High SNR and High Frame Rate Analog Front-End ICs with Multi-Functional Active Stylus for Capacitive Touch Screen Panels

Abstract

The capacitive touch systems (CTSs) have been broadly used in various applications such as smart phones, tablet PCs, and electronic whiteboards due to their high durability, high sensitivity, and multi-touch capability. The CTSs are increasingly adopted in the high-performance touch applications, for which an analog front-end (AFE) IC with high signal-to-noise ratio (SNR) and high frame rate is demanded so as to respond to the fast movements of fingers and styli. In addition, styli have been required to represent various expressions. To rapidly respond to the movements of fingers and styli, the parallel driving method (PDM) is employed in the CTSs to realize the high SNR and high frame rate. The PDM simultaneously drives the orthogonal codes to multiple transmitter (TX) electrodes. The CTS with a small number of TX electrodes has a small size of the orthogonal codes, thus enabling a high SNR and high frame rate. On the other hand, the CTS with a large number of TX electrodes has a large size of the orthogonal codes, thus suffering from a low frame rate. Styli are categorized into the passive and active types depending on whether the stylus includes the battery or not. Several passive styli, such as the conductive, electromagnetic resonance (EMR), and electrically coupled resonance (ECR) types, have been researched. A conductive stylus has been broadly used because its low-cost implementation, but suffers from limited drawing expressions and a low SNR. An EMR stylus can express pressure, but requires an additional EMR sensor to detect the stylus, thereby increasing the thickness, power consumption, and implementation cost of the CTS. An ECR stylus also can express pressure by using a pressure-to-capacitance transducer without an additional sensor, but requires an additional readout circuit to detect the resonant signal from the ECR stylus, thereby increasing the complexity of the AFE IC. Active styli can express pressure without any additional sensor or readout circuit, but cannot represent more expressions. Therefore, this dissertation proposes a high SNR and high frame rate AFE IC and a multi-functional active stylus, which are suitable for high-performance touch applications. First, this dissertation presents a CTS using an adaptive chopper stabilization (ACS) method to accomplish high noise immunity. The ACS method rapidly detects external noises and adaptively removes them, thus accomplishing high noise immunity in the proposed CTS. To prove the proposed ACS method, the AFE IC was fabricated using a 0.35-μm CMOS process technology with 18 V high voltage devices and was measured with a 46-inch touch screen panel (TSP). When external noises are not induced into the TSP, the SNR of the AFE IC is measured to be 45.8 dB. When external noises are induced into the TSP, the SNR of the AFE IC using the ACS method is measured to be 44.5 dB. Therefore, the proposed ACS method adequately eliminates the external noises induced into the TSP. In addition, the SNR of the AFE IC using the ACS method is increased by 22.0 dB compared with the SNR achieved without using the ACS method. Second, an AFE IC using a multiple frequency driving method (MFDM) is proposed to achieve a high SNR by detecting a noise spectrum and locating the frequencies of excitation signals in low-noise regions. Furthermore, the proposed MFDM increases the frame rate by concurrently sending excitation signals having multiple frequencies to the TSP. The AFE IC extracts the coordinates of the finger and stylus as well as the pressure and tilt angle of the active stylus with the force gauge and gyro sensor, respectively. The proposed AFE IC and active stylus were fabricated using a 0.13-μm standard CMOS process. The measurement results show that the AFE IC achieves a frame rate of 3906 Hz, and the SNRs of 61.0 dB and 50.1 dB when a finger and active stylus are used, respectively. Furthermore, the active stylus can express the pressure and tilt angle with 6-bit resolution. A CTS with the proposed AFE IC is demonstrated with a 65-inch TSP using 10 fingers and the active stylus with the pressure and tilt angle. Finally, a multiple-way interactive CTS (MI-CTS) with the palm rejection of the active stylus is proposed in an attempt to allow simultaneous interaction between the CTSs on a real time basis, while reducing the computational load. The proposed MI-CTS is realized with 3609 CTSs using Wi-Fi communication (802.11ac protocol) when 1 finger and 4 styli are used. The proposed active stylus and AFE IC were fabricated in a 0.13-μm standard CMOS process. The proposed active stylus achieved the palm rejection without heavy computational load. In addition, the proposed MI-CTS were successfully demonstrated with an 85″ and two 32″ TSPs.