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Electrode design in capacitive touch sensor apps

07 Sep 2012  | Cheok Thng

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In a simulation for a pair of shifted electrodes with equal size, the change of capacitance due to a finger 2mm above the touch pad is as much as 31%, an amount that is unambiguous to the sensor IC (figure 3). But when the finger moves to the ground plane, 2mm above, the same amount of signal can also be read. With similar signal levels, the sensor IC cannot differentiate the touch on the touch-pad, from the touch on the ground plane.

Figure 3: Simulation of a finger touching above an electrode.

Methods to control sensitivity of electrodes
As the objective is to make the touch-pad the only sensing port, methods to sensitize the touch pad, and to desensitize the ground plane is very much needed.

One common method is by putting the touch pad closer to the user, and hiding the ground plane deeper inside the housing.

Another method which is the center piece of this article is by having a bigger ground plane compared to the touch pad. Automatically, the sensitivity will shift to the smaller plate which is the touch pad.

Since the total charges on both plates are the same, the plate with the smaller area will have a higher charge density, more concentrated flux lines, stronger e-field, more polarization force, and therefore more sensitive to influence from the external object (figure 4).

Figure 4: The electric field at the smaller plate is stronger than the electric filed at the bigger plate.

Numerical simulation for touch pad with big ground plane
As seen in the next simulation for a pair transducers with unequal plate size, a touch 2mm above the touch-pad yields a 62% change in capacitance (figure 5), compared to an 7% change for the touch above the ground plane (figure 6). With such distinctive signal levels, an appropriate threshold level will differentiate the touch on the touch-pad, from the touch on the ground plane.

Figure 5: A strong signal is produced when the finger is above the touch pad.

Figure 6: A weak signal is produced when the finger is above the big ground plane.

Mutual capacitances behind capacitive sensing
A rigorous explanation for the unbalanced sensitivity is by means of mutual capacitance formed between the human and touch-pad CTF, and mutual capacitance between human and ground plane CGF.

Figure 7: A Mutual capacitances between electrodes and human.

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