26.5 A 977μW Capacitive Touch Sensor with Noise-Immune Excitation Source and Direct Lock-In ADC Achieving 25.2pJ/step Energy Efficiency

Capacitive touch systems play a crucial role in user interfaces across many applications, such as mobile devices, desktop PCs, and interactive whiteboards [1] –[6]. In capacitive touch systems, various sources of noise present significant challenges for achieving a high signal-to-noise-ratio (SNR) while keeping good energy efficiency, including display noise, fluorescent lamp noise, and charger noise (Fig. 26.5.1, top). Although fully differential sensing can cancel out display noise, channel mismatch still results in leakage into the signal chain. Moreover, fluorescent lamp noise and charger noise affect the touch-related channels and cannot be eliminated by fully differential sensing circuits. However, the noise sources exhibit a distinct spectral characteristic, with the energy concentrated on a limited number of frequencies and harmonics. In traditional approaches, noise mitigation involves setting the excitation signal frequency in a noise-free frequency band, implementing high-order bandpass filters, or using the lock-in architecture at the receiver side, as illustrated in Fig. 26.5.1 (left, middle, bottom) [3] –[5]. Unfortunately, these approaches lead to increased power consumption. Considering the growing importance of energy efficiency in the battery-powered operation, the factors pose significant challenges for low-noise capacitive touch systems.