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 (C
undesirable characteristics. For STM8T141 devices, the specific power mode selected and
the proximity sensitivity will also directly influence this decision.
) for their applications by investigating the most important
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November 2011 Doc ID 15600 Rev 2 1/8
www.st.com
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 (C
charged to a stable reference voltage (V
for STM8T141 devices and VDD for general
REG
purpose STM8/STM32 devices). The charge is then transferred to a known capacitor
referred to as the sampling capacitor (C
the C
capacitor reaches an internal reference voltage (V
S
V
for general purpose STM8/STM32 devices). The number of transfers required to reach
IH
). This sequence is repeated until the voltage on
S
for STM8T141 devices and
TRIP
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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2/8 Doc ID 15600 Rev 2
AN2966 Capacitor characteristics
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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.
2.1 Dielectric 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 C
capacitor. This phenomenon effectively
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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 C
phenomenon is illustrated in
Figure 1. The residual charge bleeds back (I
capacitor. This
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RESIDUAL
) through
the insulation resistor (IR) to cause a voltage offset on the CS capacitor.
Figure 1. Model of dielectric absorption
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
Ta bl e 1 for a comparison of dielectric absorption factors for the different types of
capacitor dielectrics.
Doc ID 15600 Rev 2 3/8
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