AN2869 datasheet(20/43 Pages) STMICROELECTRONICS

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Surface electrode design

AN2869

20/43

Doc ID 15298 Rev 6

3.3

Linear sensor

A linear sensor is a set of contiguous capacitive electrodes connected to the device and

placed in a single line. Touch sensors are typically linear, running only along a single axis.

They can be made up of a set of electrodes, depending on the required size and resolution.

Linear sensors use differential capacitance changes between adjacent capacitive

electrodes to determine the finger or conductive object position with greater resolution.

The size and targeted application tend to dictate the linear touch sensor layout. However,

some general rules apply to any kind of layout:

●

To ensure that the conductive object couples to more than one element, each element

must be small enough for the finger to overlap its outside edge. However, it must also

be large enough to have correct sensitivity even through the application overlay.

●

The extremities must be a half electrode and both should be connected so that the

linear touch sensor is well balanced (see Figure 13).

There are 2 kinds of linear sensors:

●

Normal patterned linear sensors

●

Interlaced patterned linear sensors

3.3.1

Normal patterned linear sensor

With a normal patterned linear sensor (see Figure 13), the linearity is limited due to the ratio

square width versus finger touch area. To improve the linearity, to get a smoother transition

between items and to increase the resolution, it is recommended to use an interlaced

electrode with crisscross teeth as shown in Figure 14.

Figure 13.

Normal patterned linear sensor with five electrodes (20-50 mm long)

Legend: e is the gap between two sensor electrodes, h is the height of the sensor electrode, and w is the

width of the sensor electrode.

The size of the square electrode and gap between electrodes are valid irrespective of the

number of electrodes.

To get larger linear sensors, the number of electrodes can be increased to eight.

This solution is mainly used by RC acquisition principle.

ai15087

2345

1 mm

15 mm

0.5 mm

8 mm

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