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Capacitive sensing for specific apps (Part 1)

Posted: 16 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:user interface? constant-current source? touch sensor?

Capacitive-sensing buttons are becoming a popular interface alternative to mechanical switches in many consumer electronics and "white goods" applications. However, designing a capacitive-sensing interface comes with its own challenges which introduce new product-development, production, and quality-control concerns. For example, the parasitic capacitance (CP) of capacitive sensing buttons can vary from board to board and also can vary due to environmental changes such as changes in temperature and humidity. Noise also varies from system to system.

Another common issue with user interface (UI) design is design portability. If the UI design of a TV front panel changes, for example, the design will require sensor retuning to accommodate changes in layout, sensor dimensions, and other factors. The laborious process of tuning adds cost to a system in terms of manpower and time, as well as delays the product's time to market.

This article focuses on different hurdles faced when designing a capacitive sensing interface and methods developers can use to overcome typical problems to make a design robust and portable.

Capacitive-sensing basics
Figure 1 shows what a typical capacitive-sensor board looks like.

Figure 1: Cross sectional view of capacitive sensing board.

The sensor capacitance with respect to circuit ground in the absence of a finger is as shown in figure 2a. This capacitance is called parasitic capacitance (CP). When a finger touches the sensor (or comes near the sensor), figure 2b, it introduces another capacitance called finger capacitance (CF) in parallel to the CP. In the presence of a finger, the total sensor capacitance (CX) is given by Equation 1:

CX = CP + CF (Equation 1)

Figure 2a: Sensor capacitance in the absence of finger.

Figure 2b: Sensor capacitance in the presence of finger.

The change in capacitance introduced by CF is used to detect a finger press.

Capacitance-measuring system
An electronic system measures the sensor capacitance by converting the capacitance into a digital value. Figure 3 shows the block diagram of a capacitive-sensing preprocessing circuit. (Note: There can be numerous different methods to measure capacitance).

Figure 3: Preprocessing circuit for measuring capacitance.

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