AN4038 datasheet(20/39 Pages) STMICROELECTRONICS

20 / 39 page

Proximity sensor design

AN4038

20/39

Doc ID 022686 Rev 1

Figure 17 shows the influence of the electrode spacing on the standard deviation of the

noise.

Figure 17.

Noise standard deviation versus Tx/Rx electrode spacing

The noise is proportional to the sensitivity and with an electrode spacing greater than 2 mm,

the noise increases slowly.

In conclusion, the largest electrode spacing increases sensitivity to the detriment of the

noise. Increased noise requires that the detection threshold is adjusted to filter out noise

and guarantee system stability. Therefore, a compromise is necessary between detection

range and system noise immunity. If the Tx/Rx electrodes are too close, the detection

distance is drastically decreased, especially when the spacing is less than 2 mm. Care

should be taken not to space the electrodes too far apart as this can create a projected

capacitance which is too small to be measured. In this case, the STM8TL5xxx cannot

compensate with the EPCC to target the reference. The EPCC is out of the recommended

range (see Section 3.4: Electrode parasitic capacitance compensation (EPCC) and can

cause an unstable proximity system. To summarize, increasing the electrode spacing

increases the detection area as long as the EPCC remains in the recommended range.

AN4038 データシート(PDF) 20 Page - STMicroelectronics