AN4038 datasheet(24/39 Pages) STMICROELECTRONICS

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Proximity sensor design

AN4038

24/39

Doc ID 022686 Rev 1

4.4

Electrode size and shape

The sensor size and shape define the detection area. The size of the sensor should be

defined to be in line with the size of the object to be detected. With a capacitive projected

technology, the size of the object to be detected has a small influence on the detection area.

The detection area is defined by the projected capacitance field which is determined by two

electrodes designs. Figure 23 shows the detection area with a hand compared to a finger

approach.

Figure 23.

Hand versus finger detection area

Generally, we can consider that the object size does not influence the detection area if its

size is in the same range or greater than the electrode field.

On the other hand, if the size of the object to be detected is smaller than the electrode size,

the detection range is lower. In fact, if the object covers only a part of the projected

capacitance field, its influence is smaller. In this case, the variation on the acquisition signal

is lower and the detection range is less than with a higher triggered object

(see Table 6).

In conclusion, the size of the object to be detected (e.g. a hand or a finger) has very little

influence on the detection area (Figure 23) but, it does influence detection distance

(

Table 6).

The shape of the sensor influences the proximity detection area (see Figure 24) which

compares three electrodes of equivalent size but different shape.

Table 6.

Detection range versus hand or finger approach

Object

Detection range (mm)

Hand

Finger

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