When using conventional mechanical switches, the way a user interacts with the capacitive sensor interface directly affects the sensor's responsiveness (sensitivity) and reliability under various operating conditions. This article introduces several common analog front-end measurement methods for capacitive sensors.

  The sensitivity of a capacitive sensor is determined by its physical structure, the method used to measure capacitance, and its ability to precisely resolve capacitance changes relative to a contact threshold level. Capacitive sensors manufactured using conventional printed circuit board (PCB) techniques typically have a measurement range of 1 to 20 pF, making it difficult to accurately detect minute variations. While several methods exist for measuring such small capacitance values, high-precision measurement using a 16-bit capacitance-to-digital converter (CDC) offers distinct advantages.

  Capacitive sensors fabricated on standard printed circuit boards or flexible printed circuits utilize the same copper material for signal traces. In both cases, the sensor's maximum sensitivity is determined by its physical dimensions, the dielectric constant, and the thickness of the overlay. For example, a 3 mm thick sensor with a 5 mm plastic overlay is less sensitive than a 6 mm thick sensor with a 2 mm plastic overlay.

  The figure illustrates the response characteristics of the sensor when an excitation signal is applied during user contact. Although the sensor capacitance varies depending on the mode of user contact, the sensor's performance remains largely consistent in both cases.

  A continuous 250 kHz square-wave excitation signal is applied to the SRC terminal of the sensor to establish an electric field within the capacitive sensor. Once the excitation signal establishes this field, the field extends partially beyond the plastic overlay, while the ClN terminal is connected to the CDC.
Figure 2 illustrates another 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 introduced, thereby increasing the RC rise time during the charging cycle.

  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; the switch then closes, discharging the capacitor and resetting the counter.

  A count of 50 indicates contact with the sensor, whereas a count of less than 50 indicates no contact. In this instance, when a user touches the sensor, the accuracy and precision depend on the frequency of the reference clock and the repeatability of the current sources driving the various capacitive sensors.