Noise wars: Projected capacitance strikes back against internal noise

November 08, 2011 //By John Carey
Noise wars: Projected capacitance strikes back against internal noise
John Carey, Director of TrueTouch Marketing, Cypress Semiconductor Corp. illustrates how projected capacitance can be used to strike back against internal noise in touchscreen ICs.

(Editor's note : Touch sensing and touchscreens are extremely popular topics with our audience; for your convenience, you can see a linked list of all articles we have published on this topic here.)

 Enabling multi-touch systems that perform with the precision today’s users expect, while still dealing with demanding environmental conditions, is no small feat. This challenge is heightened given that the internal environment is rapidly changing. In the war for touchscreen dominance, new battlegrounds are emerging.

One current trend is the push to make phones thinner. This means direct lamination of capacitive touch sensors to the display, migration of the sensor inside the display, and many other challenges with antennas and ground loading. Gone are the days where it was acceptable to just throw a shield layer onto the sensor structure to block display noise. This adds too much cost and thickness.

Beyond displays, the prevalence of USB charging connectors has commoditized battery chargers, pulling every last cent from these devices. Capacitive touchscreen ICs are now expected to sense picocoulombs of change in the presence of up to 40Vpp AC noise.

All of these factors add up to requirements for touchscreen ICs that are far more complex than what was required only last year. New innovations are required, and so begin the noise wars.

Charger Noise

Charger noise is one of the most talked-about noise sources related to capacitive touchscreens. This is noise that is physically coupled into the sensor through the battery charger during the presence of touch. It can be seen as degraded accuracy or linearity of touch, false or phantom touches, or even a touchscreen that just becomes unresponsive or erratic. The culprit is typically an aftermarket, low-cost charger.

While the OEM chargers designed to work with a particular phone have tighter specifications on noise, the widespread adoption of USB connectors for charging circuits has created a massive aftermarket opportunity. Fighting to compete

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