AN2966 Datasheet, PDF(2/8 Page) STMicroelectronics – Capacitor selection guide for STM8T141 capacitive sensor

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AN2966 Datasheet, PDF (2/8 Pages) STMicroelectronics – Capacitor selection guide for STM8T141 capacitive sensor

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Charge transfer acquisition principle overview

AN2966

Charge transfer acquisition principle overview

The STM8T141 capacitive sensor uses the charge transfer acquisition principle to sense

changes in capacitance. The electrode capacitance (CX) is charged to a stable reference

voltage (VREG). The charge is then transferred to a known capacitor referred to as the

sampling capacitor (CS). This sequence is repeated until the voltage on the CS capacitor

reaches an internal reference voltage (VTRIP). The number of transfers required to reach the

threshold depends on the size of the electrode capacitance and represents its value.

To ensure stable operation of the solution, the number of transfers needed to reach the

threshold is adjusted by an infinite impulse response (IIR) filter which compensates for

environmental changes such as temperature, power supply, moisture, and surrounding

conductive objects.

Since the CS capacitor is an integral part of the design, it is important to consider the non-

ideal effects of capacitors.

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