Design capacitive touch systems for robustness and manufacturability
Combining hardware cancellation and signal processing
A further advantage of an on-chip compensation implementation is that the compensation circuit can be dynamically adjusted by control algorithms. A dedicated optimization algorithm will run at power up and adjust the compensation circuit to achieve a pre-determined target. This target can be chosen to allow for maximum sensitivity, or to achieve an optimized point within a variety of parameters including:
* Sensitivity
* Power consumption
* Sampling frequency
* Noise suppression
The control algorithm will continuously monitor sensor performance and adjust the compensation circuits dynamically, should variations occur due to age, temperature, changes in the mechanical stack-up or even the addition of a static object to the touch panel area. The Azoteq control algorithm is referred to as Auto-ATI (Automatic Antenna Tuning Implementation).
In controllers, which are primarily designed to be used standalone, the control algorithm settings are predetermined and no intervention from a host is required. On multi channel devices, the algorithm can run autonomously. However, the host has a choice to perform partial intervention by only setting target values, or the host may take over the full control algorithm.
Advantages of parasitic cancellation and auto tune algorithms
The foremost advantage of parasitic cancellation is that sensor sensitivity is maximized, even in environments with severe parasitic capacitance. In real life cases, the reduction of parasitic capacitance is one of the biggest challenges faced by designers. With compensation circuits, the designer has much greater degrees of freedom, resulting is much smaller PCB’s and optimal performance within a product where the enclosure may add a significant parasitic capacitance.
On-chip compensation circuits allow for accurate compensation without adding to the BOM cost or PCB real estate. The on-chip circuits are calibrated during the semiconductor manufacturing and require no design skills.
Once the on-chip circuits are combined with control algorithms, the sensor may auto-tune to achieve a predetermined target. The auto-tuning algorithm will further ensure that all sensors perform similarly over manufacturing variation.
Control algorithms that allow for host intervention can be dynamically tuned to optimize the sensor. This implies that the sensor may be optimally configured for a long proximity range and low power in a standby state, whilst tuned for a high response rate in operational mode.
Table 1 – Experimental setup
In Table 1 above and Table 2 below, five scenarios are depicted. In scenario 3 an artificial parasitic load is added which is effectively more than an average touch. In scenario 4 an auto tune has been performed by the sensor and sensitivity is recovered, despite the large load.
Table 2 – Readings with the combination of techniques discussed
Conclusion
One of the biggest challenges designers of capacitive touch solutions have to deal with is parasitic capacitance that reduces sensitivity. With a hardware compensation circuit, most of the unwanted parasitic capacitance can be removed. With such a circuit implemented on-chip, no cost is added to the BOM and the designer does not need any special skills to use the compensation circuits.
The compensation circuit can be combined with an auto-tuning algorithm that will tune the sensor to optimal performance automatically. A further benefit is that all touch sensors will have similar performance, even over variations in manufacturing.
Capacitive sensitivity is greatly increased which, allows greater degrees of freedom in PCB design, the overlay thickness and limitations on the touch sensor housing. The Azoteq implementation of on-chip compensation circuits means smaller PCB’s that function optimally in environments with severe parasitic capacitance. These controllers offer a superior touch performance and unrivalled proximity detection.
Jean Viljoen joined Azoteq in 2000 as a DSP Application Engineer and was one of the key architects who created the ProxSense product line in 2004. In 2005 Mr. Viljoen moved into a marketing role and also took on responsibility for the application group at the Paarl Development Centre. Since 2010 Mr. Viljoen has been the Marketing Manager for Europe and Asia for the ProxSense product line. He holds a Bachelors degree in Electrical and Electronic Engineering from the University of Stellenbosch and an MBA from the University of Stellenbosch Business School. He can be reached at jean.viljoen@azoteq.com
This article provided courtesy of Embedded.com and Embedded Systems Design Magazine. Sign up for subscriptions and newsletters. Copyright © 2011 UBM--All rights reserved.
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