pen position signal and a pen reference signal, respectively, M is a scaling factor, and S is an offset.

A graphic tablet is a device that produces X and Y coordinate information when a user of the tablet holds a pen at a selected point on the tablet. In some tablets, a voltage gradient is established along the X dimension (and separately for the Y dimension), increasing linearly from zero volts at the left margin to a maximum value Vmax at the right margin. In one step a voltage Vx is sensed at the point where a user positions a pen. The voltage Vx varies as a function of the pen position along the X dimension, but it also varies with other factors such as the height that the pen is separated from the surface of the tablet by intervening layer paper. To normalize the position voltage, in a second step the tablet is given the voltage Vmax at all positions so that the pen senses a reference voltage Vr. The quotient Vx/Vr multiplied by a scaling factor M is the fractional distance of the pen between the minimum and maximum points of the Vx voltage gradient. A term S shifts this range to the range of the left and right margins of the tablet.

A conventional analog to digital converter receives the voltage Vx (or Vr) and converts it to a proportional current Ic = k1 Vx that charges a capacitor at a linear rate dV/dt = Ic/C. After a fixed time interval Delta T, the capacitor voltage is voltage Vc = Delta T k1 Vx/C. Conventionally, the capacitor is discharged to zero volts at a fixed rate but over a variable time that is a function of the original capa...