Reduce Display Noise In Capacitive Touchscreens
Smart phones primarily use two types of displays: the active-matrix organic light-emitting diode (AMOLED) and the liquid crystal display (LCD). AMOLED displays are normally best-in-class for viewing quality (with a few exceptions). They also produce far less noise for the touchscreen controller than LCDs. AMOLED displays are somewhat more expensive and harder to manufacture, though, so LCDs dominate the market despite the noise they produce—and savvy designers can use smart methodologies to reduce that noise and make LCDs even more competitive.
To understand why LCDs produce noise, we need to first understand at a basic level how they work (Fig. 1). Starting from the very bottom of the display, white light is generated and reflected upwards. Each pixel comprises three subpixels: red (R), green (G), and blue (B). Each subpixel represents a liquid crystal (LC) sandwich, with a conductive material called indium tin oxide (ITO) on the top and bottom layers and LC material in the middle. The top layer, common to all subpixels, normally is called VCOM. The bottom layer, dedicated to one subpixel, is called a subpixel electrode. When a voltage is applied across the LC sandwich, the LC material twists the polarity of white light.
Above the sandwich, a polarizer layer allows only a particular polarity of light through. If the light polarity lines up with the polarizer polarity, the subpixel is at a maximum brightness. If the light polarity is opposite from the polarizer, the subpixel is at a minimum brightness.
Each subpixel has a color filter layer (R, G, or B) that works like a stained-glass window. By applying voltage across each of the three subpixel LC sandwiches, the pixel can be set to any RGB-derived color. Each subpixel also has a thin-film transistor (TFT) used as an on/off switch for the voltage applied to the LC sandwich. This allows efficient sequencing of the pixels across the screen during a full-screen image update (Fig. 2).
The pixel is turned on at the gate of the TFT. The source of the TFT is connected to a color digital-to-analog converter (usually 8-bit R, G, or B) output. The drain of the TFT is connected to the ITO subpixel electrode. LC material cannot withstand a dc voltage, so the bias voltage must be ac. This is where two distinct types of LCDs start to emerge: ACVCOM and DCVCOM.
ACVCOM actively drives both VCOM and subpixel electrodes with a differential voltage. Because the VCOM layer is driven with an ac waveform, it is called ACVCOM. DCVCOM drives the common layer with a dc value while the subpixel is driven with an ac signal that swings around the dc value. There are various performance and cost advantages and disadvantages in using ACVCOM versus DCVCOM, but we won’t cover them in this article.
ACVCOM is known for causing lots of noise because of the large sheet of ITO (VCOM) that is actively driven. DCVCOM is known for being a quieter display, though this isn’t necessarily true. In the past there was a thin layer of air called an “air gap” between the sensors and LCD surface. But today phones are thinner, and the air gap is largely eliminated.
Direct lamination (DL) of the ITO sensors directly on the surface of the LCD is becoming popular and worsens noise coupling. Even more aggressive, the industry is moving toward designs that require touchscreen controllers to sense the VCOM and subpixel electrodes directly. These designs are called “in-cell” and require special synchronization between the touchscreen and LCD controller to scan a touchscreen without noise. Most LCDs in smart phones are moving away from ACVCOM and toward higher-quality DCVCOM and AMOLED displays. In addition, most smart phones are moving toward DL or some form of in-cell to decrease manufacturing cost and produce a thinner phone.
Now let’s take a look at how noise is coupled into touchscreen sensors. Figure 2 shows two capacitors that are responsible for coupling noise from the LCD circuit to the touchscreen circuit. The first capacitor is CLC. This capacitance is formed between the subpixel and VCOM surface areas, with the LC material acting as a dielectric. For DCVCOM displays, the ac signal driving the subpixel is coupled up to the VCOM layer as noise where it is broadcasted across the entire panel. It may seem that the VCOM layer is a good ac ground because DCVCOM holds this node at a dc potential. But in reality it only attenuates the noise because the VCOM layer is made of ITO, which has considerable resistance.
This is where the second coupling capacitor comes into play, CSNS. CSNS is formed between the VCOM layer and capacitive sensors. The remaining noise voltage on the VCOM layer couples up to the capacitive touchscreen sensors through CSNS and into the pins of the touchscreen controller. For ACVCOM displays, VCOM is driven with an ac waveform, directly coupling to the touchscreen sensors through CSNS.
Measuring and characterizing LCD noise is straightforward: directly cover the surface of the display (without the touchscreen sensors attached) with a conductive material and connect an oscilloscope probe. A facedown copper plate works well to cover the entire screen. A large coin or piece of copper tape also works, but be aware that the noise magnitude can decrease with the size of the conductor. It’s best to cover the entire surface to minimize coupling error into the scope.
The ACVCOM waveform typically features a strong fundamental frequency and looks similar to a square wave (Fig. 3). ACVCOMs operate typically between 5 kHz and 25 kHz. The fundamental frequency normally corresponds to the rate at which each row of LCD pixels is updated (line frequency).
The DCVCOM waveform looks like several sharp high-frequency pulses (Fig. 4). DCVCOMs do not have a strong fundamental frequency like ACVCOM, but their harmonic content can easily range between 50 and 300 kHz. The short pulses correspond to the subpixel electrode drive signals.
DCVCOM noise characteristics greatly depend on the image displayed. The worst case image is normally a pattern that alternates black and white pixels in a “checkerboard” pattern across the entire display (looks like a gray color). Be sure to try several different images while characterizing DCVCOM displays.
Reducing Display Noise
There are several options to reduce display noise effects on the touchscreen controller: shielding (eliminating noise magnitude), avoid noise frequency, digital filters, touchscreen sensor design, and synchronization.
• Shielding: A shield is a solid sheet of ITO that covers the entire display. It’s placed between the display and the touchscreen sensors. The shield is tied directly to circuit ground so the display noise is shunted to ground instead of the touchscreen controller. A shield layer is normally very effective at reducing noise, but overall it’s not very desirable because it adds manufacturing cost and slightly reduces image quality.
• Frequency: One of the best options for reducing display noise is to pick an operating frequency for the touchscreen controller that is different than the LCD noise frequency. It helps to use a touchscreen controller that can handle large peaks of noise without saturating the touchscreen sensing circuit. In addition, a narrow band receiver helps to be able to tune around the noise spikes. Sometimes it’s helpful to produce an FFT on the captured waveforms to understand the best place to set the touchscreen operating frequency (Fig. 5). Automated tools to help select a good operating frequency are often made available by the touchscreen controller manufacturer. This includes tools that allow sweeping of the touchscreen operating frequency while monitoring noise.
• Digital filters: Digital filters are very helpful for reducing noise. There are many linear and non-linear filters to choose from, all with various tradeoffs. For linear filters, a traditional infinite impulse response (IIR) or finite impulse response (FIR) filter works well in reducing noise but can result in sluggish finger tracking across the screen. Several adaptive modifications to these filters result in much better finger tracking. Other non-linear filters can help, especially for impulse noise that contains high but infrequent noise spikes. A few filters can intelligently identify LCD noise and subtract it from the signal. Touchscreen controllers that have hardware filtering are a plus so the touchscreen controller processor can save time and power.
• Touchscreen sensor design: Several sensor design patterns can greatly reduce display noise. One popular type is the Manhattan, named after New York’s Manhattan borough due to its perfectly straight horizontal and vertical lines (Fig. 6). All true multi-touch touchscreen controllers will drive a transmit (TX) sensor and receive on a receive (RX) sensor. In a Manhattan sensor design, the TX sensors are wide and positioned below the RX sensors. The RX sensors are narrow to eliminate parasitic capacitance and reduce noise coupling. The Manhattan sensor allows the TX sensors to shunt much of the noise so it doesn’t reach the RX. Several more sophisticated variants of the Manhattan are used in the industry.
• Synchronization: Synchronization between the touchscreen controller and LCD is also an option for reducing display noise. In fact, it’s absolutely required for in-cell designs. A touchscreen controller can synchronize by listening to the horizontal and vertical sync signals coming from the LCD driver, called HSYNC and VSYNC respectively. Or, in the case of ACVCOM, some touchscreen controllers can pick up the noise from the touchscreen sensors and start a scan without the need for HSYNC and VSYNC. Synchronizing to ACVCOM is fairly straightforward because the fundamental is very strong and low frequency. DCVCOM is more difficult because the noise is higher frequency, which requires precise timing between touchscreen controller scans and quiet periods in the display.
As phones get thinner, touchscreen controllers are exposed to more display noise due to tighter capacitive coupling between the display and touchscreen sensors. This warrants a greater focus on how displays work, where exactly the display noise comes from, how to measure display noise, and what options exist for reducing display noise.
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© 2013 Penton Media Inc.
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