A noise-immune stylus analog front-end using adjustable frequency modulation and linear-interpolating data reconstruction for both electrically coupled resonance and active styluses

As the demand for intuitive and interactive displays is increasing in mobile devices such as smartphones and tablets, a pressure-sensitive stylus pen solution is needed for advanced user experiences [1-5]. Figure 10.4.1 compares electromagnetic resonance (EMR), active, and electrically coupled resonance (ECR) stylus systems, and shows cross-sectional views of sensor panels. EMR stylus systems have been successfully commercialized for high-end devices due to their battery-less, light, and pressure-sensing features [1-2]. However, EMR stylus systems require an extra sensing panel for electro-magnetic coupling. The active stylus system does not require the additional sensing panel but needs a battery located inside the stylus. On the other hand, ECR stylus systems are cost-effective without needing either the additional sensing panel or a built-in battery. Furthermore, ECR stylus systems measure pen pressure by sensing a stylus' resonant frequency change upon pressure without additional circuitry [3]. Due to the external noise injection to the stylus system being more than ten times larger than the pen signal as shown in Fig. 10.4.1, noise immunity is a key performance factor for commercialization. This paper proposes an analog front-end (AFE) for both ECR and active stylus systems with high noise immunity that results in improved signal-to-noise ratio (SNR) by applying a fully differential architecture, adjustable frequency modulation (AFM), and linear-interpolating data reconstruction (LIDR).