september 2013 docid015600 rev 3 1/8 AN2966 application note capacitor selection guide for stm8t141 capacitive sensor introduction capacitors feature some non-ideal characteristic s that unfortunately limit their use in certain applications. the objective of this application no te is to help designer s in selecting the right sampling capacitor (c s ) for their applications by investigating the most important undesirable characteristics. fo r stm8t141 devices, the specific power mode selected and the proximity sensitivity will also di rectly influence this decision. table 1: applicable products type applicable products microcontrollers stm8t141 www.st.com
charge transfer acquisition principle overview AN2966 2/8 docid015600 rev 3 1 charge transfer acquisi tion principle overview the stm8t141 capacitive sensor uses the c harge transfer acquisition principle to sense changes in capacitance. the electrode capacitance (c x ) is charged to a stable reference voltage (v reg ). the charge is then transferred to a known capacitor re ferred to as the sampling capacitor (c s ). this sequence is repeated until the voltage on the c s capacitor reaches an internal reference voltage (v trip ). 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 impuls e response (iir) filter which compensates for environmental changes such as temperature, power supply, moisture, and surrounding conductive objects. since the c s capacitor is an integral part of the design, it is important to consider the non- ideal effects of capacitors.
docid015600 rev 3 3/8 AN2966 capacitor characteristics 8 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 di electric absorption (da). the effect of these non-ideal characteristics on th e 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 det rimental 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 dependant on the relaxation time between transfers and the discharge time of the c s capacitor. this phenomenon is illustrated in figure 1 . the residual charge bleeds back (i residual ) through the insulation resistor (ir) to cause a voltage offset on the c s 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 abs orption 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 absorpt ion factors for the different types of capacitor dielectrics. , 5 & 6 & 5 ( 6 , ' 8 $ / , 5 ( 6 , ' 8 $ / d l
capacitor characteristics AN2966 4/8 docid015600 rev 3 2.2 non-zero temperature coefficient to ensure trouble free operation over the fina l application operating temperature range, it is important to select a capacitor featuring a stable temperature coefficient. dielectrics like pet, pen, pps and npo usua lly have higher temperat ure characteristics than normal ceramic capacitors and are thus recommended. 2.3 dissipation factor the dissipation factor is an indication of the energy loss, usually in the form of heat. capacitors with a high dissipation factor ge nerally cause self-heating which affects the capacitance. this change in capacitance in tu rn affects the number of charge transfers needed to reach the internal reference voltage threshold. this also emphasizes the need to choose a dielec tric with a stable temperature coefficient. please refer to table 2 for a comparison of the dissipation factors for the various dielectrics.
docid015600 rev 3 5/8 AN2966 capacitor comparison 8 3 capacitor comparison table 2 compares the most important characteri stics that need to be reviewed when selecting a c s capacitor. the pps (polyphenylene su lfide) dielectric and the npo ceramic capacitors performs excellently in all categories. the pet (metallized polyest er) and the pen (metallized polyphenylene naphthalate) capacitors also perfo rm quite well and can be used in all touch sensing applications. tantalum capacitors should be avoided as t hey have a very high dissipation factor and a high effective series resistance (esr). x7r ceramic capacitors can be used in certain applications when a less sensitiv e proximity level is required. stm8t141 capacitive sensor have selectable low power modes with zoom in which the performance of the x7r dielectric is not acceptable due to its high dissipation factor and capacitance change over temperature. da has also a considerable influence on application operation in low power modes with zoom, due to the fact that the time between charge transfers varies. table 2. characteristics of film smd capacitors 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 c/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
conclusion AN2966 6/8 docid015600 rev 3 4 conclusion as explained, the sampling capacitor characteri stics play an important role in the correct and stable operation of a capacitive sensing ap plication. consequently , it is necessary to select it carefully. the recommendations for the stm8t141 capacitive sensor are summarized below: ? when the stm8t141 low po wer modes with zoom are used, pet, pen, pps or npo capacitor types must be used. ? if the stm8t141 is used for proximity detection, pet, pen, pps or npo capacitor types should be used. ? if the stm8t141 is used for touch detection, all capacitor types except tantalum can be used.
docid015600 rev 3 7/8 AN2966 revision history 8 5 revision history table 3. document revision history date revision changes 04-may-2009 1 initial release. 14-nov-2011 2 document updated to include only stm8t141 and touch sensing library-based capacitive sensors. other changes include: section 1: charge transfer acquisition principle overview : renamed and content rewritten. section 2: capacitor characteristics : renamed. section 2.1: dielectric absorption or soakage : replaced ?v trip ? by ?internal reference voltage threshold?. section 2.2: non-zero temperature coefficient : last sentence updated. section 2.3: dissipation factor : added ?charge? to ?charge transfers?; replaced ?v trip ? by ?internal reference voltage threshold?. section 3: capacitor comparison : layout and small text changes. section 4: conclusion : added. 17-sep-2013 3 template updated to v5.3. other changes: touch sensing recommendations removed.
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