Surface charge transfer acquisition principle overview

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Surface charge transfer acquisition principle

overview

STM32F0, STM32F3 and STM32L0 series use a surface charge transfer acquisition

principle. This principle is briefly described below.

The surface charge transfer acquisition principle consists in charging a sensor capacitance

(Cx) and transferring a part of the accumulated charge into a sampling capacitor (Cs). This

case). The number of charge transfers required to reach the threshold is a direct

representation of the size of the electrode capacitance.

When the sensor is touched, the sensor capacitance to the earth is increased so the Cs

voltage reaches the threshold with less count and the measurement value decreases. When

the measurement value falls below a defined threshold, a detection is reported.

Noise injection disturbs the measurement proportionally to its amplitude and depending on

its frequency. Worst case is generally found at a noise frequency close to the charge

transfer frequency (assuming no techniques are used to spread this frequency).

Figure 3. Charge transfer equivalent capacitance model