The minimum overlay thickness is mentioned due to the property of CSX sensors called signal disparity. Under normal operation, the raw counts that you see increases when the finger touches the sensor. But, with signal disparity, the raw counts will decrease when the sensor is touched.
You can use a thicker overlay if the application requires it, without increasing the Tx-Rx separation. This will result in a slightly lower signal. You can always use an overlay of larger thickness. A glass overlay (permittivity = 8) will give a better performance than acrylic (2.8 - 3) for the same thickness. The sensor separation is chosen keeping the signal level and linearity in consideration.
This is good news, as was mentioned in the 2nd point of the answer to an earlier post Using capsense to sense plastic objects , the overlay can be thicker than 0.5mm.
I do not understand exactly what signal disparity is, does it refer to the same concept as the ''turn around'' mentioned in this youtube video, where the capacitance starts increasing when the finger is too close to the sensing element? Even in this video, the author does not explain this phenomenon very well as he first mentions it is due to the finder moving away from the element, causing a spike increase in capacitance (at 2:13 of the video) But we then see someone approaching a finger too close to the sensor, causing the ''turnaround'' capacitance increase.
Bragadeesh had already mentioned this increase in the same previous cypress post:
That being said, I am interested in clearly understanding the way it works, I could not find any literature or documentation referring to mutual capacitance sensing ''turnaround'' or ''signal disparity''. Do you have some theory to share, or documentation references?
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Yes, the term signal disparity is the same as the "turn around" phenomenon mentioned in the video.
You can refer CapSense design guide, section 7.5 Effect of Grounding in CSX Method. Signal disparity (or turn around) is the condition that is explained under 7.5.2 CbodyDG<<Cfs.
As you can see, due to thin overlays causing an increased Cfs, the current drawn from the Tx increases when a finger touches the sensor. This causes the effective Cm as seen by Rx to increase and results in a dip in the raw count. Under normal operation, the Cbody takes current from the Tx and causes current flowing to Rx to decrease, thereby decreasing the Cm and increasing raw counts.
Please let us know if there are any further queries.