Most touchscreens, which have been used for decades, are based on resistive technologies. That means the user has to actually press together two electrodes, each consisting of a conductive material deposited onto a thin layer of plastic (PET) or glass.
Of course, these aren’t really “touch” screens at all. A simple touch can’t be detected; the screen needs pressure to be applied. This means such screens have been relatively unreliable. Furthermore, because the so-called touchscreen must be placed on top of the display panel, it’s vulnerable to normal wear-and-tear as well as damage from sharp objects.
Despite these problems, the benefits of good touchscreens are significant. Electrical and electronic systems are becoming more complex, with many functions not even dreamt of just a decade ago.
Touchscreens will also offer creative freedom for the system designer, without the limitations imposed by mechanical switches, or the packaging complexity that’s created by mechanical solutions. You certainly don’t need to have a hundred holes in a front panel to be able to control a hundred functions with a touchscreen.
True touchscreens, which are based on capacitive-sensing techniques with sensors that trigger on the lightest touch, are leading a revolution in human interfaces for electronic systems. This is particularly the case for domestic electrical appliances and portable consumer devices, such as mobile phones and MP3 players.
The ideal touchscreen needs to meet the following requirements:
• Precise and consistent operation with no need for constant calibration
• The ability to detect multiple concurrent touches
• Immunity to electromagnetic interference and ESD
• The simplest possible mechanical construction
• Low cost, particularly in the most price-sensitive electronic products
• Good optical properties
• Low power consumption
• Easy interfacing to host system electronics
• Long-term reliability in real-world environments
Projective capacitive touchscreens demonstrate the best combination of these characteristics today. Chargetransfer sensing for projective touchscreens, developed by Quantum Research, is one example of how to implement projective capacitive touch. It’s now being widely adopted in many consumer electronic and electrical products.
One or more etched layers of transparent, conductive, indium tin oxide (ITO) creates multiple horizontal and vertical electrodes that are controlled by the capacitive sensing chip. The ITO layers are located behind the lens, protecting them from damage. The chip can offload the data to a host processor, process the XY location of touch, or provide an output related to the user “gestures” observed on the surface.
Usually, both of the electrode sets are driven using single-ended sensing methods; i.e. there’s nothing unique about the circuitry for a row versus a column. This is referred to as “single-ended” sensing.
In some methods, however, one axis is driven using a set of ac signals, and the response through the screen is detected back via the other axis electrodes. This is referred to as “transverse” sensing, because the electric fields propagate in a transverse manner from one electrode set (e.g., rows) to the other set (e.g., columns) via the dielectric of the overlying panel.
In either case, position is determined by measuring the distribution of the change in signals amongst the X and Y electrodes. Mathematical algorithms determine XY coordinates of the touch event by processing these changed signal levels (Fig. 1). Resolutions up to 1024 by 1024 through panels a few microns thick are possible.
A significant further advantage of projected capacitive touchscreens is the potential to detect multiple concurrent touch events. This capability substantially expands the range of features that can be supported to provide a much richer interface.
In the real world, making a highquality capacitive measurement can by compromised by the noise from the system or from external sources.
Electrical interference takes the form of radiated, conducted, or earth-referenced noise. Radiated noise is induced via antennas in mobile phones, magnetic components in power supplies, and the LCD panel.
The LCD, which is very close to the ITO electrodes (and sometimes, bonded to them in a gap-free stackup), generates electrical noise as pixels are scanned. Conducted noise comes from power-supply rails or linear regulators (Fig. 2).
Earth-referenced noise can be a problem when the touchscreen is connected to noisy computers. Electrical noise occurs over many frequencies: up to 100Hz from ac power supplies, from 1 to 100kHz from LCDs, and from one to tens of MHz from radio circuits.
To tackle these issues, most projected capacitive touchscreen technologies use at least two sensing layers of ITO, plus a shield layer, to achieve noise-free operation.
Charge transfer sensing has characteristics that eliminate, or at least minimise, noise. It uses sampling capacitors, typically of 4 –10nF, to detect charge levels on the ITO electrodes. These are connected to the sensor chips via series resistors.
The combination creates effective low-pass filters to remove high-frequency radiated noise, and the chip can be shielded for added protection. The low-frequency elements of radiated noise and earth-referenced noise are removed using dedicated processing algorithms within the sensor chip.
In addition to using high-performance LDOs and supply-rail filtering, conducted noise is tackled through a sensor pattern that gives the best possible signal-to-noise performance. This pattern is also effective against earth-referenced noise.
The combination of techniques described here for tackling EMC issues delivers a remarkable potential outcome: It becomes feasible to produce a reliable touchscreen for many applications with just a single sensor layer, rather than two or three. Constructing the touchscreen layer stack-up can be as simple as that shown in Figure 3.
Reducing the number of ITO layers simplifies the construction of a touchscreen, greatly reducing costs. It also enhances light transmission through the screen to around 90%, which improves clarity and contrast, and reduces the level of backlighting needed. Less backlighting then lowers system power consumption.
INTERFACING, ESD, RELIABILITY
The charge-transfer sensing technique enables simple interfacing from the touchscreen to the sensor chip. Typically, a single-layer sensor will need just 8 to 10 routed sensor signal lines (4-X and 3-Y lines).
An added advantage of this technology is that the sensor chip can be located up to 60mm away from the edge of the ITO patterns. Therefore, mounting a chip on the flexible connecting “active tail” won’t add to the complexity—the chip can be mounted on the main PCB to improve flexibility.
ESD can’t be eradicated, but is managed to avoid inadvertent touches being registered. Dedicated functions that report ESD event status to the host system. The sensors also include power-on-reset and a watchdog timer. Smart mechanical design helps steer ESD away from the touch sensor.
Finally, long-term reliability is ensured when using charge-transfer sensing because the sensor chips automatically recalibrate on powerup. This rids any problems with the build-up of contaminants or moisture on the touchscreen surface.
ChRIs ARD is marketing director, Touch Technology Division, Atmel.