AN3150 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
Datasheet sections
- 1 Capacitive sensing overview
- 1.1 Capacitive sensing system equi valent circuit
- 1.2 Capacitive sensing acquisition method
- 2 Factors influencing proximity sen sitivity
- 3 Capacitive system ground
- 3.1 Connected applications
- 3.2 Portable applications
- 4 Proximity sensing electrode d esign
- 4.1 Ring, spiral, plain electrode design
- 4.1.1 Electrode surface size influence
- 4.1.2 Electrode shape influence
- 4.1.3 Influence of ground
- 4.2 Proximity detection range vers us electrode size
- 4.2.2 Test results
- 4.3 Proximity detection range ve rsus electrode shape
- 4.3.1 Test description
- 4.3.2 Test results
- 5 Temperature effect
- 5.0.1 Test description
- 5.0.2 Test results
- 6 Front Panel
- 7 Conclusion
- 8 Revision history
and only wakes up when it detects a user in its proximity. fundamental characteristic for power sensitive applications.
- Hardware tools: STM8T141-EVAL, ST8T143-EVAL, ST-TSLINK, STM8T14X-SB
- PC application: STM-STUDIO, STVP
Table 1. Applicable products
1 Capacitive sensing overview
1.1 Capacitive sensing syst em equivalent circuit
Figure 1. Equivalent capacitive circuit of a surface capacitive sensor third terminal on power outlets. capacitance on the application PCB are the two elements of CX.
Figure 2. Equivalent measured capacitance in the application surroundings.
- Minimizing the C F effect, also known as improving the application grounding.
- Minimizing the CX value. This value is impacted by the electrode and the overall application design. It can be improved by using an active shield. ai17441V2 Application CX Electrode 1 1 CT CF Cmeasured = CX + CT CF Cmeasured CX 1+() 1C T⁄ 1C F⁄+()⁄=
AN3150 Capacitive sensing overview
1.2 Capacitive sensing acquisition method
The principle of charge transfer is to charge the electrode capacitance (CX) using a stable power supply. When CX is fully charged, part of the accumulated charge is transferred from CX to an external sampling capacitance, referred to as CS. The transfer cycle is repeated until the voltage across the sampling capacitor CS reaches the end of acquisition reference voltage (VTRIP). The change in the electrode capacitance is detected by measuring the number of transfer cycles composing a burst (see Figure 2). Throughout this document the following naming conventions apply:
- The charge transfer period (tTRANSFER) refers to the charging of CX and the subsequent transfer of the charge to CS.
- The burst cycle duration (tBURST) is the time required to charge CS to VTRIP.
- The sampling period (tSAMPLING) is the acquisition rate.
Factors influencing proximity sensitivity AN3150
2 Factors influencing proximity sensitivity
The following factors influence proximity sensitivity:
- Capacitive system ground – A battery supplied portable system versus a well-grounded system offer different system sensitivity
- Electrode (CX) – Size: a bigger the electrode size increases the coupling between the user and the electrode and thus improves the sensitivity. – Shape: the shape of the electrode is directly linked to the detection area expected for the application. – Ground and supply proximity to sensor and tracks. – Route and distance between electrodes and proximity sensor.
- Sampling capacitor (C – Increasing the C S capacitor increases the resolution of the CX measurement.
- Panel – Materials and thickness can modify the sens itivity. The better the dielectric value, the better the sensitivity of the sensors (glass is better than plastic). – Avoid air gaps between the sensor and the overlay material. For example, use a spring with a conductive surface pressing against the overlay where big air gaps exist or attach the touch pad directly to an overlay using non-conductive glue/double-sided tape.
- Power supply – Variation in the power supply level can disrupt the sensor measurement and cause unwanted proximity detection.
- Temperature – Temperature variations have an effect on the proximity sensor measurement Special care should be taken of the above factors to ensure stability of the sensor systems. Increasing the sensitivity of the sensor excessively (by increasing the electrode size or the C S) can cause an unstable sensor system and trigger unwanted proximity detection.
3 Capacitive system ground
Figure 3. Detection range comparison between portable and grounded system
3.1 Connected applications
From a capacitive sensing point of view, connected applications form two groups. some home appliances and white goods applications. includes most consumer products, for example USB powered devices. the value is fixed, it cannot be misinterpreted as a touch or device proximity.
Figure 4. Equivalent connected to earth application schematic short from a touch sense capacitance measurement. CLG1 and CLG2 are the capacitances between the power lines and the application ground. a physical capacitor across the power supply. capacitances, are the main components of the feedback capacitance CF.
3.2 Portable applications
and the application ground, see Figure 5: Equivalent portable application schematic.
Figure 5. Equivalent portable application schematic
Proximity sensing electrode design AN3150
4 Proximity sensing electrode design
Each application has its specific proximity detection requirement. The design of the proximity sensing electrode is the result of the performance target requirement integrating application constraints. To help proximity sensor integrators, the following chapters provide experimental results that can be used as guidelines to design the proximity sensing electrodes. These tests have been performed on STM8T141 or STM8T143. The absolute performance values like the detection distance, depend on the detection threshold setting or system gain that are specific to the application.
4.1 Ring, spiral, plain electrode design
The performance results presented in this application note were measured using electrodes with basic shapes. The differences between these electrodes were obtained by changing only one parameter at a time in order to clearly show each parameter impact. The differences between electrodes highlight the influence of:
- Electrode size with 5 mm, 10 mm, and 20 mm electrodes.
- Electrode shape with plain, spiral, and loop electrodes.
- Backside influences with grounded or non conductive electrode backside layers. The PCB layouts used for these tests are shown in Figure 6.
- Electrodes 1, 4 and 7 include square spiral electrode shape
- Electrodes 2, 5 and 8 include square loop electrode
- Electrodes 3, 6 and 9 include full plain square electrode
Figure 6. PCB layout of test electrodes Characteristics of electrodes 1 to 9 are summarized in Table 2. Table 2. Electrode descriptions and characteristics
1 Spiral, 400mm2, no plane
2 Loop, 400mm2, no plane
3 Plain, 400mm
4 Spiral, 100mm2, no plane
5 Loop, 100mm2, no plane
6 Plain, 100mm2, no plane
7 Spiral, 25mm2, no plane
8 Loop, 25mm2, no plane
9 Plain, 25mm2, no plane
- For the smallest electrode, C X values are not relevant due to their very low values.
4.1.1 Electrode surface size influence
- It increases the ‘useful’ capacitance (CT) by increasing the coupling between the user close by and the electrode. This in turn improves the sensitivity.
- It also increases the parasitic capacitance (CX) between the electrode and ground and consequently reduces the sensitivity. (Figure 7) shows the influence of the sensing electrode size for a loop, spiral and plain pattern.
Figure 7. Detection range versus electrode size (performed on STM8T141) also increasing the noise susceptibility.
4.1.2 Electrode shape influence
The sensing electrode shape has also a direct influence on the sensor performance. a single wire design are options to consider to shape the detection volume. Application constraints like mechanical environment will modify the sensing electrode field. smaller when the electrode is close to a metal object than away from it.
Figure 8. Detection range versus electrode shape sensitivity. The CX capacitance is smaller, therefore the acquisition gain (CS/CX) is higher.
4.1.3 Influence of ground
on the back side of electrode 4, 5 and 6 of (Figure 6). Figure 9. Detection range versus GND plane reduces significantly the detection distance. GND field plane is also called “passive shield”. Please refer to AN 3230 and 4010 for practical applications of passive shield.
4.2 Proximity detection ra nge versus electrode size
case where sometimes the GND is very close to sensing electrode.
4.2.2 Test results
Figure 12. Detection distance versus dielectric thickness Figure 12. show that the detection distance is strongly influenced by the GND plane Figure 13. Detection distance versus electrode size
Figure 13. show that the surface size is not able to compensate the influence of the GND
- It increases the ‘useful’ capacitance (CT) by increasing the coupling between the close- by user and the electrode. This in turn improves the sensitivity. It also increases the parasitic capacitance (CX) between the electrode and the GND plane, and consequently reduces the sensitivity.
4.3 Proximity detection rang e versus electrode shape
different dielectric thickness have been evaluated.
4.3.1 Test description
Figure 14. These test have been performed with a 0.8mm and 1.6mm dielectric thickness is covered by the test object. Figure 14. Test boards form factors
4.3.2 Test results
Figure 15. Detection distance versus electrode form factor Figure 15. shows that the detection distance is not influenced by the length-over-width ratio rectangle electrode will be easier to cover with a human hand than the 70.7mm x 2.8mm. For all electrode shapes, the effect of the GND plane is similar.
5 Temperature effect
System”. Please refer to the product specification for more information.
5.0.1 Test description
5.0.2 Test results
Figure 16. Signal versus temperature Depending on the system sensitivity, the signal drift can be important.
6 Front Panel
Each material type is defined with a dielectric permittivity constant, known as epsilon (εr). The higher εr, the easier the electrical field is transmitted through the material. between the electrodes and the finger. good dielectric performance of the glass panel. Figure 17. Influence of panel on detection distance
7 Conclusion
This proximity sensor guideline gives some indications of which parameters to consider when developing a proximity application. The deal is to find the best compromise between sensitivity, stability, and immunity to noise. Depending on the application constraints, environmental change, board design constraints, and power supply, many considerations should be take into account, including:
- Define electrode design.
- Evaluate environmental constraints
- Use a shielding to bring detection directivity.
- Determine device internal parameters to optimize system performance and stability. The best practice is to develop a prototyping device as similar as possible to the final design in order to be able to already integrate all system constraints during engineering phase.
8 Revision history
Table 3. Revision history 03-Mar-2010 1 Initial release. shape, Chapter 5: Temperature effect. capcitor”, “power supply variation influence”. Updated Section : Introduction: development tools description. 28-May-2013 3 Updated Table 1: Applicable products.