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 is briefly examined in the following sections.

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 C_S capacitor. This phenomenon effectively creates a memory effect in the capacitor. The size of the offset voltage is dependent on the relaxation time between transfers and the discharge time of the C_S capacitor. This phenomenon is illustrated in the figure below.

The residual charge bleeds back (I_RESIDUAL) through the insulation resistor (IR) to cause a voltage offset on the C_S capacitor. 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 2 Characteristics of film SMD capacitors for a comparison of dielectric absorption factors for the different types of capacitor dielectrics.

To ensure trouble free operation over the final application operating temperature range, the selected capacitor must feature a stable temperature coefficient.

Dielectrics like PET, PEN, PPS, and NPO usually have higher temperature characteristics than normal ceramic capacitors and are thus recommended.

The dissipation factor is an indication of the energy loss, usually in the form of heat. Capacitors with a high dissipation factor cause self-heating that lay affect the capacitance. This change in capacitance in turn affects the number of charge transfers needed to reach the internal reference voltage threshold.

This also emphasizes the need to choose a dielectric with a stable temperature coefficient. Refer to Table 2 Characteristics of film SMD capacitors for a comparison of the dissipation factors for the various dielectrics.

The table below compares the most important characteristics that need to be reviewed when selecting a C_S capacitor.

The PPS (polyphenylene sulfide) dielectric and the NPO ceramic capacitors performs excellently in all categories. The PET (metalized polyester) and the PEN (metalized polyphenylene naphthalate) capacitors also perform quite well and can be used in all touch sensing applications.

Tantalum capacitors must be avoided as they have a very high dissipation factor and a high-effective series resistance (ESR). X7R ceramic capacitors can be used in some applications when a less sensitive level is required.

As explained, the sampling capacitor characteristics play an important role in the correct and stable operation of a capacitive sensing application. Consequently, it is necessary to select it carefully.

The recommendations for STMTouch touch sensing library-based applications are summarized below:

• If the solution uses an MCU low-power mode to reduce overall power consumption, PET, PEN, PPS, or NPO capacitor types must be used.
• If the solution uses linear or rotary touch sensors, PET, PEN, PPS, or NPO capacitor types must be used.
• If the solution uses only touchkey sensors, all capacitor types except tantalum can be used.