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Reducing noise, signal error in touchscreen applications

Posted: 21 Jun 2010     Print Version  Bookmark and Share

Keywords:capacitive sensing  touchscreen applications  sensing systems 

Capacitance measurement, widely used to detect proximity of human touch, creates a characteristic capacitive sensing chain like most of environment-sensitive proximity sensing systems. A key part of this sensing chain is the capacitive sensor.

This sensor is the low-layer hardware and software, which drives and monitors the charging process at the same time. Many low-cost systems use only single threshold logic with simple driving, using the constant current charge sensor or charging through a parallel resistor.

Hardware sensors typically provide a fixed or configurable oversampling feature, where multiple subsequent measurements are taken and processed as one value (either a sum or an average). Oversampling is time-consuming but offers precision. Software sensors provide discrete sample values, where oversampling may be controlled.

For this article, sensing is the "waiting" time when the driven capacitive signal reaches a known detection threshold (VIH). When oversampling is counted, the waiting time varies in the microseconds range.

Capacitance-to-time conversion (CTC) means that a capacitance added (by a finger) to the base capacitance of the measuring circuitry increases the measured conversion time. The base capacitance of the measuring circuitry is called parasitic capacitance, which determines the minimum conversion time, called baseline.

Depending on the layout, route thickness, and other board- and input pin-specific parameters, the baseline may easily reach tens or even hundreds of microseconds. The finger capacitance can extend this conversion time further.

Without suitable elimination of parasitic capacitance, this baseline can be very high and may even be higher than the finger capacitance being measured. A problem with capacitive measurement is instability and difficulty to estimate hardware parameters and conditions with almost random parasitic capacitance in the range of tens or hundreds of picofarads.

Today's systems have adjustable sensitivity through a single configurable current or voltage source. This setup always requires minimal or at least stable parasitic capacitance in the measuring chain.

These systems don't allow independent control of sensitivity and charge time for various parasitic capacitances and for different physical conditions. This means that higher sensitivity always means longer conversion time with all consequent disadvantages.

View the PDF document for more information.





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