Analysis of Conducted Emission with influences of operating frequencies and amplitudes of a self-oscillating capacitive touch sensing circuit

Abstract

In the rapidly evolving field of human-machine interfaces (HMI), particularly in the realm of touch screen technologies, capacitive touch sensing has gained prominence due to its superior flexibility and cost-effectiveness compared to other touch interfaces, such as resistive-based methods, infrared touch sensors, and surface acoustic wave sensors. However, this advancement comes with increased emission and susceptibility to Electromagnetic Interference (EMI) and similar disturbances, notably due to factors like operating sensing frequency and voltage. The previous research underscored the challenges of Electromagnetic Emission and some drawbacks of operating capacitive sensors at higher excitation frequencies. Characteristics of traditional capacitance to digital circuits like sigma-delta capacitive sensing circuits operate at higher frequencies, thus producing challenges in terms of emission and susceptibility. This paper offers a detailed assessment of the conducted electromagnetic emissions in a self-oscillating capacitance-to-time converter. The study primarily investigates how conducted emission characteristics change in response to the sensing circuit's operating frequency and voltage variations. The oscillating capacitive sensing circuit conducts sensing with a single clock cycle, thus mitigating some of the issues associated with the traditional capacitive sensing circuits, such as sigma-delta capacitive sensing, which generally require a higher frequency of operations. The results indicate that as the sensing frequency and the operating voltage decrease, the conducted emission of the sensor improves; this phenomenon can be particularly beneficial in high EMI environments like the automotive industry, where capacitive touch sensors are placed close to sensitive electronics.