Noise Wars: Projected Capacitive Screens Defend Against Internal Noise

1. Capacitive touch screens are widely used, but they are prone to false and erroneous responses due to noise in the product.

2. Noise originates from the internal DC/DC converter subsystem and display driver.

3. Whether dealing with noise from the display, charger, antenna or other sources, the touch IC must provide the same level of user experience.

Today's users expect multi-touch systems to provide precise operation while still meeting increasing environmental standards. It is not easy for designers to meet these requirements. As the internal environment of multi-touch systems changes rapidly, the battle for touch screen dominance is also affecting the emergence of new battlefields.

One of the current trends is to make mobile phones thinner and thinner. To achieve this goal, it means laminating the capacitive touch sensor directly on the display, moving the sensor into the display, and overcoming many other challenges, such as antenna and ground loading. In the past, simply putting a shielding layer on the sensor structure to block the display noise is no longer acceptable, it will add too much cost and thickness.

In addition to displays, the ubiquity of USB charging plugs has commoditized battery chargers. Today’s capacitive touchscreen ICs can detect capacitance changes in the picocoulomb range in the presence of up to 40V peak-to-peak AC noise. All of these factors have raised the demands on touchscreen ICs, making them even more complex than they were even last year. New innovations were needed, and so began the noise war.

Charger noise

Charger noise is physically coupled to the sensor through the battery charger when there is a touch. Its effects include: reduced touch accuracy or linearity, false touches or ghost touches, and even an unresponsive or unreliable touch screen. The culprit is usually a low-cost retail charger. Although OEM-supplied chargers generally have stricter noise specifications, the widespread adoption of USB plugs in charging circuits has created a huge business opportunity for the retail market. To compete in this market segment, manufacturers in the retail market are working to make their chargers cheaper. These low-cost electronic products can charge a phone, but may inject enough noise into the touch screen to make the phone unusable.

Two common battery chargers are ringing-choke and flyback. Flyback chargers typically use a PWM circuit, while low-cost self-oscillating ringing-choke chargers use a variation of the flyback design (Figure 1).

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Figure 1. A flyback converter charger typically uses a PWM circuit (a), while a low-cost, self-oscillating, self-excited charger uses a variation of the flyback design (b).

Self-excited converters have neither microcontrollers nor capacitors, lack PWM control, use lower-cost transformers, fewer diodes, and smaller-value polarized input capacitors. These reductions equate to cost savings for the manufacturer, but leave the customer with a noisy system. Some self-excited converter chargers are almost broadband noise generators because they radiate up to 40V peak-to-peak noise in the range of 1kHz to 100kHz. Most chargers have a tendency to circulate noise with many harmonics. A good example is the so-called zero-wait charger, which has a noise output of 10V to 25V peak-to-peak (Figure 2).

Figure 2. The noise of the “zero wait charger” is not the same at 0 (a), 50 (b), and 100% (b) load.

The output of the charger depends on the state of the battery itself. To address this phenomenon, many OEMs have come together to create EN (European Regulation) specifications to control the maximum noise level radiated by a charger at any frequency. EN 62684-2010 and EN 301489-34v1.1.1 are responsible for these noise levels (Figure 3).

Figure 3. EN specifications specify the maximum noise level that a charger should radiate at any frequency. EN 62684-2010 and EN 301489-34v1.1.1 govern these noise levels.

From 1kHz to 100kHz, the output noise of the charger should be no more than 1V peak-to-peak, and as the frequency increases, the noise level will decrease exponentially from this level. However, no products on the retail market meet such strict specifications. Therefore, OEMs now want to use touch ICs to cope with such high noise. Some specifications require 40V peak-to-peak from 1kHz to 400kHz, and 95V peak-to-peak suppression in the range of 50Hz to 60Hz. Fortunately, specialized algorithms and methods can meet these stringent requirements and provide battery chargers with noise suppression capabilities exceeding 95V peak-to-peak. Some methods are used to achieve these levels, such as nonlinear filtering, frequency hopping, and other hardware techniques.

Display Noise

Projected capacitive touch display systems present many challenges because they can generate a lot of noise that is directly transmitted to the capacitive touch screen sensor. To make matters worse, OEMs are demanding thinner and thinner industrial designs for their phones, which means moving the touch screen sensor closer to the display or even inside the display. For many years, the industry has used a protective shield to protect the sensor from the noise generated by the display. Although this solution is effective, it increases the cost and thickness of the phone. The industry has also used a solution that maintains a 0.3mm air gap between the display and the sensor to dissipate the noise from the display through the natural properties of air. However, as phones become thinner and thinner, neither of these solutions is suitable for today's designs.

Fortunately, the noise radiated by the display is less than that of the charger, but it is still difficult to deal with. When using traditional TFT (thin film transistor) LCD, the common electrode can be driven by DC voltage or AC voltage. The AC common electrode layer can usually reduce the operating voltage of the display driver and maintain a constant voltage across the liquid crystal. The AC common electrode layer is used for relatively low-cost displays, has higher power consumption, and has weaker noise characteristics than the DC common electrode layer (Figure 4).

Figure 4. The AC electrode layer is used in relatively low-cost displays, has higher power consumption and higher noise than the DC common electrode layer.

The noise characteristics of a typical AC common electrode display are about 500mV~3V peak-to-peak at 10kHz~30kHz, while DC common electrode displays are usually quieter. The method of measuring a display is very simple. Connect an oscilloscope to the copper strips on the top of the display, connect the ground to the circuit ground of the display, let the display work, and you can capture the waveform.

The use of AMOLED (active-matrix organic light-emitting diode) technology is gaining popularity in cell phones because it allows for wider viewing angles, brighter colors, and deeper contrast. AMOLED displays are also quieter, but this comes at a price (Figure 5). The AMOLED display in the figure outputs a spike of 30 mV peak-to-peak, which is 1% of the noise of an AC common-electrode display, greatly simplifying the design of touch screens. The integration of the sensor with the physical display creates an on-cell (on the pixel) and in-cell (in the pixel) structure, which also simplifies this type of display. However, AMOLED displays are much more expensive than traditional LCDs.

Figure 5. A typical AMOLED has relatively small display noise.

On-cell designs typically deposit the sensor layer on the color filter glass of the display, keeping it as close to the display chemistry as possible because it is inside the stack. Noise and parasitic loading increase. However, AMOLED technology is inherently quiet, making it a good platform to build on-cell or in-cell sensors underneath the color filter glass design.

When designing sensors, a widely accepted sensor structure is to use a two-layer sensor with the transmit lines in the bottom half of the sensor and the receive lines in the top half. The receive lines are sensitive to display noise, but the wide transmit lines at the bottom of the sensor form a barrier to the noise generated by the display. This effectively creates a shield in the sensor (Figure 6).

Figure 6. Touch screen sensors using MH3 (a), diamond (b), and patented technology (c) using different stacking methods and materials.

In an MH3 dual-layer stack, the bottom layer of ITO (Indium Oxide) acts as a shield against display noise. Unfortunately, this approach is rarely used for glass-based sensors because it adds thickness and cost. The industry is working to build sensors on a single-layer substrate without shielding. To achieve a true single-layer sensor without shielding, the touchscreen IC is required to be immune to display noise. This task is difficult because display noise can easily reach 3V peak-to-peak in both AC common electrode and DC common electrode displays.

Display noise can be reduced even with direct lamination, where the sensor structure is pressed against the top surface of the display with no air gap or shielding, also known as display-integrated design. An example is Cypress Semiconductor’s Display Armor approach to preventing display noise. Here, the touch IC integrates a built-in touch device listening channel that eliminates display noise through advanced algorithmic decisions to determine which information is noise and which is data. By detecting the source of noise and locking to the waveform, capacitance measurements can be made when it is quiet. These methods of reducing display noise have resulted in advanced and thinner capacitive touch screens at a lower cost.

In addition to noisy displays and chargers, capacitive touch screen designers face many other challenges. For example, antennas are a huge noise source challenge. Space is becoming increasingly tight in mobile phones, and various components (such as antennas and touch screen sensors) actually overlap each other. Such design challenges can cause trouble when dealing with this part of the touch screen. Fortunately, the same innovations that help reduce display and charger noise also help reduce noise from other sources (such as antennas). Whether using simple IIR (infinite impulse response) filters, advanced nonlinear filtering methods, built-in noise reduction hardware, frequency hopping functions, or any other method, capacitive touch screens have achieved some of the most advanced performance embedded in devices.

Obviously, noise rejection is one of the biggest concerns for designers. Whether dealing with noise from the display, charger, antenna, or other sources, the touch IC must achieve the same level of user experience. Innovations are happening every day in capacitive touch technology, and touch ICs are also constantly engaged in the noise war.