Dedicated Analog Channels Deliver High Noise Immunity Multitouch

Dedicated Analog Channels Deliver High Noise Immunity Multitouch

Cypress Semiconductor's TrueTouch has moved into its 5th generation and it might be hard to improve upon it because its noise immunity is so far above the competition and Cypress' previous generation of multitouch capacitive controllers. One of the major additions to the technology is the use of individual ADC channels for each scan line allowing simultaneous data capture in one pass.

It works with a range of touch sensor platforms including single layer approaches (see “Single Layer Multitouch Cuts Cost In Half”). In fact, the noise immunity improvements are especially important in these thinner, direct laminated displays. The technology currently targets displays from 1.5- to 14-in.

Improving Touch Sensing

Normally a capacitive touch system has a multichannel ADC that cycles through the sensor inputs. This works because the scanning rate is faster than the rate needed to report a touch position. This simplifies the touch subsystem but the downside is that the system is more prone to noise especially as the scanning frequency increases. A faster scan provides better system response.

Cypress allocates one ADC per channel along with its own attenuator to prevent saturation. It also lets the system significantly increase the scan rate to 500 Hz. It also allows Cypress to employ adaptive frequency hopping (AFH). This combination puts the signals past the 3rd harmonic of a typical charger that normally are below 150 Hz.

The controller also employs DSP filtering on each channel before the data makes it to the internal processor. This provides exceptionally clean data allowing the on-board processor to address functional details such as multitouch support instead of having to clean up the data first.

The other advantage of individual ADCs is that all data is acquired at the same time. The faster rate also means the the system can utilize mutual and self capacitance techniques that react differently. This is important as water begins to play a part especially if a sensor is wet or even slightly damp. Most sensing systems simply do not work but Gen 5 just takes it in stride. Its water rejection support can even even detect the presence of moisture and ignore it. The instant baseline recalibration helps after water is cleared from the screen. Gen 5 essentially doubles the signal to noise ratio (SNR) when water is involved.

Another feature is the controller's high drive capability. Higher currents provide improved accuracy and it is also needed as the system ground plane moves closer to the sensors. This is the case as displays get thinner and utilize single layer sensors since the controller must overcome the parasitic capacitance of the ground plane.

Chargers Can Be Bad For Touch Screens

Did you ever have a device with a capacitive touch device that would not work or worked poorly when plugged into the charger? That was probably because the noise immunity with the charger was very poor. It is one area where TrueTouch Gen 5 excels. It delivers a minimum charger noise rejection up to 40 VPP up to 500 kHz (Fig. 1) but it is really more like 60 VPP with proper tuning.

Figure 1. Without major tweaking, TrueTouch Gen 5 delivers a minimum of 40 VPP up to 500 Khz.

The design handles inputs as small as 0.5mm without jitter. This allows a stylus to deliver very accurate input in addition to handling multiple fingers.

The Gen 5 controllers all incorporate a 32-bit, Arm Cortex microcontroller. This allows the use of an efficient host/controller packet interface. The controllers will handle a range of sensor stacks including in-cell and on-cell stacks. Accuracy is 1mm with a fast sense rate of 150 Hz. The smallest version is available in a 5mm by 5mm, 44 connection QFN package.

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