AN4038 datasheet(28/39 Pages) STMICROELECTRONICS

28 / 39 page

Proximity sensor design

AN4038

28/39

Doc ID 022686 Rev 1

Figure 27 shows the theoretical detection distance versus the distance between the ground

backplane and the sensor for two dielectric materials: air and FR4.

Figure 27.

Detection distance versus distance between ground backplane and

electrode for two dielectric materials (air and FR4)

It is strongly advised never to put ground planes behind the sensor and to keep a clearance

distance between it and any ground object with an air thickness greater than 2 mm. For air

thicknesses under 2 mm, the ground backplane strongly decreases detection range.

The closer the surrounding ground is to the electrode plates, the lower the sensitivity is.

Table 7 shows that at a surrounding clearance of 2 mm, the detection distance is drastically

reduced.

In addition, the ground surrounding reduces the electrode detection area significantly.

Figure 28 shows the detection area and detection ranges obtained with a finger using the

same electrode with and without ground surrounding.

Table 7.

Detection range versus electrode ground surrounding

Surrounding clearance (mm)

Detection distance (mm)

211

515

No ground

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