# Analysis of Multi-Level Simultaneous Driving Technique for Capacitive Touch Sensors
## 1 Introduction  
The Internet of Things and wearable sensor technologies have accelerated the development of various sensors and their integrated design methodologies. Sensors estimate absolute or relative physical quantities, although their final outputs have different contexts such as the distance, location, and orientation of the target device. Sensors are usually exposed to noise from a variety of sources inside and outside the system. Therefore, noise immunity is a primary metric for quantitatively evaluating sensor systems. In the case of capacitive touch sensors, for overcoming various types of environmental and dynamic noise including display noise, simultaneous driving techniques can achieve a high signal-to-noise ratio (SNR) with a limited response time [1; 2]. Because technological advancements continue to yield thinner and more compact panel designs, display noise has become one of the most problematic types of noise that deteriorate the SNR [3; 4; 5]. For example, if the shielding layer between the display and the touch panel is removed to design more compact and less expensive display modules, the incident display noise for the touch sensor also increases. The SNR decreases with a decreasing thickness of the display and touch panels. High transmission (TX) signals of sensors that are strong to the increased display noise and other external noise sources generate flickers on the display and affect the display's quality and reliability. TX signal levels can also be constrained in ultrathin panels because of increased parasitic capacitance and power consumption [1; 3]. Thus, when placed in close proximity, the display panel and touch sensors compete for their own specifications and performance levels in newer technologies. This paper presents a new simultaneous TX driving and multiplexing technique for capacitive touch sensors that employs multi-level Hadamard (_M-H_) matrices for communication systems [6; 7]. In a capacitive touch panel where many capacitive elements are forming between transparent channels, alternating signals for TX have used to sense the capacitances in a sequential manner. Orthogonal codes used in TX driving fully overlap the sensing time for multiple TX channels and then increase the SNR of the capacitive signal at the receiver [1]. In this work, the theoretical sensor model [8] will be employed to extract SNRs for _M-H_ matrices that are applicable to capacitive touch sensors. This attempt provides unified and quantitative views for modeling and evaluating multiplexing techniques for different sensor systems. Unlike two-level bipolar Hadamard (_B-H_) sequences with orders of 2 and 4 [1; 9; 10; 11; 12; 13; 14], the proposed application can theoretically be used to establish any order of orthogonal sequences. Odd number orders of multiple TX lines and the corresponding code sequences can be applied, which yields a better \(H\) matrix to satisfy the given system requirement for the scan rate or the reporting rate of the sensor to maximize the SNR. Another important feature of these \(H\) matrices is that regular column sum of the _M-H_ matrix and the acquired capacitive signals can be configured due to their capacity efficiency. Although the cost and complexity must be considered in the TX driver design, it can be a competitive option in newer module designs. In our system model, modulating sequences for orthogonal frequencies [15; 11] can also be regarded as another type of orthogonal code for simultaneous driving including multi-level sequences. The orthogonality between sub-carrier frequencies still provides higher SNRs in longer modulating sequences. However, the most frequency-dependent noises (such as the ones from the fluorescent light circuit, battery charger and the display panel) constrain the selectivity of the TX frequencies. Moreover, the slowest TX frequency increases the entire code length and the full scan time of the touch-screen. This paper is organized as follows.