Capacitive sensing in battery-powered devices – Design considerations

Technology News | August 13, 2014

By eeNews Europe

Mobile devices such as tablets and cellphones generally 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 micrometer 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)

Optimize 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 below to see how the parallel plate capacitor model may be translated into a device.

Figure 3 (a) 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:

In most cases, increasing the electrode width will have a positive effect on the trigger distance. When the width is extended towards the device ground so that “trigger distance 2” dominates, then the intended effect of the larger electrode will be lost. This effect is shown in Figure 4 .

Optimize device ground for optimal sensor performance

Device ground and earth references should be identified as part of the electrode placement and design process.

Earth references should only be considered as an element that potentially gives the device an improved reference (increased capacitance) to the user/phantom. This effect improves the sensitivity and may lead to increased trigger distance. It is recommended that the device is tested in the more isolated case (as shown in Figure 1 where C2 is very small when preparing for SAR qualification.

Device ground has an important role to play in the isolated case. In this case the device ground distance to the electrode has a direct effect on the possible maximum detection distance. In cases where the design of the battery position, printed circuit board (PCB) ground and mechanical structure is fixed, the electrode distance from these elements should be maximized as shown in Figure 4 . When there is freedom to change the device ground reference areas, the ground plane can be adapted (moved or decreased) for a specific trigger distance.

Figure 4 Effect of sensing pad size and sensing pad to device ground distance

Sensor integrated circuit (IC) placement

Select where to place the sensor IC according to the following criteria:

Place sensor IC next to electrode (see Figure 5) if:
Electrodes have to be small (example: 20 mm x 1 mm) (multiple electrodes are sometimes required to surround a radio frequency emitting device)
Detection distance has to be large (detection distance ≈ electrode length along edge)
Sensing through a metal aperture
Large metal objects are close to the area intended for the electrodes
Place sensor IC away from electrodes with a shielded cable to cover the distance between the sensor IC and the electrode (see Figure 5) if:
Detection distance may be small (detection distance < 0.5 * electrode length along edge)
Electrodes may have a large size compared to the required detection distance
No large metal structures are close to the electrodes
Sensor IC can compensate for the capacitive load introduced by the shielded cable

Figure 5. Description of different sensor IC placement strategies

Capacitive sensors measuring proximity signal levels are sensitive to temperature changes. It is generally recommended to place the sensor IC on an isolated board to protect the IC and sensitive lines against rapid temperature changes.

Electrode placement

Electrode placement is a key design element. The electrode placement area is usually pre-defined by radio frequency (RF) antenna placement strategies. The sensor electrode placement within this area is important for effective non-iterative design. Devices normally have a thin edge, with a back cover on the one side and the display on the other.

Figure 6. (a) Side section view description and (b) electrode focus area description

Devices are qualified in SAR testing on the minimum trigger distance of all the angles tested. The thin edge (as shown in Figure 6 ) typically qualifies with the smallest trigger distance. Although compensated for, capacitive loading of the electrodes (electrode-to-ground coupling) reduces trigger distance, especially in the far proximity trigger distance (> 20 mm). For this reason it is best to find an optimum middle way in the size of the electrode, while focusing the electrode towards the thin edge and other parts that are important in the SAR tests. Figure 7 shows the SAR test done at various angles (A, B and C), highlighting the importance of electrode placement.

Figure 7. Electrode focus areas in relation to phantom body test angles

Electrodes should surround the RF antenna in order to offer a solution that practically protects the user. The device is also tested by covering only part of the antenna with the phantom, leaving the other part open, as shown in Figure 8.

Figure 8. SAR test showing the need to surround the antenna with the electrode
Troubleshooting

Extending trigger distance

A general recommendation is to design the electrode solution for optimal trigger distance while leaving at least one more sensitive threshold option. Extending the trigger distance can be done in any of the following ways:

    Extend the electrode along any edge where SAR testing is done. The effect of coupling to more of the phantom body in SAR testing without significantly enlarging the coupling to the device ground is a powerful method of extending trigger distance.
    Increase the coupling to the outer part of the device. Attaching electrodes with adhesive eliminates air gaps and optimizes the capacitive field of influence by extending it towards the medium with less restrictive dielectric properties. A spacer element is recommended to prevent mechanical instability.
    Increase testing speeds so that they are closer to typical human behavior. Depending on the algorithm in the solution used, this may expose improved trigger distances (specify: mm/sec).

RF interference

During integrated tests involving RF and capacitive sensing frequencies, interference may be minimized by:

Testing for interference with the test device as close as possible to the intended end product. Keep in mind that all additional wiring may act as a receiver for RF energy. Although a series resistor usually makes capacitive sensors relatively immune to RF interference, power lines may suffer from RF interference when long unshielded lines are used

Conclusion

By following the guidelines mentioned here, capacitive proximity sensing can be implemented with minimal design cycles and without the need to use an overly sensitive proximity threshold. By optimizing the electrode design for the specific device, along with ground considerations, a directed proximity field with good signal-to-noise ratio can be achieved.

Note:

See application note AZD080 for equations on how the difference in sensitivity between C1 and C3 is derived.