In this Product How-To design article, Jean Viljoen of Azoteq details the EMI and other challenges of designing capacitive touch user interfaces and how the company’s ProxSense auto tuning technology maximizes sensitivity, reduces PCB layout constraints and limits overlay thicknesses, negating the need for calibration.
The user interface of personal electronics has become critical to the success of the product. Capacitive touch sensing has become the user interface technology of choice.
Designers face challenges like electromagnetic susceptibility, parasitic capacitance, enclosure effects, variety of overlay materials and demands on low power consumption. Antenna or electrode tuning simplifies the design and improves signal to noise ratio (SNR) and reliability.
A touch controller with auto tuning maximizes sensitivity, greatly reducing the conventional constraints on PCB layout, limits to overlay thicknesses & materials and negating the need for calibration during manufacture.
The principle of capacitive sensing is based on the measurement of a disturbance introduced to the long-term steady state capacitance of the sensor. The electrode projects an electric field which is designed to be disturbed by a user touching the designated area. The capacitance of this electrode relative to the surrounding environment is continuously sampled.
Capacitive sensing sensors can achieve very high sensitivity. Due to the nature of small variations in capacitance, the sensitivity is usually very dependent on the influence of external factors. These include:
* Sources of parasitic capacitance related to the PCB layout
* Sources of parasitic capacitance related to the touch sensor housing
* Sources of external noise, including EMC disturbances and noise from the power supply
* The thickness and type of the overlay material
* Presence of air-gaps between the sensor electrode and the touch surface
Popular methods to address these influences and described in touch sensor design application notes include:
* Specific PCB routing suggestions
* Prescriptions on the power supply stability
* Suggestions for larger copper key area to improve sensitivity
* Use of hash-grounds to find an optimal point between improving immunity whilst trying not to introduce too much parasitic capacitance which reduces sensitivity in turn
* Use of active driven shields
* Limits on the thickness and type of allowable overlay materials
Whilst these methods can improve the design, they are often required to make the touch sensor system work at all. Designers will typically require multiple iterations of the touch sensor PCB layout as the design moves towards production and stability and sensitivity problems are discovered as late as pre-production, which causes costly delays.
In an age where designers are well conversed in MCU and embedded design, the design principles for sensitive and robust touch sensing remains black magic to inexperienced designers.