N-trig's Dual-Mode Pen and Touch

How it all works

By Geoff Walker

N-trig Innovative Technologies, an Israeli startup, has developed a dual-mode pen and touch digitizer aimed at the Tablet PC market. N-trig first showed a public demo of the technology embedded in a Motion Computing Tablet PC at Microsoft's WinHEC 2006 conference. The first product in which the technology will be used is Motion's LE1700 slate; it is expected to be utilized in several other new Tablet PCs later in 2007.

As shown in Figure 1, the heart of N-trig's sensor consists of a two-layer (X-Y) grid of transparent ITO (indium tin oxide) conductors that's shared between finger-touch and pen use. When the touch panel is used in finger-touch mode, it operates exactly like a standard projected-capacitive touch panel. In this mode, the touch panel responds only to fingers. When the touch panel is used in active-pen (high-resolution) mode, it operates as an electrostatic digitizer. This is very different from the standard Wacom electromagnetic digitizer that's used on almost all current Tablet PCs. Since the Wacom is electromagnetic, it requires a low-impedance sensor using copper traces on a printed circuit board; this means the sensor is opaque and must be located behind the liquid-crystal display (LCD). Because the N-trig is electrostatic, it can use a high-impedance sensor using transparent conductors that can be located in front of the LCD.

Figure 1: Architecture of N-trig's dual-mode pen and touch digitizer.

In operation, a pulse from the excitation coil around the periphery of the touch panel energizes the active cordless pen, producing a concentrated electric field at the tip that's detected by the transparent sensor grid. ASICs containing differential amplifiers and switches resolve individual sensor line signals into the differences between the closest sensor lines. The differences are amplified and then converted from analog to digital by high-speed A/D converters in the ASICs. The controller processes the digitized samples, calculates the pen location and outputs ASCII data packets.

Geoff Walker is the founder and principal of Walker Mobile, LLC, a technical marketing consulting firm specializing in mobile computing.