Since the advent of the iPhone in 2007, the range of applications for inductive capacitive touch screens has continued to expand. Despite this, the integration of inductive capacitive touch screens into devices still presents significant challenges, especially in environments where liquid crystal displays (LCDs) and peripheral devices are interfering and noisy. One of the effective solutions is to use a high signal-to-noise ratio (SNR) touch screen controller to combat noise. A high SNR controller will have other advantages, as described in more 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 at the same load, the SNR can be obtained by calculating the square of the root mean square (RMS) of the amplitude. The value of the power ratio (PS/PN) is usually large and is usually described in logarithmic (dB). The SNR can be expressed as:
SNRdB = 10log10(PS/PN) = 10log10(RMSS/RMSN)2= 20log10(RMSS/RMSN)
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 devices: the touch screen sensor and the touch screen controller. There are many types of touch screen sensors, and their names indicate their shapes and structures, such as triangles, diamonds, snowflakes, bars, and so on. For example, "diamond" is a diamond-shaped grid structure, and "bar" 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 desired system performance and touch screen controller chip.
The touch screen sensor style and layer structure ("stacking") is typically determined based on the touch screen controller structure to maximize SNR. For example, in a diamond pattern with a single layer of mutual capacitance with crossover (bridge), the distance between the touch screen surface and the X and Y layers of the ITO is the same, which reduces the gain error and makes the SNR of the rows and columns very close. However, it is still necessary to add a shield to prevent the sensor from being disturbed by LCD noise. Using a high SNR touch screen controller can reduce the cost of the touch screen sensor, relax design constraints, and use more styles and layer structures. As will be discussed below, the high SNR touch screen controller can also provide additional benefits such as easier access to the touch center, reduced sensitivity of the touch screen to ambient noise, and the use of gloves or pointed pencils.
Controller architecture
Self-capacitance and mutual-capacity 1 are two main capacitive touch screen sensing detection technologies. The characteristics of self-capacitance and mutual capacitance are summarized as follows:
Self-contained
Early technology still in use today.
Limited to "ghost points" (incorrect touch locations relative to real touch locations), usually a touch or two touch.
Diamond patterns are most common.
The LCD noise suppression is poor.
Simple, low cost controller.
Mutual capacity
A new generation of designs that are capturing the market.
Real two or more touches.
Higher precision.
Sensor style design is more flexible, which helps maximize SNR.
Better noise suppression.
More complex, high cost controllers.
Many applications require only one or two contacts, so the self-contained solution is more attractive, especially when the touch position of the user interface is controllable to eliminate "ghost points". The typical SNR of a self-contained solution exceeds 30 dB, and it is usually necessary to add a shielding layer between the LCD and the bottom of the touch layer of the sensor, which increases the cost and reduces the display brightness.
Other techniques can be used in a self-contained scheme to further increase the SNR. This includes (a) increasing the number of samples per channel; (b) increasing the sensor drive voltage, which increases the amplitude of the signal at a fixed noise (such as noise from the LCD); (c) sampling at different frequencies to avoid fixed frequency interference, Such as avoiding 60Hz (this is called "frequency jitter"). However, this technique usually reduces the frame rate and increases power consumption, both of which are undesirable.
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