Design Challenges of Inductive Capacitive Touch Systems from a Controller Perspective

Since the iPhone appeared in 2007, the application scope of inductive capacitive touch screens has been expanding. However, there are still great challenges in truly integrating inductive capacitive touch screens into devices, especially in liquid crystal displays (LCDs), peripheral devices that generate interference and noisy environments. One effective solution is to use a touch screen controller with a high signal-to-noise ratio (SNR) to combat noise. A high SNR controller will also have other advantages, which will be described in detail below.

SNR is defined as the power ratio of the signal (useful information) to the noise (unwanted signal). If the signal and noise are measured under the same load, the SNR can be obtained by calculating the square of the root mean square (RMS) amplitude. The value of the power ratio (PS/PN) is usually large and is usually described in logarithms (dB). SNR can be expressed as:

A high SNR means that the measured signal strength is higher than the background noise.

Overall touch performance

The overall touch performance is determined by two components: the touch screen sensor and the touch screen controller. There are many types of touch screen sensors, and their names vividly describe their shapes and structures, such as triangle, diamond, snowflake, stripe, etc. For example, "diamond" is a grid structure of diamonds, while "strip" is a grid of rows and columns, like a city street. Some sensor types use one layer of ITO, while others require two or three layers, depending on the required system performance and touch screen controller chip.

The touchscreen controller architecture often determines the touchscreen sensor pattern and layer structure ("stack") to maximize SNR. For example, in a single-layer mutual capacitance diamond pattern with crossovers (bridges), the distance from the touchscreen surface to the X and Y layers of the ITO is the same, which reduces gain error and makes the row and column SNRs very close. However, an additional shielding layer is still needed to protect the sensor from LCD noise. Using a high-SNR touchscreen controller can reduce the cost of the touchscreen sensor, relax design constraints, and use more patterns and layer structures. As will be discussed below, a high-SNR touchscreen controller can also provide additional benefits, such as easier to find the center of the touch, reduced sensitivity of the touchscreen to ambient noise, and allowing the use of gloves or fine conductive pens.

Controller Architecture

Self-capacitive and mutual-capacitive[1] are two main capacitive touch screen sensing technologies. The characteristics of self-capacitive and mutual-capacitive are briefly summarized as follows:

Self-contained

●Early technology still in use today.

●Limited by "ghost points" (false touch locations relative to true touch locations), usually one or two touch points.

●The diamond shape is the most common.

●Poor LCD noise suppression.

●Simple, low-cost controller.

Mutual Capacitance

●A new generation of designs that is taking over the market.

●True two-point or multi-point touch.

●Higher precision.

●More flexibility in sensor pattern design, which helps maximize SNR.

●Better noise suppression.

●More complex, high cost controller.

Many applications require only one or two touch points, so self-capacitive solutions are more attractive, especially when the touch position of the user interface can be controlled to eliminate "ghost points". Self-capacitive solutions have a typical SNR of more than 30dB, and usually require a shielding layer between the LCD and the bottom of the touch layer of the sensor, which increases cost and reduces display brightness.

Other techniques can be applied to the self-capacitive solution to further improve SNR. These include (a) increasing the number of samples per channel; (b) increasing the sensor drive voltage, which increases the signal amplitude in the presence of fixed noise (such as noise from the LCD); and (c) sampling at different frequencies to avoid fixed frequency interference, such as avoiding 60Hz (this is called "frequency dithering"). However, this technique usually reduces the frame rate and increases power consumption, both of which are undesirable.

From the above discussion, it is clear that in order to maximize SNR and support two or more touch points, mutual capacitance is the most promising sensing technology. The system block diagram of Figure 1 summarizes the implementation of mutual capacitance, that is, an excitation signal is added to one pole of the touch screen sensor capacitor, and the other pole is connected to the analog front end (AFE) of the touch screen controller. The output of the AFE is converted into a digital format and further processed in the digital signal processor (DSP).

Figure 1. Block diagram of a mutual capacitance system

Design Challenges

There are many technical challenges when integrating capacitive touch screen sensors into touch-enabled devices. The following situations can all benefit from a touch screen controller with high SNR.

Sensor layer design: There are a variety of touch screen sensor layer structures today, corresponding to different requirements for materials, thickness, performance and cost. As shown in Figure 2. Single-layer or multi-layer substrate, "up" or "down" architecture, changes in X and Y layer thickness, changes in optical clear adhesive (OCA) thickness and other factors will affect the signal amplitude generated by the sensor. Since high SNR touch screen controllers can handle a wider dynamic range of touch screen sensor signals, the impact caused by structural differences will be weakened. This gives designers more freedom to choose the sensor layer structure.

Figure 2. Layer structure of a mutual-capacitive touchscreen sensor (not to scale).

Thick protective cover: Some applications, such as bank ATM machines, may require a thick glass cover to prevent the display from being damaged. However, thick glass covers will reduce the touch signal strength and reduce the accuracy of touch position detection. This is because the distance between the finger and the touch screen sensor becomes larger, resulting in a larger capacitance range and lower signal amplitude, making it difficult to determine the exact touch position. Wearing gloves will also have the same effect.

LCD VCOM type: LCD VCOM is the "common mode voltage" and is the reference voltage of the LCD screen. Depending on the system requirements, AC VCOM or DC VCOM may be used. AC VCOM is alternating, while DC VCOM is a constant voltage. The former method will generate more noise.

Air gap between touchscreen sensor and protective lens: One of the most common problems reported by users of touch devices is a broken protective lens. To make the product thinner, capacitive touchscreen sensors can be pressed onto the back of the protective lens, but when replacing a broken protective lens, the touchscreen sensor must also be replaced, which increases repair costs. To avoid this cost, as well as the cost of low yields in the lamination process, a gasket is usually used to separate the touchscreen sensor from the protective lens.

However, when there is an air gap between the touch screen sensor and the protective lens, it is difficult for the touch screen sensor to detect a finger touch because the dielectric constant of air is low and the signal strength generated by a finger touch is also low. One way to solve this problem is to increase the sensitivity threshold of the touch system, but this is dangerous because the sensor will receive some stray signals, such as LCD or other environmental noise, making it difficult for the touch screen sensor to distinguish touch actions from the noise.

Industrial design requirements: Some device manufacturers make the touch screen sensor directly on the display to make the overall design thinner. But this is also risky because the touch screen sensor is placed directly on the noise source. One solution is to add a shielding layer between the touch screen sensor and the display. However, adding an extra layer of ITO will increase the overall material cost and affect the light transmittance.

Integrated touch screen sensor: In order to reduce production costs, LCD manufacturers began to make touch screen sensors directly on the color filter under the polarizer. This method does not require external sensors and lamination, but the touch screen sensor is closer to the display, further increasing the noise received by the sensor.

Touchscreen controller location: Capacitive touchscreen controllers are usually located on the touchscreen cable (chip on wire or PCB), and sometimes directly on the touchscreen sensor (chip on glass). However, for testing purposes, some designs require the touchscreen controller to be placed on the system board. This may require a long flexible printed circuit (FPC) to connect the touchscreen sensor and controller. The long FPC will act as an antenna and easily absorb noise, making it difficult for the touchscreen controller to process the weak signal from the touchscreen sensor.

Other noise sources: The main noise sources of mobile devices are LCD screens, LCD inverters, WiFi antennas, GSM antennas, and various high-speed circuits in the device. Environmental noise also has a great impact on the touch system. For example, some AC power supplies will generate strong noise, which will be transmitted through the AC adapter. Similarly, when the device is placed near a strong noise source such as a desktop fluorescent lamp, the touch system will mistake the noise for a valid touch behavior.

Under normal conditions, for normal-sized fingers (>7mm), high-SNR controllers do not have much advantage over low-SNR controllers. The advantage only becomes apparent in strong noise environments, such as when using a writing pen or a gloved finger input, when the signal is very weak. Low-SNR controllers cannot distinguish the signal from the noise. If the sensor threshold is lowered to increase the detection sensitivity, the touch system will be easily triggered by mistake, causing misoperation, which is absolutely not allowed in practical applications.

Application Challenges

Touch accuracy: Touch accuracy is an important indicator in touch screen sensor design. For example, in virtual keyboard applications, characters are compactly arranged in a small area, and it is critical to accurately respond to touch actions and avoid mis-entering characters. One way to improve accuracy is to add more sensor channels to the controller to support a higher touch screen sensor grid density. But this will come at a cost because both the touch screen sensor and the touch screen controller require more pins. In addition, more sensor channels require more traces to be added to the touch screen border, which increases the border width.

A high SNR touchscreen controller can enhance detection accuracy because it has a stronger ability to detect weak signals and collects sample data from a larger peripheral range. The larger detection range provides more reference points so that the touch position can be accurately calculated. Figure 3 reveals the impact of the touchscreen controller SNR on the accuracy of the line drawn by a robotic arm holding a 4mm metal sheet. The line drawn by the high SNR controller is obviously smoother than the line drawn by the low SNR controller. Note that these measurements were recorded from the same touchscreen sensor and the same post-processing software to ensure a fair comparison.

Figure 3. A robot arm holding a 4mm metal sheet and drawing a straight line. The left side uses a high SNR touch screen controller;

On the right, a low SNR touchscreen controller is used.

Stylus: Users of resistive touch screens have long been accustomed to using a stylus with a sharp point. The tip of a typical resistive touch screen stylus is less than 1mm in diameter and is usually made of non-conductive plastic. It is difficult for a capacitive touch system to detect such a small, non-conductive device because the signal it can provide to the touch screen controller is very weak. Many touch systems on the market use stylus tips with large diameters (3-9mm), making writing and drawing difficult because the thick tip makes the writing traces blurry.

A high SNR touchscreen controller can detect a stylus with a 1mm diameter tip, as long as the stylus is wrapped in a conductive material (a relatively small sacrifice). Figure 4 illustrates the effect of touchscreen controller SNR on the detection of a stylus with a 2mm conductive tip. A low SNR controller will have a difficult time distinguishing a small tip stylus from the background noise, especially on the noisiest parts of the screen. Using a 1mm tip stylus with a low SNR will result in the useful signal being buried in the background noise, rendering the stylus unusable.

Figure 4. Capacitance profiles of a 2mm conductive stylus on a 4-inch screen, with a high-SNR touchscreen controller on the left and a low-SNR touchscreen controller on the right. The stylus is located at the top of the green cone; the height of the white plane represents the background noise. The increase in signal-to-noise ratio effectively reduces the background noise amplitude, as shown on the left. If the stylus in the right image moves to the left side of the screen, the signal will be overwhelmed by the noise and the stylus will not work.

Non-contact detection: Proximity detection is increasingly being adopted in touchscreen applications. For example, by increasing the sensitivity of the touch system, when using an e-book, the user can turn the page with a gesture without actually touching the screen. However, the increased sensitivity of the touch system is also easily triggered by environmental noise, and designers have been working hard to find the best balance to maximize the proximity distance without causing false triggers. Mitsubishi has done some interesting research in this area, and they have built a touch system that automatically adjusts the sensitivity based on whether the touching finger is suspended or actually touching.

Glove operation: In medical applications, touch screens need to work with surgical gloves. Similarly, car touch screen GPS needs to work with gloves in winter. Most gloves are made of dielectric materials, which makes it difficult for touch screen sensors to detect touch actions. Increasing the sensitivity of the touch screen controller may cause false triggers when the user is not wearing gloves. The only solution is to require the application (or user) to select different sensitivities according to the situation.

in conclusion

High SNR capacitive touch screen controllers have many advantages. They can meet a wide range of design and application requirements such as writing pens, small fingers and gloves. It can help improve touch accuracy without the need for specialized ITO sensor styles or additional sensor channels. It can meet the requirements of various displays and different backlights while maintaining good touch performance, and it provides more flexible options for sensor design and production. It enables the touch system to work in a strong noise environment and reduces the noise of the device itself from the LCD, WiFi antenna, GPS antenna and AC adapter. It gives device OEMs more freedom to choose components. Finally, from a performance point of view, it provides precise touch accuracy. In short, high SNR touch screen controllers can help end users achieve more reliable applications.