Capacitive sensors typically have capacitance values ​​ranging from 50 fF to 20 pF, making it difficult to accurately detect small capacitance changes, thus requiring a suitable analog interface. Like traditional mechanical switches, the user experience with capacitive sensors is directly related to the sensor's sensitivity to contact under various reliability conditions. The sensitivity of a capacitive sensor is determined by its physical design, the method used to measure capacitance, and its ability to accurately compare capacitance changes relative to a preset contact threshold level. Capacitive sensors manufactured using conventional printed circuit board (PCB) processes typically have capacitance values ​​in the 50 fF to 20 pF range, making it difficult to detect minute capacitance changes. While several methods exist for measuring these minute capacitance values, the advantages of using high-precision measurement techniques equipped with a 16-bit capacitive-to-digital converter are significant. Capacitive sensors can be designed on conventional low-cost PCBs or developed on standard PCBs or flexible PCBs using the same copper material as signal traces. In both cases, the maximum sensitivity of the sensor is determined by the sensor's physical dimensions and the dielectric constant of the plastic screen printing compound, including the dissipation factor and the thickness of the screen printing material. For example, a 3mm diameter sensor with a 5mm plastic screen print is less sensitive than a 6mm diameter sensor with a 2mm plastic screen print. The goal is to develop capacitive sensors with correct response and that meet ergonomic requirements. In some applications, the sensor may need to be small, resulting in a smaller capacitance change due to user contact. Figures 1 and 2 illustrate a common approach to designing capacitive sensors on a circuit board. Figure 1: Capacitive sensor design considering field structure. Figure 2: Another capacitive sensor design approach. The figure above shows the sensor behavior under excitation when a user touches the sensor. In these two approaches, the sensor capacitance changes differently depending on user contact; however, the sensor performance is comparable in both cases. Exciting a capacitive sensor. The example in Figure 1 applies a continuous 250kHz excitation square wave to the source terminal (SRC) of the sensor to establish the electric field of the capacitive sensor. The electric field created by the excitation in the sensor partially passes through the screen-printed plastic. The capacitor input terminal (CIN) is connected to the input of a capacitance-to-digital converter. Figure 2 shows another example of a capacitive sensor design. A constant current source is applied to port A of the sensor, while port B is grounded. When a user touches the sensor, the capacitance of their finger is superimposed. This increases the RC rise time during the charging cycle. The conventional method for measuring the capacitance of a capacitive sensor and a probe sensor is shown in Figure 3. Figure 3: The conventional method for measuring capacitance uses a comparator and a 555 timer/counter to continuously charge the capacitive sensor to the comparator's reference threshold level using a constant current source. Whenever the capacitive sensor reaches the reference threshold level, the comparator goes high; this high level turns off the switch, discharges the capacitor, and resets the counter, as shown in Figure 4. Figure 4: The sensitivity threshold level of the conventional comparator and 555 timer/counter is determined by counting the clock cycles taken for the capacitive sensor to charge to the reference level (REF) on the comparator. This value is then compared to a preset threshold detection setting; for example, a count of 50 indicates sensor contact, while a count less than 50 may indicate no contact. In this example, the accuracy and precision of the measurement when the user touches the sensor depend on the frequency of the reference clock and the repeatability of the current source over a wide range of capacitive sensor values. The optimal method for measuring capacitance is shown in Figure 5, which employs a high-resolution 16-bit analog-to-digital converter (ADC) and a 250kHz excitation source. The AD7142 analog front-end excitation source in Figure 5 is a continuously outputting 250kHz square wave that establishes an electric field and electric field lines across the screen-printed material in the capacitive sensor. The precise 16-bit ADC can detect touches with a measurement resolution of 1fF whenever the user touches the sensor. No external components are required for adjustment; automatic calibration ensures no malfunctions due to temperature or humidity changes or failure to count certain touches. After the capacitive sensor output data is digitized, the detection threshold level for each sensor can be easily programmed individually by setting the corresponding 16-bit register. The threshold levels can be programmed approximately to 25% and 95.32% of the sensor's full-scale (FS) output value, as shown in Figure 6. Figure 6 shows the setting of the AD7142 sensitivity threshold level. A reliable capacitive sensor originates from the analog front-end, which must measure small output changes caused by user contact with the capacitive sensor. The new highly integrated capacitor-to-digital converter integrates a high-performance analog front-end with a low-power, high-resolution Σ-σ converter, enabling capacitive sensor system design engineers to benefit greatly from the latest advancements in mixed-signal technology.