ST AN4310 Sampling Capacitor Selection Guide for Touch Sensing Applications User Guide
- June 12, 2024
- ST
Table of Contents
ST AN4310 Sampling Capacitor Selection Guide for Touch Sensing
Applications
Product Information
Product Name: AN4310 – Sampling capacitor selection guide for touch sensing
applications on MCUs
Product Type: Application note
Manufacturer: STMicroelectronics
Applicable Products
Product Series
STM32F0 series
STM32F3 series
STM32L0 series
STM32L1 series
STM32L4 series
STM32L4+ series
STM32L5 series
STM32U5 series
STM32WB series
STM32WBA series
Product Usage Instructions
Charge Transfer Acquisition Principle Overview
The product utilizes the charge transfer acquisition principle for touch
sensing applications. This principle is explained in detail in the user
manual.
Capacitor Characteristics
Dielectric Absorption or Soakage
Dielectric absorption, also known as soakage, can impact the operation and
accuracy of capacitive sensors. It is caused by charge trapped in the
dielectric material, leading to a voltage offset on the sampling capacitor
(CS). This offset voltage affects the sensitivity and may result in false
proximity detections. To mitigate this, it is recommended to choose a
capacitor with a low dielectric absorption factor. Refer to Table 2 for a
comparison of dielectric absorption factors for different types of capacitor
dielectrics.
Nonzero Temperature Coefficient
To ensure trouble-free operation across the application’s temperature range,
it is important to select a capacitor with a stable temperature coefficient.
Capacitors with dielectrics like PET, PEN, PPS, and NPO are recommended as
they have lower temperature characteristics compared to normal ceramic
capacitors.
Sampling capacitor selection guide for touch sensing applications on MCUs
Introduction
Capacitors feature some non-ideal characteristics that unfortunately limit
their use in some applications. The objective of this document is to help
designers in selecting the right sampling capacitor (CS) for their touch
sensing applications by investigating the most important undesirable
characteristics.
STMicroelectronics is providing free STMTouch touch sensing firmware
libraries, directly integrated into the corresponding STM32Cube package (such
as STM32CubeL4).
Table 1. Applicable products
| Type | Product series |
|---|---|
| Microcontrollers | STM32F0 series, STM32F3 series, STM32L0 series, STM32L1 |
series, STM32L4 series, STM32L4+ series, STM32L5 series, STM32U5 series, STM32WB series, STM32WBA series.
Charge transfer acquisition principle overview
An MCU-based touch sensing applications may use the charge transfer
acquisition principle, supported by STMTouch touch sensing libraries, to sense
changes in capacitance. The electrode capacitance (CX) is charged to a stable
reference voltage (VDD for general purpose Arm®-based STM32 microcontrollers).
The charge is then transferred to a known capacitor referred to as the
sampling capacitor CS. This sequence is repeated until the voltage on CS
reaches the internal reference voltage VIH. 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
that compensates for environmental changes such as temperature, power supply,
moisture, and surrounding conductive objects.
Since CS is an integral part of the design, the non-ideal effects of
capacitors must be considered.
Note: Arm is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.
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.
Dielectric absorption or soakage
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 dependent on the relaxation time
between transfers and the discharge time of the CS capacitor. This phenomenon
is illustrated in the figure below.
The residual charge bleeds back (IRESIDUAL) through the insulation resistor
(IR) to cause a voltage offset on the CS 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 for a comparison of
dielectric absorption factors for the different types of capacitor
dielectrics.
Nonzero temperature coefficient
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.
Dissipation factor
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 for a comparison of the dissipation factors for
the various dielectrics.
Capacitor comparison
The table below compares the most important characteristics that need to be reviewed when selecting a CS capacitor.
Table 2. Characteristics of film SMD capacitors
Characteristic| PET| PEN| PPS| NPO| X7R|
Tantalum
---|---|---|---|---|---|---
Operating temperature (°C)| -55 to 125| -55 to 125| -55 to 140| -55 to 125|
-55 to 125| -55 to 125
ΔC/C with temperature (°C)| ±5| ±5| ±1.5| ±1| ±1| ±10
Dissipation factor (%)
| 1 kHz| 0.8| 0.8| 0.2| 0.1| 2.5| 8
10 kHz| 1.5| 1.5| 0.25| 0.1| –| –
100 kHz| 3.0| 3.0| 0.5| 0.1| –| –
Dielectric absorption (%)| 0.5| 1| 0.05| 0.6| 2.5| –
ESR| Low| Low| Very low| Low| Moderate to high| High
Reliability| High| High| High| High| Moderate| Low
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.
Conclusion
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.
Revision history
Table 3. Document revision history
| Date | Version | Changes |
|---|---|---|
| 15-Jul-2013 | 1 | Initial release. |
| 11-Jun-2014 | 2 | Added support for STM32L0 Series and STM8AL Series. |
20-Oct-2015
| ****
3
| Updated Table 1.
Added support for STM32L4 Series.
23-Jan-2019
| ****
4
| Updated:
- Title of the document
- Table 1. Applicable products
23-Aug-2021
| ****
5
| Updated:
- The Introduction section.
- The Applicable products table.
- The Charge transfer acquisition principle overview section.
10-Jan-2023
| ****
6
| Updated Section Introduction in the Table 1. Applicable products to incorporate the STM32WBA series.
Updated the whole document with minor changes.
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