AN4038 datasheet(35/39 Pages) STMICROELECTRONICS

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AN4038

Conclusion

Doc ID 022686 Rev 1

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Conclusion

This proximity sensor guideline gives some indications of the parameters to consider when

developing a proximity application.

Results from the tests presented in this application note, advise designers to find the best

compromise between sensitivity, stability, and noise immunity.

7.1

Practical example

An example is given below of how to design two electrodes which have a size constraint.

●

Electrode #1, size constraint = 50 mm2

●

Electrode #2, size constraint = 300 mm2

In accordance with the recommendations given in this application note, to obtain the best

detection range and the least noise amplitude, the following parameters for Electrode #1

and Electrode #2 are proposed.

Electrode #1

●

Because of its small size, increase the spacing between the Tx/Rx electrodes as much

as possible. This should improve detection distance and increase noise. The best

compromise is to favor the best detection distance. Take care not to space the

electrodes too far apart as this can create a projected capacitance which is too small. If

this happens, STM8TL5xxx devices are not be able to compensate part of the

projected capacitance with the EPCC to achieve the target reference. Consequently,

the EPCC will be out of the recommended range (see Section 3.4: Electrode parasitic

capacitance compensation (EPCC) and can lead to an unstable proximity system.

Electrode spacing selected: 4 mm.

●

Increase the Rx/Tx electrode length to improve the detection distance and decrease

the noise by increasing the projected capacitance. Electrode length selected: 7mm

●

Chose an electrode width which does not penalize the detection distance and does not

increase noise. Electrode width selected: 1.5 mm.

●

Reduce the target reference as much as possible to reduce noise, but check the EPCC

value is in the recommended range. The target reference selected is 1000 and the

threshold setup is in accordance with the noise standard deviation recommendation.

Electrode #2

●

With a big electrode, increase the Rx/Tx length to improve detection distance and

reduce noise. Electrode length selected: 35 mm

●

Increase the spacing between the Tx and Rx electrodes to improve the detection

range. Note that increasing the spacing increases noise: always give preference to

increasing electrode length before electrode spacing. Electrode spacing selected:

5 mm.

●

Chose an electrode width which does not penalize the detection distance or increase

noise. Electrode width selected: 1.5 mm.

Figure 34 shows the parameters chosen for electrodes #1 and #2.

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