2.2 MSP CapTIvate PCB Design Guidelines for Mutual Capacitive Sensors
A good capacitive sensor design can improve a product's HMI (human machine interface) performance and robustness. This video introduces the fundamentals of mutual capacitance as well as suggested guidelines for button, slider and wheel sensor PCB layout.
This video covers mutual capacitive:
- PCB layout principles
- Button sensor
- Slider and wheel sensors
Mutual capacitance involves measuring a change in capacitance just like self-capacitance, with one big difference: we define both plates of the capacitor, instead of utilizing earth ground as the second plate. Note: Mutual capacitance may also be referred to as projected capacitance.
Mutual capacitance electrodes actually consist of two separate electrode structures, and they require two pins from the microcontroller- a transmit electrode, and a receive electrode. When a user touches an area on the panel where a Tx meets an Rx, the mutual capacitance between those Tx and Rx electrodes is reduced. This is because the user's interaction has the effect of disturbing the electric field propagation between the two electrodes. Users are coupled to earth ground, and the human body is a conductor. Placing a finger in between two mutual capacitance electrodes has roughly the same effect as placing ground between them- it reduces electric field coupling between them, which reduces the capacitance. Typical changes in mutual capacitance due to a touch are small- usually less than 1pF.