ST AN2966 Application note

AN2966

Application note

Capacitor selection guide for STM8T141 and touch sensing library-based capacitive sensors

Introduction

Capacitors feature some non-ideal characteristics that unfortunately limit their use in certain applications. The objective of this application note is to help designers in selecting the right sampling capacitor (CS) for their applications by investigating the most important undesirable characteristics. For STM8T141 devices, the specific power mode selected and the proximity sensitivity will also directly influence this decision.

November 2011

Doc ID 15600 Rev 2

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Charge transfer acquisition principle overview	AN2966

1 Charge transfer acquisition principle overview

The STM8T and touch sensing library-based capacitive sensors use the charge transfer acquisition principle to sense changes in capacitance. The electrode capacitance (CX) is charged to a stable reference voltage (VREG for STM8T141 devices and VDD for general purpose STM8/STM32 devices). 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 for STM8T141 devices and VIH for general purpose STM8/STM32 devices). 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 nonideal effects of capacitors.

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AN2966	Capacitor characteristics

2 Capacitor characteristics

The most common short comings of capacitors are the following:

●Series resistance

●Series inductance

●Parallel resistance (leakage current)

●Non-zero temperature coefficient

●Dielectric absorption (DA) or soakage

●Dissipation Factor

The three most important characteristics that need to be examined are non-zero temperature coefficient, dissipation factor and dielectric absorption (DA). The effect of these non-ideal characteristics on the operation of the system will be briefly examined in the following sections.

2.1Dielectric absorption or soakage

Dielectric absorption (DA) or soakage can be detrimental to the operation and accuracy of capacitive sensors that rely on a stable reference capacitor.

DA is caused by the charge that is soaked-up in the dielectric and remains there during the discharge period. The charge then trickles back out of the dielectric during the relaxation period and cause a voltage to appear on the CS capacitor. This phenomenon effectively creates a memory effect in the capacitor. The size of the offset voltage is dependant on the relaxation time between transfers and the discharge time of the CS capacitor. This

phenomenon is illustrated in Figure 1. The residual charge bleeds back (IRESIDUAL) through the insulation resistor (IR) to cause a voltage offset on the CS capacitor.

Figure 1. Model of dielectric absorption

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This offset voltage influences the sensitivity of the system by reducing the number of transfers needed to reach the internal reference voltage threshold and may cause false proximity detections to occur.

By choosing a capacitor with a low dielectric absorption factor, a higher sensitivity level can be selected, ensuring a more stable and reliable design with improved proximity detections. Refer to Table 1 for a comparison of dielectric absorption factors for the different types of capacitor dielectrics.

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