Because it uses a traditional mechanical switch, users directly connect the capacitive sensor interface to the sensor's responsiveness (sensitivity) under various operating conditions (reliability). This article will introduce some general-purpose capacitive sensor analog front-end measurement methods. The sensitivity of a capacitive sensor is determined by its physical structure, the method of measuring capacitance, and its ability to accurately compare changes in capacitance relative to a contact threshold level. Capacitive sensors manufactured using traditional printed circuit board (PCB) methods typically have a measurement range of 1–20 pF, making it difficult to accurately detect minute changes. While several methods exist for measuring these minute capacitance values, high-precision measurement methods using a 16-bit capacitance-to-digital converter (CDC) still offer significant advantages.
Capacitive sensors fabricated on standard printed circuit boards or flexible printed circuit boards use the same copper material for signal lines. In both cases, the maximum sensitivity of the sensor is determined by its physical dimensions, dielectric constant, and coating thickness. For example, a 3mm thick sensor with a 5mm plastic coating is less sensitive than a 6mm thick sensor with a 2mm plastic coating.
Our goal is to develop capacitive sensors with accurate response and ergonomic design. In some applications, the sensor may be very small, resulting in minute capacitance changes at the user's contact surface.
Figures 1 and 2 illustrate two common methods for designing capacitive sensors on printed circuit boards. The figures show the sensor's response characteristics when an excitation signal is applied during user contact. Although the sensor capacitance varies depending on the user's contact method, the sensor's performance is largely similar in both cases.
A continuous 250kHz square wave excitation signal is applied to the SRC terminal of the sensor to establish an electric field within the capacitive sensor. After the excitation signal establishes the electric field in the sensor, this field partially extends beyond the plastic coating, and the ClN terminal is connected to the CDC. Figure 2 shows another example of a capacitive sensor design, in which a constant current source is applied to terminal A of the sensor, while terminal B is grounded. When a user touches the sensor, additional finger capacitance is added, thereby increasing the rise time of RC during the charging cycle.
Students should master basic concepts, principles, and techniques, especially gaining a deep understanding of technical difficulties and key points to lay a solid foundation for the course. Regarding hardware, one should be able to visualize the components inside a microcomputer, laptop, or mobile phone, understand the connections between these components, and their operational processes. By referring to performance indicators, one can evaluate the quality and applicability of a computer.
The application of computers lies in software, and learning and mastering programming is of paramount importance in this course. Programming serves as a bridge between humans and computers. Through continuous programming practice, students learn programming skills and the integration of application software, thereby continuously improving and refining their programming level, skills, and thinking methods. The extension and expansion of computer functions are accomplished through interface technology, which is also an essential technology and skill for practical engineering applications. Students should master the methods of consulting interface chip datasheets, be able to read them, understand and master the functions and parameters of interface chips, be able to connect interface chip pins to the computer's CPU pins, and be able to complete tasks such as address allocation, function control, and information exchange between the computer and peripherals.
A constant current source continuously charges the capacitive sensor until it reaches the comparator's reference threshold level. When the capacitive sensor reaches the reference threshold, the comparator outputs a high-level pulse, then closes the switch, discharging the capacitor and resetting the counter. A count of 50 indicates that the sensor is in contact, while a count less than 50 indicates no contact. In this example, when the user contacts the sensor, its accuracy and precision are related to the frequency of the reference clock and the repeatability of the current source driving the various capacitive sensors.
A reliable capacitive sensor interface begins with an analog front-end that must be able to measure minute changes in output caused by a user touching the capacitive sensor. Now, new, highly integrated CDCs allow design engineers to benefit from integrated low-power, high-resolution mixed-signal technology.
