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Dealing with internal noise in touchscreens

Posted: 18 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:multitouch? projected-capacitive? touchscreens? noise?

Enabling multitouch systems that perform with the precision today's users expect, while still dealing with demanding environmental conditions, is not an easy task. 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.

Figure 1: Architectural differences between the flyback and ringing-choke charger topologies.

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 in this space, aftermarket manufacturers are dropping every last cent out of these chargers. The result of low-cost electronics is a charger that will charge your phone but may inject so much noise into your touchscreen that it becomes unusable.

Two of the most widely used types of battery chargers are the ringing-choke converter and the flyback converter. The fly-back converter charger typically uses a pulse-width modulation (PWM) circuit whereas the ringing choke converter is a very low-cost, self-oscillating variant of the flyback design. Figure 1 illustrates the architectural differences between the two topologies.

It is clear that, with the ringing choke converter, much has been pulled out from the flyback converter. There is no longer an MCU nor a Y-cap, while there is a lack of PWM control, a lower-cost transformer, fewer diodes, and lower-capacitance polarized-input capacitors. This equates to quite a bit of cost savings for the manufacturer. The result for the end-customer, however, is a very noisy system.

Some ringing-choke converter chargers are on the verge of becoming classified as broadband noise generators, as they are putting out as much as 40Vpp noise ranging from 1kHz up through nearly 100kHz. Most end up having more periodic noise tendencies with many harmonics. A good example is the "Zero Charger," which has become a well-known challenge in the industry. This device has been measured to output anywhere from 10 to 25Vpp. Figure 2 is a look at measurements of this device with varying loads.

Figure 2: Noise measurements of the "Zero Charger" device at varying load. (Click on image to enlarge.)

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