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Key specs in reliable capacitive touchscreens

Posted: 27 Apr 2010     Print Version  Bookmark and Share

Keywords:capacitive touchscreen  touchscreen design feature  touchscreen specs 

Anyone can bake a cake, but while some chefs bake dry, uninspired bricks of dough, there are other chefs who make cakes we would die for. The ingredients may be the same, but the outcomes are very different.

This is also the case between average electronic products and world-class, market-changing products. One of the most recent technical sensations is the capacitive touchscreen. But what makes some touchscreen-based products amazing, while others get such poor reviews?

This article explores key touchscreen performance parameters, critical touchscreen design features, significant design trade-offs and key issues product designers must consider when choosing their touchscreen supply chain. Don't get caught making an uninspired product; instead, create something people would like to have.

Perhaps the single most significant technology change to affect the performance of today's touchscreens has been the shift from resistive to capacitive touchscreens. Industry analyst iSuppli forecasts that nearly 25 per cent of the mobile handsets with touchscreens will have shifted from resistive to capacitive screens by 2011, while Jeffries and Co. has increased their projections for capacitive touchscreens in 2010 from 100 million units (Mu) to 188 Mu. The market is exploding, in large part because of the benefits that capacitive-touchscreen technology brings.

While traditional resistive touch panels detect a finger or stylus touch when a flexible top layer of clear material is pressed down to contact a lower conductive layer of material, projected capacitance screens have no moving parts. The projected capacitance sensing hardware consists of a glass top layer, followed by an array of X, Y, and insulating layers of indium tin oxide (ITO) on a glass substrate. (Some sensor suppliers create a single-layer sensor that includes both X and Y sensors in a single layer of ITO with small bridges where they cross.)

As a finger or other conductive object approaches the screen, it creates a capacitor between the sensors and the finger. This capacitor is small relative to the others in the system (about 0.5pF out of 20pF), but it is measurable using several techniques. For example, one technique used with Cypress Semiconductor's TrueTouch parts involves rapidly charging the capacitor and measuring the discharge time through a bleed resistor.

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