The sensitivity of a capacitive sensor is determined by its physical structure, the method of measuring capacitance, and the 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 to 20 pF, making it difficult to accurately detect small changes. While there are several ways to measure the tiny values of these capacitors, a high-precision measurement method using a 16-bit capacitor-to-digital converter (CDC) still offers clear advantages.
Capacitive sensors built on standard printed circuit boards or flexible printed circuits all use the same copper material for signal lines. In both cases, the maximum sensitivity of the sensor is determined by the sensor's physical size, dielectric constant, and coating thickness. For example, a 3mm thick sensor with a 5mm plastic coating is not as sensitive as a 6mm thick sensor with a 2mm plastic coating.
Our goal is to develop capacitive sensors that respond correctly and meet ergonomic requirements. In some applications, the sensor may be small, resulting in small capacitance changes on the user interface.Figures 1 and 2 show two common approaches to designing capacitive sensors on printed circuit boards. The figure shows the response characteristics of the sensor when an excitation signal is applied during user contact. Although the sensor capacitance will vary depending on the user's contact pattern, the performance of the sensor is not much different in the two 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 an electric field in the sensor, the electric field will partially extend out of the plastic film, and the ClN end is connected to the CDC.
Figure 2 shows another capacitive sensor design example, which adds a constant current source to the A terminal of the sensor and connects the B terminal to ground. When the user touches the sensor, additional finger capacitance is added, thereby increasing the RC rise time during the charge cycle.
A constant current source continuously charges the capacitive sensor to the comparator's reference threshold level. When the capacitive sensor reaches the reference threshold, the comparator will output a high pulse, then close the switch, discharge the capacitor and reset the counter.
A reliable capacitive sensor interface starts with an analog front end that must be able to measure the small output changes caused by user contact with the capacitive sensor. Now, new highly integrated CDCs allow design engineers to benefit from integrated low-power, high-resolution mixed-signal technology.
