Avoiding EMI in capacitive touch screens

Touch screen technology has been incorporated into practically every consumer device with a screen, from the large screen of the iPad down to the smallest digital camera or GPS screen. Millions of devices incorporate touch technologies employed to create interesting user experiences with touch, multi-touch and gesture recognition. Capacitive multi-touch technology is the most widely adopted technology in these high-end consumer devices as it offers a raft of benefits over its competitors. However, the technology does face some technical challenges.

One of the biggest challenges is EMI – there are many sources of electromagnetic interference around and capacitive technology is particularly susceptible to them. The most commonly cited noise source is the LCD itself, which of course is permanently in close proximity to the touch sensor. Every LCD panel is different, and noise produced by LCDs has a correspondingly wide range of intensities. OLED displays are thought of as a noise-friendly choice by comparison.

Another common source of noise is charging devices. Since the standardisation of mobile phone chargers to use micro USB, manufacturers no longer have control over the quality of the charger that is plugged into their touch screen mobile phone, and unqualified chargers can introduce significant noise.
So how do high end touch controller manufacturers mitigate EMI in their products?

SUPERIOR ALGORITHMS
Atmel recently released the maXTouch mX122E series of capacitive touch controllers optimised for 2.8 to 3.5 inch touchscreens such as digital cameras and GPS devices. They allow up to four simultaneous touches – a level of performance previously only seen in the smartphone market –touch processing rejects unintentional touches caused by a gripping hand, while interpreting light touches correctly for gestures made on the device.The part can support a narrow passive stylus input even when the user’s hand in resting on the screen in a natural writing position.

Atmel says these devices offer the most advanced signal processing and algorithms to mitigate noise from after-market chargers and other sources.

“The common technique to guard against display noise is to add in a physical shield layer between the sensor and the display, which adds both cost and thickness to the design,” explains Helen Francis, Atmel’s Senior Marketing Manager. “With noise processing on Atmel’s latest E series devices, this is no longer required and Atmel can support shieldless designs for devices ranging from handsets to tablets. This results in thinner, lighter, more appealing products.”

“For many competing solutions RF noise is also a challenge,” she adds. “However, the Atmel touchscreen controller operates outside this frequency range, so this is easily avoided.”

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The Atmel maXTouch mXT112E allows up to four simultaneous touches, a level of performance that has previously only been seen in the smartphone market

DYNAMIC RANGE
Maxim has developed the MAX11871 (part of its TacTouch series), which it says has such a high dynamic range that it can detect even a touch from a gloved hand, notoriously difficult for capacitive touch. This product can detect and track up to 10-finger simultaneous touch.

Maxim says the MAX11871’s analogue front-end is built from the ground up for capacitive touch by the company's high-resolution data-converter IC design team, and provides near 60dB signal-to-noise ratio (SNR) performance, equivalent to a 1000:1 ratio between touch and no-touch. This is ten times more than existing products.

“Noise is the number one problem for our handset customers [when selecting a capacitive touch controller],” says Bart DeCanne, Business Director for application-specific data converters at Maxim.
“EMI immunity is really a problem of dynamic range – you want to detect a weak signal, like a touch from a gloved hand or stylus, in the presence of a large noise,” he explains. “Most touch controllers on the market have a broadband analogue front end controller – but you need to scale back the gain of the front end controller or it will be overloaded [with noise] and the weak signal will be lost.”
Maxim’s impressive SNR enables detection of very weak (in the femto-farad range) touch variations, such as from a hand waving near the screen (proximity detection), touch from a fine-tip stylus or ballpoint pen, or a gloved hand. Also, it means that the touch point can be further away from the sensor, allowing for thicker cover glass for improved ruggedness.

“We also have a system which rejects noise at frequencies other than the frequency we accept from the touch screen,” he says, referring to the MAX11871’s proprietary architecture that rejects noise (by over 40dB) from external sources such as AC USB chargers, LCDs, or CFL lights.

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Maxim’spart provides near 60dB signal-to-noise ratio (SNR) performance, equivalent to a 1000:1 ratio between touch and no-touch


FREQUENCY HOPPING
STMicroelectronics’ solution to display noise is based on synchronising the acquisition of touches with the display blanking time, and the company’s latest multi-touch controller, FingerTip, provides both frame and line synch methods. This single chip solution is for touchscreens up to 10” in diameter and features a 32-bit DSP engine to help drive its EMI immunity.

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ST has used its know-how in analogue and mixed-signal interface technology to develop an innovative analogue front-end that is able to detect variations of capacitance in atto-Farads (10-18 F)

The company also cited conducted noise from chargers, present on both power supply and ground, as a problem since it creates false touches.Giuseppe Noviello, Director of Technical Marketing for STMicroelectronics’ Sensor Business Unit, explained that false touches can also result from noise injected by the user’s fingers touching the panel.

“In the case of finger noise, it is the human body that captures radiated noise, like the noise generated by fluorescent lamps or coming from other low frequency (below 1MHz) sources and transfers it to the screen,” Noviello says. “There is no easy countermeasure because it is in the same frequency range as the panel touch acquisition circuits and even if it’s periodic, it is affected by wide jitter and most of the time it is modulated.”

According to Noviello, the common solution for conducted noise is based on frequency hopping and a heavy firmware heuristic algorithm to eliminate suspected false touches. Using the MCU to create filters can reduce or eliminate the unwanted touches, but it will reduce the scan rate and response time. Frequency hopping also cannot deal effectively with noise when there is modulation and jitter.
FingerTip, like other solutions, uses frequency hopping to scan the panel in a noise free area in the frequency domain. This area may be pretty narrow because of jitter and modulation, causing corruption of the valid signals coming from finger movement. FingerTip uses a narrow band digital demodulation method with proprietary DFT filtering that is able to strongly attenuate the adjacent noise component. All the signal processing is done in the analogue front end using a state machine; the processor is not involved so the response time is not affected and the raw data coming in is already filtered to an SNR level that can provide the required accuracy and jitter, even in the presence of tens of volts of noise injected on the panel.

“In order to overcome these limitations, ST is adopting a series of techniques that come from its strong know how in capacitance to voltage conversion already used in MEMS analogue front end design,” Noviello says.

In a MEMS device, the mechanical sensor outputs capacitance variations as low as atto-Farads (10-18F), when the capacitance of the system is a million times bigger. The detection of such small signals requires sophisticated analogue filtering techniques in order to get fast response time with a strong SNR.

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ST’s MEMS sensors and FingerTip technology share a similar architecture, in which a sensing element is connected to a high-performance capacitance-sensing circuit.

“The capacitive touch screen also sends small capacitance variations created by the finger touch to the analogue signal conditioning circuit,” says Noviello. “Like the MEMS case, these variations are very small, typically in the range of femto-Farads (10-15F) but the panel capacitance to earth is a thousand times bigger. The analogue front-end resolution must be enhanced to a significant level for a touch to be recognised, and FingerTip’s A-D converter and signal conditioning circuits use similar methods to MEMS devices to achieves the best SNR.”


Capacitive touch screen controller manufacturers need to protect against noise of many different types from many different sources. Advances in design for EMI immunity allow today’s touch screens to pick out even the smallest signals from an ocean of electrical noise. Manufacturers are continuing to refine their approaches to EMI in capacitive touch and are introducing new techniques based on their experience in analogue electronics and MEMS.