Experience in solving noise problems in capacitive touch screen applications

Touchscreen devices can be subject to interference from many different noise sources throughout the day, both internal and external. Charger and display noise are two of the most common problematic noise sources today. As charging devices on the market become thinner and noisier, this challenge will only become more difficult to manage. In addition, many other everyday objects can generate noise and cause interference, such as radio signals, AC power, and even fluorescent light ballasts. In the presence of noise, the position reported by a low-performance capacitive touch system can be distorted, affecting accuracy and reliability.

Today’s touchscreen controllers use a variety of different methods to improve the signal-to-noise ratio and filter out bad data from the noise, including on-chip generation of high-voltage transmit signals, specialized hardware acceleration, high-frequency transmit, adaptive frequency hopping techniques, and saturation prevention techniques. However, touch technology continues to evolve in terms of how touch controllers can take advantage of these features, dynamically adapt to the noise present in the system, and accurately track touches under changing environmental conditions.

The effects of injected noise include large jitter (large variations in the reported touch coordinates for a non-moving finger), false reports of finger touches when no finger is touching the screen, not reporting the presence of a finger when it is touching the screen, and even completely locking the device. In the case of a touchscreen phone, this means that the phone cannot be unlocked (because the finger manipulation cannot be reported), or the wrong number is dialed due to jitter or false touches (it is a big problem to call your boss late at night when you meant to call a friend). Figure 1 shows the results obtained by testing finger tracking (for example, drawing a circle with a finger) using the best-selling smartphone on the market today. As noise increases, the position of the finger on the panel is reported incorrectly (in blue), and false touches are detected on the panel (in other colors).

How a touchscreen controller handles the effects of noise can have a significant impact on the quality of the user's touch interface experience. Poor touch performance under noisy conditions can lead to customer dissatisfaction and increased returns. Because there are differences between various types of noise, the touchscreen controller needs to be able to detect, distinguish and deal with these noises, especially the two most problematic noise sources: charger and display noise.

Chargers and common-mode noise

A major problem with capacitive touchscreen devices is that touch performance degrades when the charger emits high levels of high-frequency noise. Some mobile devices have responded to the noisy charger problem by providing limited touch functionality when plugged in, or by displaying a message that the charger cannot be used when the device is connected to a charger that is not designed for the device. These solutions are imperfect at best. A quick look at online forums and message boards shows that the problem of touchscreen devices being affected by charger noise is widespread and has caused some consumers headaches.

The rapid adoption of USB as a standard charging interface in mobile devices has spawned a large number of low-cost aftermarket chargers. Many chargers focus more on cost than performance, using cheap components or lacking specific components that can help reduce common-mode noise.

Common mode noise is created when the power and ground supply voltages of a device fluctuate relative to ground, while maintaining the same voltage difference between them. This fluctuation affects the performance of the touch screen only when a ground-coupled finger touches the screen. The finger has the same potential as ground, and the fluctuations in the phone's power and ground relative to it cause noise to be injected into the touch screen through the finger. The amount of charge injected depends primarily on the peak-to-peak voltage of the noise.

此外，电荷的传输量还受另外两个因素的显著影响：手指和触摸屏之间的接触面积，以及触摸屏覆盖透镜的厚度。这两个因素的影响可通过平行板电容器的电容方程式来理解：

Higher capacitance means more noise injected into the touch screen. In this case, one side of the capacitor parallel plate is formed by the finger contact area and the other side is formed by the receiving electrode of the touch screen sensor. First, as the area of ​​contact between the finger and the touch screen increases, the capacitance also increases proportionally. However, because the receiving electrodes are composed of very narrow rows or columns, it is the diameter of the finger that actually matters (see Figure 2).

Some OEMs use smaller fingers (e.g. 7mm) to test their devices’ immunity to charger noise. However, this does not cover all use cases. A typical finger diameter is 9mm and a typical thumb diameter is 18 to 22mm. If you only test a 7mm finger, you will not be able to ensure common use cases like unlocking the phone or scrolling through a list with your thumb. In fact, if we analyze the difference in diameter, a 22mm thumb injects more than 3 times the charge of a 7mm finger!

The distance (d) between the finger and the receiving electrode is primarily determined by the thickness of the touchscreen cover lens (see Figure 3). Typical cover lens thicknesses range from 0.5 mm to 1.0 mm. This means that a device with a 0.5 mm cover lens has half the "d" of a 1.0 mm cover lens device and twice the capacitance. In other words, a 0.5 mm cover lens injects twice as much noise as a 1.0 mm cover lens. As device form factors move toward thinner and lighter form factors, the thickness of the cover lens and the ability of the touch controller to withstand the increased noise caused by thinner lenses becomes increasingly important.

Although chargers are subject to several product certifications, there are no requirements for common-mode noise. In 2010, a group of mobile phone OEMs agreed to develop a general specification, EN62684, to manage the maximum peak-to-peak voltage allowed for chargers over a frequency range. The specification requires that the noise generated by the charger must not exceed 1Vpp (from 1kHz to 100kHz), and a lower voltage strength is required above 100kHz. Typical off-the-shelf chargers do not follow this guidance.

While lower noise chargers generate noise in the 1–5Vpp range, higher noise chargers can fluctuate in the 20–40Vpp range, which results in a large amount of charge transfer. The amount of injected charge depends on the voltage amplitude of the noise (Q=C*V). Despite the large amount of noise, the touchscreen controller must still be able to detect a finger that causes a small charge change.

Capacitive touch screen phones also face a new type of common mode noise, that is Mobile High Definition Link (MHL), which is a standard interface for transmitting audio and video from mobile phones to HDTVs. The phone is connected to the HDTV through an MHL adapter, which converts the phone's USB interface to the TV's HDMI interface. This common mode noise comes from the TV power supply and is transmitted to the phone through the HDMI and USB cables.

Challenges of thinner and lighter devices

Nowadays, thin and light is fashionable. The aggressive introduction of thinner and thinner touch screen devices, especially touch screen mobile phones, faces two problems: one is the increase of noise coupled from the display to the sensor; the other is the increase of parasitic capacitance of the sensor.

The noise generated by the display is much lower than the noise from the charger, but it has a significant impact on touch performance because it is very close to the touch sensor. Although AMOLED displays are quiet (but more expensive than LCDs), the majority of LCD displays on the market today are noisier ACVCOM and DCVCOM types. The common electrode VCOM layer of such displays is a source of noise. Let's return to equation (1) and this time determine the capacitance generated by the parallel plate capacitor between a given receive electrode and the display's VCOM layer in a touch sensor. Here, area "A" is the total area of ​​the receive electrode, and since the display covers the entire screen, distance "d" is the distance between the receive electrode and the VCOM layer.

Previously, touchscreen devices used air gaps or shielding layers to protect the touch sensor from display noise coupling to the receive electrodes. However, these solutions add thickness and cost (the shielding layer for a 4-inch display adds up to $1.00 in cost). Now, as devices become thinner, both the air gap and shielding layers are eliminated, and the touch sensor is directly connected to the display using optically clear adhesive (OCA). This causes the sensor's receive electrode to be closer to the noisy VCOM layer, which shortens "d", increases capacitance, and couples more noise. Since the OCA (dielectric constant is 3) replaces the air gap (dielectric constant is 1), the capacitance is further increased. The next development trend of thin and light products is that part or all of the touch sensor needs to be integrated into the display, which is called in-cell (embedded touch) or on-cell (external touch). Such a display integrated protocol stack will bring the sensor's receive electrode closer to the display's VCOM layer, thereby coupling more noise .

The second problem with the development of thinner and lighter products is the increase in parasitic capacitance (CP) of the touch sensor. In order to find a way to make the overall protocol stack thinner, the ITO substrate layer (made of glass or PET) needs to be thinner and thinner. This shortens the distance between the sensor's transmitting and receiving electrodes, thereby increasing the capacitance. The increase in CP requires longer charging and discharging when scanning the touch panel, which reduces the maximum frequency of scanning the panel. The problem with this is that we want to scan more frequently because the noise in the higher frequency band is generally smaller. In addition, longer scanning time also means increased power consumption and lower refresh rate.

Solving the noise problem

Because there are many sources of noise, touchscreen controllers need to adapt to the different levels and types of noise present in the system at a given time. To ensure the most robust noise immunity, the first factor to focus on is the signal-to-noise ratio (SNR). There are several different features that can be used to improve the SNR.

One of the main ways to improve the signal-to-noise ratio is to use very high transmit voltages to scan the touchscreen's sensor. The raw SNR is proportional to the transmit voltage, so the higher the better. In the past, high voltage transmits have been a challenge for many touchscreen controllers and have only been supported by using external high voltage analog supplies (which sometimes significantly increase power consumption and are not supported by most consumer handheld devices) or by using large and expensive external components such as switching regulators. Both of these methods add additional cost to the device. Now, new touchscreen controllers can generate high voltage transmits on-chip through internal charge pumps.

Another way to improve SNR is to use a dedicated hardware acceleration mechanism. Although it is very important to ensure touch performance under noisy conditions, taking up a lot of CPU resources to run noise filtering algorithms will reduce the refresh rate and increase power consumption. By using proprietary hardware that can work in parallel with the CPU, the target refresh rate and power consumption can be maintained while improving the signal-to-noise ratio under noisy conditions. Cypress's Tx-Boost technology is a typical example, which can increase the existing SNR by 3 times.

The scanning frequency of the touch sensor has a great impact on the touch performance in a noisy environment. If the noise frequency is close to the frequency of scanning the panel, the touch data may be corrupted. In this case, we can use adaptive frequency hopping technology to change the scanning frequency to a level where the noise amplitude is low enough to avoid data corruption. However, the effect of frequency hopping is limited, depending on the range of selectable transmission frequencies and the frequency range where noise exists. Some chargers emit a lot of noise across the entire frequency range, making it difficult to find an interference-free area. The fundamental frequency of large charger noise is 1kHz to 300kHz, and the harmonic amplitude is lower at higher frequencies. We can solve this problem by using high-frequency scanning in the range of 300kHz to 500kHz, completely avoiding the highest amplitude noise band and the first few harmonics. In addition, this method can also improve the noise immunity of the display when it is far away from the LCD noise frequency range.

While there are many techniques to improve SNR, these improvements will not prevent touch data corruption if the noise is really high enough to completely saturate the receive channel of the touchscreen controller. Signal processing relies on an analog front end that outputs a linear result. If the output is constantly locked to a maximum value due to a large amount of charge coupled to it by a noise source, the touchscreen may not be usable at all. To solve this problem, we can increase the range of the receive channel so that it can handle the larger amount of charge. This usually adds additional chip area, which means more capacitance. Another way to solve this problem is to split the original signal before the receive channel to reduce the noise, but we must also be aware that this will also separate the signal from the finger itself.

Display and charger noise are not new issues, but noisier chargers and thinner displays are issues that touchscreen controllers must address to improve noise immunity. To cope with higher amplitude noise, today's controllers use a combination of features to improve the signal-to-noise ratio and avoid noise as much as possible. At the end of the day, consumers want consistent touch performance on their devices, regardless of whether it's connected to a charger or near a noisy fluorescent light. As the noise challenge continues to change, touchscreen controllers will continue to evolve to ensure consistent performance.