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Capacitive sensing in battery-powered devices

Posted: 02 Sep 2014     Print Version  Bookmark and Share

Keywords:Specific Absorption Ratio  Capacitive sensing  proximity sensing  SAR sensors  battery ground 

Mobile devices such as tablets and cell phones typically require proximity sensors for Specific Absorption Ratio (SAR) qualification and on-ear detection. Capacitive sensing may be used to meet both of these needs. Self-capacitance technology is popular for proximity sensing in mobile devices. It is of great importance to note that the sensing electrode, as well as its size, is not the only design variable. There also is a strong focus on the effective implementation of SAR sensors with the qualification process in mind.

Overview of capacitive proximity sensing
Capacitive sensing is one of the very few cost-effective technologies that pass the SAR qualification tests. Capacitive sensing addresses all the limitations of other sensor technologies.

It is important to note some key points when implementing a capacitive sensor in a complex and compact design that requires optimal performance.

Dependence on battery ground: All sensor measurements are taken in relation to battery ground (device ground). Variance between human body ground (well coupled to earth) and device ground will affect the performance. The illustration below shows these potential variables.

Figure 1: Circuit element description showing device ground effect on sensitivity.

Extreme sensitivity: The graph below shows the theoretical values of a parallel plate capacitor. A similar case will be true for a human (infinite ground plane) approaching a capacitive sensor (charged electrode). Keeping this type of sensitivity in mind (low femto-Farad deltas per mm) it is easier to understand that mechanical instability and typical device placement may also trigger the sensor. Mechanical instability refers to movement in the micrometre range of the flexible printed circuit (FPC) or device cover in relation to a battery itself, or to another large ground structure in the device.

Figure 2: Capacitance estimation of a small 1 mm x 20 mm electrode at varying distance from the phantom body (ground plane).

Optimise electrode size
Electrode design (sizing and placement) cannot be done separate from ground reference considerations. This is because an electrostatic field is formed between the electrode and ground reference in the same way as a field is formed in a parallel plate capacitor. See the illustrations in figure 3 to see how the parallel plate capacitor model may be translated into a device.

Figure 3 (a) is an example of how the parallel plate capacitor model may be translated into device testing, (b) an example of typical coupling to the device ground (open folded capacitor), (c) a combination view, emphasizing that the two effects together determine the trigger distance.

If the trigger plane (phantom body, hand, etc.) is larger than the electrode itself, a good rule of thumb for the trigger distance is:

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