Abstract
Flexible and thin displays for smart devices have a large coupling capacitance between the sensor electrode of the touch screen panel (TSP) and the display electrode. This increased coupling capacitance limits the signal passband to less than 100 kHz, resulting in a significant reduction in the received signal, with a driving frequency of several hundred kilohertz used for noise avoidance. To overcome this problem, we reduced the effective capacitance at the analog front-end by connecting a circuit with a negative capacitance in parallel with the coupling capacitance of the TSP. In addition, the in-phase and quadrature demodulation scheme was used to address the phase fluctuation between the signal and the clock during demodulation. We fabricated a test chip using the 0.35 µm CMOS process and obtained a signal-to-noise ratio of 43.2 dB for a 6 mm diameter metal pillar touch input.
Highlights
In recent years, ultra-thin and flexible displays have attracted a great deal of attention
The touch screen panel (TSP) on such a thin display must be very thin, which makes the distance between the touch sensor electrode and the display electrode extremely short, resulting in a large capacitance between the two electrodes
This paper focuses on how to improve the overall bandwidth of a touch sensor that has been decreased by the increase in the display coupling capacitance of mutual capacitive touch sensors with charge amplifier (CA) circuits, even though the TSP has a sufficiently large passband using metal-mesh electrodes
Summary
Ultra-thin and flexible displays have attracted a great deal of attention. In TSCs that use a charge amplifier (CA) scheme because of its excellent immunity to low-frequency noises, the frequency characteristics of the amplifier in the CA stage can limit the bandwidth of the touch sensor [2,4,5,6,7] This paper focuses on how to improve the overall bandwidth of a touch sensor that has been decreased by the increase in the display coupling capacitance of mutual capacitive touch sensors with CA circuits, even though the TSP has a sufficiently large passband using metal-mesh electrodes. The final section summarizes and discusses the measurement results for a test chip
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