Principles and applications of A31T214 Shield CH
Principles of Shield CH operation
▶ When the sensing electrode is surrounded by GND:
When a drop of water or excess moisture is formed between the sensing electrode and the GND, a dielectric is formed between the two to increase the Cap. In this case, the Cap has the same electrical characteristics as the finger touch, and can be mistaken for a normal touch.
[Figure 1] Touch Electrode Surrounded by GND
▶ When the sensing electrode is surrounded by the shield electrode:
Even when a drop of water or excess moisture is formed between the sensing electrode and the shield electrode, similar to the above case, a dielectric is formed between the two electrodes to increase the Cap. In this case, however, the shield CH electrode is driven by the same waveform as the sensing electrode, so the voltage difference between the two electrodes is always maintained at 0V.
Even if an unexpected Cap is formed, there is no change in the charge and the water between the sensing electrode and the shield electrode does not affect the touch operation at all.
[Figure 2] Touch Electrode Surrounded by Shield CH
Application methods and examples of Shield CH
When water is formed between the sensing electrode and the GND or between other sensing electrodes, malfunction occurs due to water formation. Therefore, when designing a PCB, you should design the shield CH (not GND or other sensing electrodes) to surround the sensing electrode.
[Figure 3] shows a DoorLock Demo PCB, which shows that the touch CH is surrounded by the shield CH. This structure in Figure 3 is designed to prevent water from contacting the GND or other sensing electrodes.
[Figure 3] DoorLock Demo PCB
Water performance evaluation of DoorLock model
Using the DoorLock Demo Board with an A31T214 IC, we checked the touch/non-touch performance while water was flowing.
A DiffData of about 200 is formed while water flows, and a DiffData of 1200 or more is formed during touches. Therefore, setting the Touch Threshold to approximately 800 ensures SNR that allows normal touch operations.
[Figure 4] DiffData under Water Condition on the DoorLock Demo Board