Due to the large number of noise sources, the contact screen controller needs to be adapted to the different noise sizes and types that exist in the system for a given time. To ensure the highest robustness in noise immunity, the first factor to focus on is the signal-to-noise ratio (SNR). There are several different characteristics that can be used to improve the signal-to-noise ratio.
One of the main ways to improve the signal-to-noise ratio is to use very high emission voltages to scan the sensors on the contact screen. The original SNR is proportional to the transmit voltage, so the larger the better. Historically, high-voltage emissions have been a challenge for many touchscreen controllers, and can only be supported by the use of an external high-voltage analog power supply (which sometimes adds a significant amount of power and is not supported by most consumer handheld devices), perhaps with the option of larger and more expensive external components such as switching regulators. Both of these options add to the cost of the equipment. Now, the new touch screen controller can generate an on-chip high-voltage emission through an internal charge pump.
Another way to improve SNR is to use a dedicated hardware acceleration mechanism. While it's important to ensure contact under noisy conditions, CPU-intensive running noise filtering algorithms will slow down the refresh rate and then improve power consumption. Cypress' Tx-Boost skill is a good example of how to improve the signal-to-noise ratio under noisy conditions by using proprietary hardware that works in parallel with the CPU, which can adhere to the target refresh rate and power consumption, and can improve the existing SNR by a factor of three.
The scanning frequency of the contact sensor can have a significant impact on the contact function in a noisy environment. If the noise frequency is close to the frequency of the scanned panel, contact data corruption may occur. In this case, we can use the adaptive frequency hopping technique to change the scan frequency to a level where the noise fluctuation is low enough to prevent data corruption. However, frequency hopping has a limited effect and depends on the magnitude of the emission frequency that can be selected and the magnitude of the frequency where the noise is present. Some chargers emit a lot of noise over the entire frequency scale, making it difficult to find interference-free areas. The base frequency of the larger charger noise is 1kHz to 300kHz, and the harmonic fluctuations are lower at higher frequencies. We can use high-frequency scanning in the 300kHz to 500kHz scale to deal with this problem, and then completely prevent the highest fluctuations in the noise band and some harmonics at the beginning. In addition, this approach can improve the noise immunity of the display at a distance from the LCD noise frequency scale.
While there are many skills to improve SNR, if the noise is indeed so high that it completely fills the acceptance channel of the touch screen controller, then the above improvements will not prevent the damage of contact data. Signal processing needs rely on an analog front-end that outputs linear results. If the output is continuously locked to the maximum due to the many charges coupled to the noise source, the contact screen may not be usable at all. To deal with this, we can increase the size of the acceptance channel so that it can handle a much larger amount of charge. This often adds additional chip area, which means that the capacitance is larger. Another way to deal with this is to split the original signal before accepting the channel, and then reduce the noise, but we must also be aware that this will also separate the signal from the finger itself.
Display and charger noise is not a new problem, but noisy chargers and thinner displays are a problem that must be faced by contact screen controllers to improve noise immunity. In order to cope with higher fluctuations in noise, today's controllers use a combination of features to improve the signal-to-noise ratio and prevent noise as much as possible. At the end of the day, consumers expect the device's contact function to remain common and not be compromised by connecting to a charger or being near a noisy fluorescent lamp. As noise challenges continue to evolve, contact screen controllers will continue to evolve to ensure that they provide a common feature at all times.
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