(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 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.
Figure 1: Architectural differences between the
flyback and ringing-choke charger topologies
(to see an enlarged image, click here).
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
(to see an enlarged image, click here).
It is easy to see the various harmonics generated from this charger, and peak voltages can range up to 25Vpp. It is also noteworthy that its response is quite dependent on the battery state itself. To fight against this phenomenon, many OEMs banded together to create EN specifications that govern the maximum noise levels a charger should emit at any frequency. EN 62684-2010 and EN 301489-34v1.1.1 govern these noise levels and can be seen in Figure 3.
Figure 3: Maximum noise levels as defined by EN 62684-2010 and EN 301489-34v1.1.1
From 1kHz through 100kHz, a charger is expected to output no more than 1Vpp noise, and the levels degrade exponentially from there as the frequency increases. While this specification is stringent, none of the aftermarket chargers conform. As such, OEMs are now asking touchscreen ICs to deal with noise at much higher levels. Some specifications require 40Vpp from 1kHz through 400kHz, with 95Vpp immunity in the 50-60Hz range.
Fortunately, there are specialized algorithms and methodologies on the market, such as Charger Armor from Cypress, which can meet stringent requirements and provide upwards of 95Vpp noise immunity to battery chargers. These levels are achieved through a variety of mediums such as nonlinear filtering, frequency hopping, and other hardware methodologies.
Displays offer many challenges for projected-capacitive touchscreen systems. This is because they can generate quite a bit of noise that can be conducted directly into the capacitive touchscreen sensor. To make matters more difficult, OEMs are demanding thinner industrial designs for their phone models, which means moving the actual touchscreen sensor closer to the display, and even inside it.
For years now, the industry has used a shield layer to protect the sensor from the noise generated by the display. This adds both cost and thickness to any phone, but is quite effective. The industry has also used a small air gap, typically about 0.3mm in height, between the display and the sensor to allow the natural properties of air to dissipate the conducted noise from the display. However, as phones become as thin as possible, neither of these options is ideal for today’s designs.
Fortunately, the levels of noise that are emitted from displays are far lower than the noise levels from chargers. That being said, they are directly coupled and so are still quite difficult to deal with.
With a traditional TFT LCD, the common electrode (VCOM) is driven by either a DC or an AC voltage. An ACVCOM layer is typically used to lower the operating voltage of the display driver while keeping constant the voltage across the liquid crystal. These are part of a relatively low-cost display, and one should expect higher power consumption in this type of display versus DCVCOM, and also a noisier profile. A quick look at a typical waveform of an ACVCOM display is shown in Figure 4.
Figure 4: Typical voltage waveform of an ACVCOM display versus time
Typical ACVCOM-type displays will have noise profiles centered around 10-30kHz, and anywhere from 500mVpp to 3Vpp (as shown in Figure 4). DCVCOM is often a bit quieter. Measuring noise from a display can be done simply by adding a little copper tape to the top of the display and connecting the oscilloscope to that tape, with ground connected to the display’s circuit ground, and running the display to catch the waveforms.
AMOLED is gaining traction in mobile phones. It has a very wide viewing angle and can provide very-bright colors and deep contrast. AMOLED displays are also very quiet, though this functionality does come with a price. Figure 5 is an example of a typical AMOLED display-noise profile.
Figure 5: Display-noise profile of a typical AMOLED shows it is relatively small.
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
(to see an enlarged image, click here).
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. You can reach him at firstname.lastname@example.org.
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