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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?

Note that the AMOLED display is outputting up to 30mVpp in the peak spikes. This is 1% of the noise from an ACVCOM display and greatly helps with touchscreen design. Integrating the sensor inside the physical display to create an on-cell or in-cell topology is also straightforward with this type of display. However, it is much more expensive than a traditional LCD.

With on-cell designs, the sensor layer is physically deposited on top of the color filter glass inside the display. This brings it much closer to the chemistry of the display, since it is now physically inside the stackup. Not only does the noise increase, but so does the parasitic loading. However, AMOLED is inherently quiet, and makes for a very good platform for on-cell or in-cell (sensor beneath the color filter glass) design.

So how do touchscreen ICs deal with display noise when they can't use an air gap or a shield layer? When designing sensors in PET, one well-accepted sensor structure is to use a two-layer sensor where the Tx lines are in the lower part of the sensor and the Rx lines are in the top. As the Rx lines are sensitive to display noise, the wide Tx lines in the bottom of the sensor form a barrier against the noise generated in the display. This effectively builds shield functionality into the sensor pattern. Figure 6 shows the various types of sensor structures.

Figure 6: Multilayer buildup and structure of three common touch-screen sensor technologies.

MH3 is the dual-layer stackup referenced, where the bottom layer of ITO acts as a shield to display noise. Unfortunately, this solution is not often used in glass-based sensors, and it still increases thickness and cost.

As such, the industry is pushing to build sensors on a single substrate layer with no shield. To enable true single-substrate-layer sensors without shielding requires the touchscreen IC to be resilient to display noise. This is not an easy task, as display noise can easily reach 3Vpp in AC or DCVCOM type displays.

Display noise can be mitigated even in direct lamination (where the sensor structure is laminated to the top of the display with no air gap or shield) or display-integrated types of designs. An example of this is Cypress's Display Armor method to combat display noise. By integrating a built-in listening channel to the touchscreen device, touchscreen ICs can eliminate display noise in two distinct ways.

One way is to make advanced algorithmic decisions on what information is noise vs. data. Another is to detect the noise source and latch on to the waveform such that capacitive measurements are made during quiet times. Either way, the result is advanced and thinner capacitive touchscreen stackups at lower costs.

Aside from noisy displays and chargers, many other challenges face capacitive touchscreen designers today. Antennas are huge sources of noise challenges. With the real estate within a phone becoming further constrained, components are literally being placed on top of each other. This is the case with antennas and the touchscreen sensor.

Such design challenges can create real issues in dealing with that portion of the touchscreen. Fortunately, the same innovations that are helping display and charger noise are also helping with other noise sources, such as antennas. Whether it is simple IIR filters, advanced nonlinear-filtering methods, built-in noise-avoidance hardware, hopping capabilities, or any other methods, capacitive touchscreens enable some of the most-advanced functionality in the whole embedded device.

While the projected capacitive touchscreen controller space is going to continue to evolve, it is clear that noise immunity is one of the biggest concerns for designers. Whether it is noise from displays, chargers, antennas or other sources, touchscreen ICs are required to perform with the same level of user experience. Innovation is happening daily in capacitive touch, and touchscreen ICs continue to wage the war against noise.

About the author
John Carey is Director of Marketing, TrueTouch Technology, for Cypress Semiconductor Corp. He holds a Master's Degree in Electrical Engineering from California State University, as well as a Bachelor's Degree in Electrical Engineering from Arizona State University.

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