AN4312 datasheet(20/36 Pages) STMICROELECTRONICS

20 / 36 page

Surface sensor design

AN4312

20/36

DocID024815 Rev 1

3.3

Linear sensor

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

placed in a single axis line. The number of electrodes is dependant of the desired size and

resolution of the sensor.

The electrodes can be arranged in 3 different ways:

•

Mono electrodes design: each channel is associated to only one electrode (see

Figure 13).

Figure 13. Mono electrodes design

•

Half-ended electrodes design: the first and the last electrodes are connected to the

same channel (usually the first one) and their width is half the width of the other

electrodes. This is to ensure that all the electrodes capacitance are identical (see

Figure 14).

Figure 14. Half-Ended electrodes design

•

Dual electrodes design: all the electrodes are duplicated and interlaced together (see

Figure 15).

Figure 15. Dual electrodes design

The half-ended and dual electrodes designs are used to increased the touch area of the

sensor.

On top of that, there are 2 manners to design the electrodes pattern on the PCB:

•

Normal pattern (see Figure 16).

•

Interlaced pattern (see Figure 17.).

These 2 patterns are described in more details below.

3.3.1

Normal patterned linear sensor

With a normal patterned half-ended linear sensor (see Figure 16), 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 patterned electrodes design with crisscross teeth as shown in Figure 17.

MS32445V1

Ch1

Ch2

Ch3

MS32446V1

Ch1

Ch2

Ch3

Ch1

MS32447V1

Ch1

Ch2

Ch3

Ch4

Ch5

Ch1

Ch3

Ch5

Ch2

Ch4

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