A 6.9mW 120fps 28×50 capacitive touch sensor with 41.7dB SNR for 1mm stylus using current-driven ?S ADCs

Capacitive touch sensors are essential for the user interfaces of smartphones and tablet PCs. Large touch-screen panels (TSPs) require high-quality touch features, resulting in an increased number of sensing channels as well as a reduction of sensing capacitance due to fine-pitch arrangements. Therefore, touch-sensor ICs demand high resolution, low power and high efficiency. State-of-the-art capacitive touch sensors [1-6] mostly rely on capacitive charge amplifiers as analog front-ends (AFEs), which convert the capacitor change into a voltage signal and often limit the system noise figure, thus consuming most of the power. This paper presents a 120fps 28×50 touch sensor that achieves 41.7dB SNR for 1mm-φ stylus, while consuming 6.9mW, which results in an energy efficiency of 0.41nJ/step, for a >4× improvement compared to state-of-the-art stylus touch sensors (see Fig. 9.7.6). This is achieved by a current-driven ΔΣ ADC architecture, which implements charge balancing between a reference charge and a differential current from adjacent channels, directly interfacing with 2^nd-order ΔΣ modulators. An area-efficient sinc² filter, as a decimation filter, enables full parallel implementation of the 2^nd-order ΔΣ ADCs, providing sufficient suppression of interference signals. In contrast to previous works, the current-driven ΔΣ ADC only needs to digitize small differential currents, which relaxes requirements of the front-end amplifier, resulting in a large power reduction.